By Brian Tomasik
First written: 11 June 2013; last update: 15 Jan. 2017


If bugs can suffer, parking lots and sidewalks probably prevent lots of bug suffering by precluding plant growth, thereby lowering bug populations for many decades into the future. That said, pavement also occasionally traps worms, slugs, and other bugs, especially after a rainstorm dries up. If we see a stranded worm, is it better to rescue it? Should we humanely squish insects trapped in our house? I discuss possible reasons for and against and conclude that further research on the population dynamics of bug suffering would be worthwhile. In any event, if you do squish bugs, make sure to do so as quickly and painlessly as possible, preferably by forcefully smushing them with a hard, flat surface like a piece of wood against a piece of scrap paper, in order to crush the whole body instantly and then drag out the guts along the paper to make sure neural connections among major body regions are destroyed (see here for an example of how I do it). I also point out that preventing photosynthesis may be a more guaranteed way to reduce insect suffering than trying to operate on insect ecosystems themselves.

I currently avoid squishing insects most of the time, because I fear that the same food they're eating might be eaten by someone else if they weren't around. Other reasons include squeamishness about causing immediate suffering, the common-sense view that if you care about insects you shouldn't kill them, and moral pluralism regarding non-utilitarian views that other compassionate people hold.

For more concrete advice, I put together a wikiHow article: "How to Avoid Hurting Insects". One of the easiest tips is to avoid buying silk, since its production boils silk worms alive.

If you choose to freeze insects as a form of euthanasia, the following paragraph from "BIAZA Recommendations for Ethical Euthanasia of Invertebrates (Version1.1-Apr2013)" (p. 8) should be kept in mind:

Ideally, surplus invertebrates or invertebrates requiring euthanasia should be taken to a veterinary practice. However, this may not be practical or possible. Most members of the public do not have access to any chemical methods therefore physical crushing or freezing is still being advised by some institutions. Freezing is increasingly regarded to be inhumane without prior anaesthesia (Pizzi, 2012). However, if there is no alternative and euthanasia of an invertebrate MUST be carried out at home it is best to refrigerate first for a minimum of four hours then place the individual in the coldest freezer possible for at least two hours (Bushell, personal communication).


It's conventionally assumed that if you care about insect suffering, you should want to avoid stepping on insects, should want to let them outside when they're trapped in your house, should want to avoid pesticide use, etc. The Jains and Buddhists probably agree. These recommendations may be right, but it's not completely clear. For one thing, the insect that you avoid killing would have died painfully in some other way. In addition, it might have gone on to have children, most of whom would have died painfully shortly after birth. So it's possible that killing the insect could be better after all. Of course, if you do kill it, you should squish it as quickly and completely as possible, rather than drowning it, vacuuming it, spraying it, or casually crushing it without completely destroying its brain. But is it better to be squished or to die naturally? And if you do squish it, will it be replaced by other insects taking its place?

We can see that the question of how to treat insects and other bugs that we interact with is not obvious. This essay aims to suggest a few theoretical considerations, but it lacks empirical details, and the framework itself may be missing important points that would be elucidated with more entomological theory and data. I begin by considering the dilemma of what to do with worms on pavement and then consider the general problem of when to (humanely) squish bugs and when not to.

Worms on pavement

The saddest thing that I see in person on a regular basis is the stranding of worms on concrete during the rainy season. This is obviously a tiny fraction of the wild-animal suffering that occurs even in my own neighborhood, but it's emotionally salient because the worms are immediately accessible, and I might be able to do something to help them. The other wild animals are out of my immediate reach.

I've asked whether dying worms on the sidewalk would prefer to be humanely squished and put out of their misery. Now I'm not certain, but I think maybe either leaving the worm alone or squishing just the head would cause the least suffering.

In this piece I ask a different question: When we see stranded worms, should we try to rescue them? That is, should we return them to a neighboring patch of soil?

Some short-term considerations:

  • Handling. When I pick up a stranded worm, it's pretty stressed out. Usually it poops in my hand. This is unfortunate, but probably it doesn't compare with the severity of everything else at stake. I don't think I cause lasting tissue damage by handling, although sometimes it can be difficult to pick up a worm without pinching it pretty hard. When the worm is wet and tough to grab with fingers, I should use a small stick or piece of bark instead.
  • Disorientation? When I put the worm back on soil, does it lose track of where it is? Does this cause long-term harm? I would guess not, but maybe I sometimes put it in a spot where it wouldn't actually be able to survive? That said, in the few cases when I've checked back, the worms have usually seemed to disappear into the soil after a few minutes. It would be good to confirm this, though.
  • Would they have made it anyway? Some worms march along confidently and briskly. They seem like they can cross the sidewalk or parking lot without a problem. Others look weaker and move slowly, and it's not clear whether they'll make it. I know for certain that some don't make it, because on mornings after a wet night, there are often stranded worms drying up in the sun. Still, we can ask what fraction of the worms would make it on their own without help. If it's most of them, this reduces the potential importance of helping them relative to the the handling and disorientation costs.

Longer-term considerations:

I assume that worms' lives are probably about neutral before death (maybe slightly negative during periods of cold, hardship, or disease) and pretty negative during the process of dying. There are a few cases to consider for the long-term impacts of rescuing them.

  • If the rescued worm would die soon anyway:
    • In this case it probably doesn't matter much whether you rescue the worm. Out of ignorance, I would assume that death by dehydration, cold, organ failure, etc. in a parking lot is about comparable to death by other means later on. (How do most worms die anyway?) The few extra hours or days of life from saving it aren't enough to matter much in the calculation.
  • If the rescued worm would live a while longer but wouldn't reproduce:
    • In this case, it seems plausibly good to save the worm. This is not because its own life would be net positive but because its existence might help a little bit to prevent the existence of other worms, assuming some sort of carrying-capacity constraints on the number of worms that can exist in an area. This worm would consume food that would otherwise be used to support other worms. That said, I don't know how strong this effect is, but it's probably real. If there weren't carrying-capacity constraints, the worm population should keep expanding until there are such constraints. The worm might also eat food that would have been eaten by other species and might affect soil ecology generally -- see the discussion below for more on this.
  • If the rescued worm would live a while longer and would reproduce: This may be a pretty bad outcome, because once the worm reproduces, it creates lots more worms that will have to endure the pain of death. There are two subcases here.
    • If there are hard carrying-capacity constraints on worm population:
      • In this case, either the current worm survives and reproduces, or someone else does. So it doesn't matter from an offspring perspective whether you rescue it; just decide whether to rescue it based on which way of dying is less painful for the current worm.
    • If not saving the worm decreases the total worm population for at least a little while:
      • In this case, we need to consider the spillover effects of fewer worms. Of course, this seems like a good thing for the worms themselves: Fewer of them means fewer babies born for some generations until the population rebounds. However, it also could have implications for other species.
        • Would the worms have eaten food that would otherwise have been consumed by even smaller critters? In this case, maybe it would actually be good to have more worms.
        • Worms contribute to the fertility of the soil and improve plant growth, nutrient cycling, etc., which is probably bad.

I'm tempted to assume that these indirect effects of having more worms cancel out in expectation, just out of ignorance rather than because I think they actually do. The risk of creating more small invertebrates seems real, but it's also easy to imagine that soil without worms might become more lifeless, which would be good.

The uncertainty here is really high. This discussion would benefit from expertise by oligochaetologists and edaphologists. It would also benefit from empirical investigation of the soil-ecological questions raised.

How long does it take a half-squished bug to die?

Entomologist Jeffrey Lockwood wrote:

My sense is that it takes a rather long time for a partially crushed worm to die, as its physiology and anatomy are such that death would not follow nearly as quickly as it would for a mammal with a more complex and concentrated set of vital organs.

This agrees with my anecdotal experience of seeing injured bugs that appear to have been debilitated for hours or perhaps days but are still barely moving.

In mammals, if the heart stops or the organism loses too much blood, brain activity ceases soon thereafter. In contrast, arthropods have open circulatory systems and don't receive oxygen via their hearts. Instead, they breathe via their tracheal systems.

Can damage to the tracheal system cut off brain activity? How long would that take? What if half-crushing a bug doesn't destroy the tracheal system? Indeed, many small insects don't actively inhale and exhale at all. I'm curious to find out more about this, but it looks like partly crushed insects are in for terribly long deaths.

Should we (humanely) squish non-predator insects?

Here's a more general framework for this question. Remember that some insects like most of the ants and bees that we interact with will not have kids, since reproduction is reserved for the queen. (On the other hand, maybe such insects would still contribute to reproduction by feeding the home nest.)

  • Ignoring effects on other species:
    • If the insect you squish wouldn't have more kids before dying then
      • if there's no hard carrying-capacity constraint such that its existence doesn't affect other insects, then just squish if and only if squishing would be less painful than dying naturally
      • if there is a hard carrying-capacity constraint such that its existence does prevent another insect,a then
        • if the other insect wouldn't have had kids either, squish if and only if squishing would be less painful than dying naturally
        • if the other insect would have had kids, don't squish because that prevents the other one from having kids.
    • If the insect you squish would have had more kids before dying, then
      • if there's no hard carrying-capacity constraint such that its existence doesn't affect other insects, then squish to prevent it from having kids
      • if there is a hard carrying-capacity constraint such that its existence does prevent another insect, then
        • if the other insect wouldn't have had kids, squish this insect to prevent its kids
        • if the other insect would have had kids also, then you'll end up with the same amount of kids whether you squish or not, so squish if and only if squishing is less painful than dying naturally.
  • Considering other species:
    • If the insect is big and prevents smaller insects from existing by stealing their resources, then never squish.
    • If the insect contributes to recycling nutrients in the ecosystem or otherwise seems to allow more other insects to exist, then always squish.

One thing I neglected to consider is that it's possible, at least for some species, that having insects around doesn't just starve out other offspring but actually prevents other offspring from being born due to hormonal, etc. effects. I don't know how common this is.

Should we take non-predator insects outside?

The analysis above discussed only the question of whether to (humanely) squish an insect, but it's also an option to let the insect outside (except during the winter).

  • Ignoring effects on other species:
    • If the insect you let outside wouldn't have more kids before dying then
      • if there's no hard carrying-capacity constraint such that its existence doesn't affect other insects, then let it outside if and only if dying outside would be less painful than dying inside
      • if there is a hard carrying-capacity constraint such that its existence does prevent another insect, then
        • if the other insect wouldn't have had kids either, let it outside if dying outside would be less painful than dying inside
        • if the other insect would have had kids, let it outside because that prevents the other one from having kids.
    • If the insect you let outside would have had more kids before dying, then
      • if there's no hard carrying-capacity constraint outside such that its existence doesn't affect other insects, then
        • if it wouldn't have kids inside due to dying prematurely, then don't let it outside to prevent it from having kids
        • if it would have kids inside, then it may have kids whether you let it out or not. The better choice in this case isn't clear. Letting the bug population grow inside will be a nuisance and might lead to you accidentally crushing the bugs. On the other hand, it's plausible that the bug population inside will taper off sooner than outside and so result in fewer long-run offspring (since we're assuming that there's no hard carrying-capacity constraint outside).
      • if there is a hard carrying-capacity constraint outside such that its existence does prevent another insect, then letting it outside is plausibly better to prevent it from starting a population inside.
  • Considering other species:
    • If the insect is big and prevents smaller insects from existing by stealing their resources, then let it outside.
    • If the insect contributes to recycling nutrients in the ecosystem or otherwise seems to allow more other insects to exist, then don't let it outside.

Keeping an insect inside also runs the risk that you or another inhabitant of your house will step on it accidentally, which might crush it incompletely and leave it suffering for hours or days.

It seems especially important to gently take outside those insects that would be able to sustain a population inside, such as ants eating food scraps or moths whose larvae eat clothes/towels. Often when I find moths on towels, I can get them outside just by gently picking up the towel, taking it out the door, and then shaking it so that the moths fly away, though sometimes I have to use a Ziploc bag to catch the moths and remove them.

Should we (humanely) kill predator insects?

Should we kill spiders? Should we destroy spider webs? Here the principal consideration will probably be the impact of the predator on its prey rather than its own welfare.

First, here's one line of reasoning:

  • If predators have no long-term impact on prey populations, probably we should kill the predators (as painlessly as we can) because their only effect is to kill prey instead of letting the prey die in other ways, and I think death by predation is worse than death by other means. One argument for predators not having an impact on prey populations could suggest that prey produce huge numbers of offspring, so the population will still reach carrying capacity even if lots of offspring are eaten. Or, even if those prey don't eat the food, some other prey will. Also, killing prematurely increases prey turnover rates, which could imply more deaths even if the population remains the same.
  • However, if predators do have a long-term impact on prey populations, we should ask whether we want more or fewer prey animals.
    • If the prey are big insects that prevent other insects from existing, it could be good to have more prey, in which case we should kill the predators.
    • If the prey are small insects, it may be good to keep the predators if doing so prevents the prey from having more babies that die shortly after birth.
    • If the prey are themselves predators, then these conclusions may be reversed, and the analysis should proceed to the organisms that they predate upon. For example, if we want more prey, we want fewer first-level predators, so we want more second-level predators, which means fewer third-level predators, etc.

Here's a second line of reasoning:

  • There's a fixed amount of plant food to be eaten by herbivorous prey. Someone has to eat it, so we need those herbivores around. Therefore, it's good for herbivorous prey to have as little obstruction as possible in doing their task of eliminating biomass.
  • Indeed, in the long run, one reason prey have so many offspring is to maintain resilience against predation. If we remove predators, is it possible that eventually the insects would evolve not to produce as many offspring? Is this effect too small to be significant except for large-scale, sustained interventions?
  • One exception to this reasoning is if the plant food would have instead been eaten by non-sentient bacteria or by much-bigger herbivores if the small insects hadn't eaten it. Another exception is if the insects themselves help to increase plant growth more than would have happened otherwise. (Do bees do this, or would non-bee-pollinated plants produce just as much biomass?)

I destroy spider webs whenever I see them. For cobwebs, this is just a matter of cleaning up to make sure insects don't get needlessly trapped; for new spider webs, it's a matter of preventing insects from getting caught and eaten. However, destroying spider webs probably only hinders spiders' predatory efforts for on the order of a day or something. Webs actually take just an hour to build, though maybe depleting the spider's silk and other dilatory effects operate as well?

If I have proper equipment, I go further and squish spiders, but I recommend this only if you can do it really humanely. I have a "smushing board," which is a piece of wood with flat ends that I use to crush certain insects in one big, instantaneous smash against paper underneath, after which I drag them along the paper to fully flatten them. The things that I crush at the moment are (a) flies that appear too weary to survive and are basically dying on the floor and (b) all spiders. I scoop the bugs off the floor or window using a piece of paper, deposit them on the smushing paper, aim, and then smush -- making sure I press until all the innards are flat, to minimize the risk that lingering ganglia are still firing in pain. It's especially important to avoid causing pain to spiders, because as predators, spiders are highly astute and probably much more aware of the pain they're enduring than the average bug is.

Predators and prey birth rates

One interesting question to explore is how predation pressure affects the reproductive rates of prey. More births per prey parent imply more total prey deaths per unit of food consumed by the prey. That is, even if the prey population size remains constant (due to food limitation, etc.), that population will contain more (painful) prey deaths per unit time (due to more non-surviving offspring) and hence plausibly more total suffering.

One possible argument is that, in the presence of predators, prey will produce more offspring per parent so that at least some of the offspring will avoid being eaten. However, this is just a hypothesis, and the reality might be the opposite.

Guo et al. (2011) found that members of a prey rotifer species "grown in predator-conditioned media had lower population abundance and slower population growth rate than controls" (p. 163). The authors speculated on why this might have been (pp. 169-70):

Compared with controls, conditioned medium from the predator, Asplanchna, reduced the abundance and offspring production of B. calyciflorus, possibly due to increased energy allocation to anti-predator defenses. We also observed significantly longer spines in B. calyciflorus in the presence of predator-conditioned medium as compared to controls. It therefore seems that exposure to Asplanchna kairomones causes B. calyciflorus to reduce investment in reproduction by lowering fecundity while increasing posterolateral spine length, both of which improve survival in the presence of predation. Thus, resource allocation to growth and reproduction seems to be flexible and responsive to predation risk.

Should we err on the side of not squishing healthy insects due to r-selection?

The above decision trees consider possibilities about population dynamics and replacement of one insect by another in the long run. They don't, however, consider potential evolutionary effects over even longer timescales.

In particular, consider why organisms use r-selection: It's because they want to produce enough offspring so that, despite a dangerous and variable environment, enough kids can survive to reproductive age. More unstable environments tend to induce more r-selection. If more healthy bugs are killed -- whether by squishing, trapping indoors, or allowing predators -- does this tend to cause more r-selection?

One naive model might say "yes." For instance, suppose that a given insect species has 1000 offspring per parent pair. Say that 900 of these die early due to poor hatching, infant mortality, predators, starvation, etc. 100 make it a little longer, but 90 of them die at an intermediate age. The remaining 10 duke it out for resources and mates, and 2 of them are ultimately successful in reproducing on average. Those insects that have at least 10 candidates in the final stage have a bigger pool from which to select the best 2 winners and hence have better long-run reproductive success. In other words, it seems that evolution wants to keep the number of candidates high. Say evolution chose to always keep at least 10 final-round candidates. Now consider what happens if we kill an extra 50 insects out of the 100 intermediate survivors. Then there are only 5 candidates duking it out in the final round. This means that in the future, evolution will push the insects to have 1500 offspring instead, so that 150 will survive to an intermediate stage, so that after you kill 50, there are still 100 that are then winnowed down to 10 final contestants. In this case, every intermediate-stage bug you kill implies 10 more that need to be born. If you kill a final-stage bug, 100 additional eggs are needed. Of course, I just made up these ratios, but they don't seem completely unrealistic.

However, it's not clear that evolution would exactly compensate for the extra offspring killed. There are other constraints on how many offspring an insect pair can have. Indeed, if that weren't true, then insect couples would presumably want to have infinitely many offspring in order to maximize self-promotion of their copies and ensure maximal selective pruning away to leave the fittest candidates. So it's actually quite plausible that the number of offspring that insects have is not dependent at all on how many of their kids are killed at what stage, at least within some reasonable range. Maybe number of offspring is limited by egg size, metabolic cost, and other factors. If so, then the concern about killing healthy bugs leading to greater r-selection would not apply.

This question -- what are evolutionary determinants of how many offspring an insect couple has? -- seems very concrete and may already have been answered in the literature. There are a number of studies on determinants of clutch size, but these typically focus on game-theoretic models of competing individuals within a species or on resource costs, rather than predation or other mortality-related factors.

If higher death rates by healthy insects do lead to more offspring produced in the long run, it's not clear what the effect size is. As demonstrated in my simplistic example, it's conceivable that one extra death of a healthy later-stage bug could apply pressure for tens or hundreds more eggs, but that was an extreme case. The other extreme is that number of eggs isn't affected at all by death rates within a reasonable range. Also, the high death rates would need to persist for a long time before evolution found it worthwhile to make this adaptation, but on the other hand, once the adaptation happened, it might also persist for a long time, beyond the point at which the higher mortality abated; this makes it hard to assess the impact of a single instance of killing a healthy bug. In general, if the previous decision trees suggest that you should be roughly indifferent between squishing a healthy bug or not, the r-selection consideration suggests erring on the side of not squishing. However, if in fact, insect populations are not at carrying capacity, then you should potentially squish healthy bugs. It's not clear what the safer approach is.

Isn't helping bugs an inefficient use of time?

Well, it partly depends what you would be doing instead. Sometimes I take a break to walk for a few minutes outside (on pavement only! -- walking on grass or forest is liable to squish bugs in the soil), in which case there's basically no cost to look for bugs that seem like they could benefit from my help -- e.g., a dying bee or worm that I can move out of the way of pedestrian feet. (Of course, if there are lots of healthy insects on the sidewalk, it may be best not to walk there because you might step on a few of them.)

The potential to affect insect suffering via crop agriculture is big -- there are probably at least 1 million insects per hectare of crop land at a given moment. Say we can get 5% net confidence that the change we advocate (e.g., farming more or less land, or switching to crops with lower biomass density) can reduce insect suffering by 10% over some duration (say, 1 year) until the situation reverts back to normal. Then we could prevent 5,000 insect-years per hectare-year in expectation. Suppose that our advocacy efforts in a year could change whether 40 hectare-years of farming take place. Assuming 8 hours of work per day on each day of the year, that implies saving 68 insects per hour, or more than one every minute. It's remotely possible you could help as many insects per minute in your own house and on your own sidewalks if there were a lot of bugs out or if looking for bugs had basically no opportunity cost, but it would be hard, and my estimate for the cost-effectiveness of the long-term approaches was almost certainly too low.

However, the problem with calculations like these is that they assume you actually will become an insect-suffering researcher/advocate and spend your time super-efficiently. It's not clear in practice whether the 2 minutes you save by not helping some worms will in fact translate to several insects saved by farther-range means. In my own case, I think assisting injured bugs a little bit may actually help to keep me on track as far as caring about the bigger-picture issues. There are two ways in which this is so.

  1. It could be tempting to reduce the cognitive dissonance of seeing suffering bugs in front of me and not helping by saying to myself, "Ah, that's probably not so bad." I could also say, "I would help, but I should work on longer-range insect helping instead." However, if I do the latter, I'm reminded of how valuable my time is, and that makes me feel bad about wasting it from time to time. Because I don't like to feel bad about occasionally wasting time, I might come up with other excuses, including brushing off insect suffering as not such a big deal.
  2. I would rather do insect advocacy than help them directly in person. However, I may find myself not helping them in either way. If I then see suffering insects that I could be helping directly, it reminds me that I could also be working on helping them indirectly. Occasionally this prompts me to do something indirect to help them.

Anyway, these could be peculiar facts of my psychology, so your mileage may vary, but I doubt I'm the only one for whom they're true.

I think it's useful to answer questions like "What should you do with insects you find in your house?" not just because doing so illustrates broader principles but also because this can provide a hook to interest more people in the general topic. Lots of people think about how to handle bugs in their house, and that's a direct, immediate example of how they affect insect suffering. From that one can begin moving to bigger-picture forms of insect suffering.

Should we focus on reducing plants?

We've seen that the ecosystem dynamics among animals are complicated. For example, we need to worry whether eliminating bigger insects allows more smaller insects to survive. However, if you think about it, plants could be seen as the root of the problem (pun intended). After all, if we had fewer plants, there would be less food, and all the levels of insects would no longer be an issue. If the food is produced, eventually some possibly sentient organism will eat it, unless it's burned or decomposed by non-sentient bacteria/fungi/etc.

So, could reducing plant photosynthesis be a more guaranteed way to reduce insect suffering? I say "photosynthesis" in particular because that's the process that creates energy. If we just cut down plants, then more will grow in their place, and the energy has already been captured in the fallen plant stalks. If, instead, we cover an area so that plants can't grow at all, that actually effects a net reduction in conversion of light to food energy.

One simple way to reduce plant mass is through land-use changes: e.g., paving hell and putting up a parking lot. This prevents plants from photosynthesizing on that land for decades into the future. There are other activities humans do that have bigger effects in this direction.

You might be able to reduce biomass on your own property by covering over your lawn with gravel, pavement, or something else to block sunlight. Covering plants with boards might work too, but I often see insects making a home underneath boards, so I hope this wouldn't actually increase insect populations by giving them more shelter. Anecdotally, it seems like many of the bugs I can see around my house feed on flowering plants -- as do most of the bugs in this video about grassland insects -- so covering over flowers would be particularly valuable. That said, covering ordinary grass is probably good too, because even if insects don't eat grass directly, flies and other bugs eat the poop of rabbits and woodchucks that do eat grass.

Plants seem like an obvious growth-limiting factor for animal populations in the long run. Are there others that could be tweaked? Maybe water availability, nutrients, climate? But it seems like these should mostly matter through their effects on plant growth.

In aquatic ecosystems, dissolved oxygen may be a limiting factor, and eutrophication may actually make deep lakes anoxic. So it's possible that in this case, more primary production doesn't mean more sentient organisms, unless there are massive numbers of very tiny but possibly sentient creatures that can survive anoxic conditions? At least on land, though, it seems pretty clear that more plants imply more animals.

What are the main limiting factors for insects?

This page on an entomology website contains an excellent overview of the variety of factors that may limit insect populations.

  • Certain factors, such as food and habitat, imply some degree of fixed carrying capacity for insect growth: if one insect thrives, it's at the expense of another. Let's call these "resource limitations".
  • Other factors -- especially predators, pathogens, and parasites -- suggest that insect populations may be limited not by resources but by mortality. Let's call these "predator limitations", in order to use a concrete name even though predators are not the only organisms that kill insects.

Another page echoes the same distinction:

The amount of resources available can affect the size that an insect population can attain. This concept is sometimes referred to as the carrying capacity of an environment (Schowalter, 2006). Resources that provide food or shelter are sometimes called “bottom up” factors. Populations can also be limited by natural enemies or infection by insect disease-causing organisms, sometimes called “top down” factors. Planting a non-preferred crop plant through rotation or resistant varieties, and conserving natural enemies combines both bottom-up and top-down factors to lower insect pest populations (Tscharntke and Hawkins, 2002).

This piece explains (in the context of vertebrate predators):

Many consider it obvious that removing predators should increase prey populations, but predator-prey interactions are far too complex to assume this. For example, if prey are limited by habitat then removal of predators may do little good. Predator control is based on the assumption that predators limit prey abundance and that a decrease in predators will increase prey productivity or abundance, or at least reduce the overall losses of prey. The success of predator removal depends on what fraction of natural mortality is caused by the predator and how other sources of natural mortality interact.

This textbook reports that predators can reduce prey populations (again, in the context of vertebrates):

To determine whether moose populations are held below carrying capacity by timber wolves, François Messier and Michel Crête determined the age and condition of 62 moose consumed by wolves during the winters of 1981-1984 in Québec. The researchers found that wolves killed a disproportionate number of older moose. Older individuals made up just 9 percent of the total population but 34 percent of wolf prey. Nonetheless, almost two-thirds of the total kills were either calves or moose of reproductive age. This observation suggests that predation could have a strong effect on the moose population size. It supports the hypothesis that predation holds populations below carrying capacity.

Wisconsin DNR (2016) says: "studies have shown that wolves have minimal negative impact on deer populations, since they feed primarily on weak, sick, or disabled individuals." However, even if this is true for wolves, my impression is that it's less true for many invertebrate predators. For instance, spider webs tend to trap flies that move around actively, not those too ailing to fly.

It's unclear whether resources or predators are more limiting on lawns, in forests, etc.:

  • One argument for resource limitation is that if we imagine reducing the size of the habitat area by 1/2, intuitively the prey and predator populations would also reduce by 1/2. If food or habitat wasn't limiting, then if we shrunk a habitat by 1/2, all the prey and predators should just cluster in the shrunken habitat area and go on just as before. Unless prey are now easier to catch due to being closer together or something, the populations of this ecosystem should be identical to the populations with the original, bigger ecosystem, which seems implausible.
  • Here are two arguments for predator limitation:
    • Most plants that insects eat don't seem to be fully consumed by insects (with some exceptions for leaves that have been fully chewed). This seems to suggest that more prey insects could exist with the same number of resources. That said, it's hard to tell for sure how much food is available for prey insects, since most prey probably only eat a small fraction of plant types, and they may be picky about which parts of a plant they eat. Maybe insects do eat most of the juiciest parts of plants, and maybe plants set up defenses against further consumption.
    • Large herbivores are at least partly limited by predators, because when apex predators are eliminated from an area, herbivore populations tend to increase. So it's plausible that small herbivorous insects would be similarly predator-limited.

The correct answer could be some combination of many factors. Whether bugs are resource-limited vs. predator-limited also depends on the location and type of bug. In my house, it seems that most flies are food-limited, for example.

This distinction between resource limitation and predator limitation is important because it influences whether killing insects reduces net insect populations:

  • If you kill prey insects outdoors, then
    • if resources like food or space are limiting, then killing one prey insect may simply allow another to take its place.
    • if mortality from predators and disease is limiting, then
      • if your killing prey insects doesn't affect predator populations, then your killing of prey insects reduces their populations temporarily relative to if you hadn't killed them.b
      • if your killing prey insects reduces the number of prey insects eaten by predators, then:
        • if there's a one-for-one replacement in which every prey insect you kill is one less prey insect that would have been eaten by predators, then you don't in fact reduce net prey populations. In this case, whether it's good or bad to kill prey depends on whether you kill them more or less humanely than predators kill them.
        • if every prey insect you kill only reduces the number eaten by predators by less than 1,c then you still produce a net reduction in prey-insect populations, in which case quick and thorough squishing could be preferable.
  • If you kill prey insects indoors or take them outside, then you probably reduce their populations, since generally indoor insect populations haven't reached carrying capacity and so will grow if not stopped.

As before, these arguments don't necessarily apply for ants, bees, and other non-reproducing insects. In their cases, killing them doesn't obviously reduce the colony's overall population except insofar as you stop the ants/bees you kill from feeding the nest.

Other ways to not harm insects

I compiled most of my advice about everyday ways to not harm insects in a wikiHow article: "How to Avoid Hurting Insects". Here are some additional points too trivial or speculative to include there:

  • If your garbage can contains any food waste, it may become home to fruit flies. This is bad because (1) you may crush fruit flies when adding in more garbage and (2) when the garbage goes to a waste compactor the remaining flies will very likely be crushed. To avoid this problem, seal food waste in plastic bags -- maybe several layers of bags. Ideally these would be bags you're going to dispose of anyway. An additional benefit of doing this is that sealed waste is more likely to decompose via bacteria rather than by insects, so fewer total insects will be born and die painfully soon thereafter.
  • Driving kills tons of bugs. Minimizing driving and using public transportationd help reduce this impact. In addition, it seems particularly important to avoid driving in the rain (when worms and slugs may come out onto the road) and at night during non-winter months (since presumably bugs come out more at night and may be attracted to your car's lights).
  • Some vegetables at the supermarket have different densities of bugs than others. For instance, celery and sometimes lettuce may contain small flies, especially near the stem. Carrots and cucumbers don't. Does this suggest that insect densities are lower on cucumber crops, or is it just an artifact resulting from the way in which insects can get caught in some vegetables but not others? Absence of insects from veggies could be due to washing. For instance, salad mixes are pre-washed, which means that they could have contained lots of insects that merely got drowned before you saw the final product. In general, exploring which vegetables were produced with less insect suffering than others is a wider problem than can be usefully evaluated merely by insect densities on what you buy.
  • Don't get or use an outdoor swimming pool. Such pools trap tons of bugs and sometimes even frogs. When I was young, I used to go to a friend's pool most days during the summer, and we had to scoop out the piles of dead bugs and sometimes dead frogs using a net. This page reports that "Springtails can become a nuisance around swimming pools when they fall in and drown in large numbers, often coating the pool surface."
  • If you have vegetables from a local farm that have tons of bugs in them, you can remove a lot of bugs at once by getting a bucket, putting it in the sink, rinsing the bugs off into the bucket, and then dumping the bug-filled water into the soil outside (quickly, so that the rinsed-off bugs don't drown in the water). Better yet, if you have a gentle hose outside, you can use that. Or use a water spray bottle. By using water to remove bugs, you avoid crushing the bugs with your fingers or letting them drown in your sink drain. In 2015, I did this with Brussels sprouts that someone I knew insisted on eating even though they had ~20 aphids per sprout.
  • Some clothing fabrics are eaten more by bugs than others: "Moth larvae generally only feed of fibers that are of animal origin, wool, silk, cashmere, angora and similar." This is an a priori reason to avoid animal-based fabrics, although it's also relevant to consider what other effects buying such products has. For example, maybe sheep grazing reduces bug populations on pasture fields?

How would I deal with insect infestations in the house? The answer depends on the kind of infestation. For ants or houseflies, I scoop them in a Ziploc bag and take them outside and then seal where they were coming in. (I listen to podcasts simultaneously if it takes a while.) To prevent fruit flies, I put all food in the fridge rather than leaving it out, cover the garbage can, wipe sticky juice off countertops, and generally remove anything they're feeding on. If you must kill the infesting insects, then humanely squishing them one by one with a piece of wood and paper seems best. Next best might be bulk squishing or drowning or something. Don't use flames, since this seems like a terrible way to die.

Wanted: An insect charity

I'm not aware of any organization in the world focused on the problem of insect suffering. I wish the Society for the Prevention of Cruelty to Insects were a real charity rather than a parody website.

I've wondered whether I could help start such an organization, but I don't know anyone who would want to work for it. I can't think of any individual person who's an insect activist, although there are a few heroic biologists like Jeffrey A. Lockwood and John E. Cooper who have devoted a decent portion of their careers to writing about insect welfare. Some vegans also discuss insect suffering at human hands, which is welcome, though it's limited in scope and ignores the vast suffering of wild insects.

However, I worry that if there were an insect-advocacy organization, it could send the wrong message. It might lead people to infer that all killing of insects is bad, even if doing so prevents net suffering in the long run. Perhaps insect-rights sentiments would bolster environmentalism because of the short-term harm (notwithstanding long-term benefit) to insects caused by habitat appropriation. For example, one person commenting on a piece I wrote about insect suffering said:

It's been quite shocking to see the difference in number of insects living in a little piece of protected nature vs a city. And this was originally farm land - can't imagine how much richer it must have been before humans fucked it up.

Of course, if we want to reduce insect populations in order to reduce insect suffering, this is exactly the wrong conclusion to draw. So I would be wary of supporting any old insect organization; it would have to be one that took a stance against "natural" suffering by insects as well.

Still, I think it could make strategic sense to focus on insects under human control, especially those killed by cooking/boiling alive for food and silk. I would worry about making entomophagy more popular, though; given that eating insects is not widespread in the West yet, it could be that any news would be good news for entomophagy companies and bad news for insects.e So even an anti-entomophagy campaign seems uncertain, though opposing silk is a safer bet.

Summary of key ideas

In a simple model where animal numbers are limited by food, the main problem is that plant energy exists, and the goal is to get rid of that energy (typically through respiration, though potentially also through fire and other forms of combustion). In this over-simplified model, if you kill an insect prematurely, the food it would have eaten will be eaten by someone else. Hence, all you're doing is delaying the population increase due to that food and causing more deaths (and hence more suffering) per unit of respiration that happens. Now, maybe the food would later be eaten by bacteria, and if you don't care about bacteria, this isn't a problem. If so, it's possible that having fewer insects could be better. But I care a little about bacteria, and I'm also not sure killing insects does reduce insect populations on balance, because some other insect might just come along and eat that food later.

In addition, I have a concern that killing insects might be bad because of r-selection, though I'm not sure how much weight to give this.

My simplified model of food as the limiting factor may not be correct, especially not for a given species, but across all forms of life it seems more plausible. (Food will almost always be eaten at least by bacteria if no one else. And then of course, bacteria may be eaten by nematodes and worms, and up we go.) But might it be the case that insects speed up nutrient cycling and hence increase total primary productivity?

I think farming can be justified if it reduces plant biomass relative to the counterfactual (in the case of my mom's garden, the counterfactual would be grass/hay). This is probably mainly accomplished by putting down covering to prevent sunlight from reaching plants. On industrial farms, herbicides probably help. The sign of pesticides isn't totally clear. They probably reduce net insect deaths on the crop fields by lowering insect populations, but there's the concern I raised about whether the respiration would just happen elsewhere by some other animal. However, in the case of crops (unlike most other biomass), that concern is partly allayed, because it's big animals (people, cows, etc.) who eat the biomass that insects aren't eating, which means there's not nearly as much suffering in the counterfactual respiration per calorie as if it were eaten by insects or bacteria. (I guess there are still reasonable numbers of bacteria eating the biomass inside human and bovine stomachs, but much of the respiration happens in the human/bovine cells.)

Postscript: Objections to preventing insect births

In this essay I've made the assumption that insect lives are on average not worth living. The reason is that most insects die potentially painfully just a few days after birth, and living just a few days cannot generally provide enough pleasure to outweigh the pain of dying. Moreover, most insects do not enjoy lives of comfort but endure hunger, disease, and other hardship. Maybe there are some lucky insects that happen to have awesome lives and quick deaths, but this is an exception.

In this section, I review various objections to my view that preventing insect births is a merciful act.

Objection: The insects have not given consent to not be born.

No one can consent to being born either. By the time you can understand what this question means, it's already too late. If we're worried about consent, isn't it non-consensual to give birth to an organism that will inevitably suffer as a result?

In any case, insects and other lower animals don't understand enough to give meaningful consent. They probably don't comprehend what it means to exist as a life form, what their future prospects are, how painful death will be, and so on. Even for really smart animals like dogs and cats, it's the owners and vets who ultimately make the call about when to euthanize them based on how much suffering they seem to be going through. We can't rely on the pets to tell us on their own, and if we don't take any action, we're responsible for letting them suffer needlessly.

Objection: Insects implicitly convey their enthusiasm for life by their actions.

What actions in particular? The fact that they actively seek food and mates? But an insect searching desperately for food out of hunger looks from the outside similar as an insect excitedly seeking food out of pleasure. We can't always discern the hedonic valence of insects from their behaviors.

Of course, we can make guesses by analogy with ourselves and by contemplating the fitness reward-punishment landscape they face. This suggests that insects may (if they're sufficiently cognitively sophisticated) suffer when hungry, enjoy eating, feel frustrated when unable to mate, feel pleasure upon mating, and feel extreme pain upon dying. The argument about net insect suffering already accounted for these assumptions.

Objection: Since insects try to stay alive, they must prefer to live.

This argument mainly only applies to the few animals that can conceptualize suicide, which may be only humans and is probably not more than the smartest mammals/birds.

If we're inferring a preference for living by the fact that animals eat, avoid danger, etc., it's true that one can from these observations construct an aggregate sort of preference to survive that's exhibited by the animal's brain and body components acting in concert, but this implicit preference may contradict the animal's negative hedonic experiences, which I think matter a lot more.

Suppose you're going to be tortured until you die. It would be better for you to die immediately. Yet, if someone runs at you with a sword, it's plausible you'll still run away.

Suppose you're going to live alone in a prison cell for a year, being given food and water. Once that year is up, you'll be brutally tortured every day for the next year. But if you die within the first year, you can escape the future torture. From a hedonic perspective it's clearly better for you to refrain from eating and drinking in order to kill yourself. But in practice, doing that would be very difficult. We shouldn't infer from the fact that you eat after attempting starvation that you actually prefer to stay alive. Of course, the situation for animals in the wild may not be as drastic as the example just painted, but it illustrates the caution we should exercise when drawing conclusions about an organism's preference for life from the instinctual survival behaviors that it demonstrates.

Objection: If insect lives aren't worth living, why don't they commit suicide?

See "If Life in Nature is so Bad, Why Don't Wild Animals Kill Themselves?" The most obvious point is that insects don't understand suicide, and beyond that, if most of the pain that insects endure comes from the process of dying itself, then there's nothing to be gained by killing themselves early; they don't have access to euthanasia.

Objection: We should leave life alone except in extreme circumstances.

This amounts to status-quo bias, which has been solidly criticized by other authors, including Nick Bostrom and Toby Ord with the reversal test.

Objection: We shouldn't play god.

Aren't you playing god by using medicines and birth control? Where do you draw the line? In any case, if you don't want to play god, at least you should oppose spreading wildlife to places where it doesn't already exist, like space and computer sims.

Objection: In the face of suffering, we should always strive to keep going and make things better.

Preventing the birth of organisms that will have short lives and painful deaths could count as "making things better." If you mean that we should make things better in ways that don't entail preventing life, then this is a fundamental assumption with which I disagree. Surely if you knew you'd have a child that would die painfully two days after birth, you would avoid starting the pregnancy?

Objection: Humans need insects to survive.

This is a valid point, and I'm not proposing to eliminate all insects on the planet. However, it's clear we can prevent a lot of insect births without having an appreciable impact on human survival. Already humans pave over vast insect habitats for seemingly trivial reasons. What I am suggesting is to include the pain of insect lives as an additional term in our objective function in order to push economic and environmental decisions slightly more toward policies that would prevent insect births, without implying drastic consequences for other things people value.

I like the following quote from Bryan Caplan. In reply to the claim that "we would have to abandon civilization (and perhaps even commit suicide) in order to stop killing bugs", Caplan says

Sorry, you're not thinking enough like an economist. Yes, we'd have to end civilization (and mankind) to utterly stop killing bugs. But we could clearly vastly reduce bug suffering with marginal lifestyle adjustments.

Caplan's argument here is for purposes of a reductio, but his modus tollens is my modus ponens.

The bottom line

Fundamentally my position comes down to this: I would dread being born as an insect, and I would wish that others would prevent my birth. I think that if many people thought seriously about the life of a typical insect (i.e., probably dying a few days after being born) they would also choose not to go through such an experience. Sometimes it can be hard to feel this way as we imagine the situation from our comfortable homes and happy lives, but if we were in the midst of being eaten or starving, we'd feel differently.

In Hinduism and Buddhism, reincarnation as an insect is generally seen as a punishment for sins in a previous life. But given that reincarnation is probably not true, we can in fact prevent this misery by reducing the number of insects that are born.


  1. The carrying-capacity effect doesn't need to operate in the immediate term. The carrying capacity is not a strict upper limit on the number of bugs that can possibly coexist simultaneously. Rather, the capacity is more about a long-term average population that can be supported. For example, say that by not squishing a bug, it lives another few weeks and eats food during that time. Suppose that food otherwise would have lain dormant for a while. In the short term, no additional bugs were prevented by eating the food, but eventually that food might have been eaten by a bug, and if it's eaten now instead of later, the population in the long term is constrained. If the food would have been eaten by something else (e.g., nonsentient bacteria), then this is a case where the bug population can go below carrying capacity.  (back)
  2. Unfortunately, even in this case, the food that the insects would have consumed remains available to feed someone else, but the other organisms fed might be big herbivores (which suffer less per calorie of food energy than insects) or bacteria (which seem to me to matter less per gram than insects, though this is debatable).  (back)
  3. For instance, suppose predators eat 50% of prey insects. If you kill 2 prey insects, predators would have eaten only 1 of them, so the number of prey insects now not eaten by predators is only 1. Hence there's a net reduction by 1 of the number of prey insects.  (back)
  4. Most of the time, public transport isn't used to full capacity, since the number of buses or trains is limited by how often they travel rather than how many people they can hold. So in most cases, the marginal contribution of your using public transport to increased driving is almost zero (and might even be vanishingly negative insofar as you increase the profits of public transportation and thereby encourage the infrastructure to improve so that more people use it?). Using public transport during rush hours when the trains are packed full might increase the frequency with which trains run in expectation by a small amount. Even if you do slightly increase the number of buses or trains that run, the number of bugs killed per passenger on such vehicles is tiny compared with the number of bugs killed per passenger for cars. Maybe another factor to consider in this analysis is whether driving contributes to paving more total roads than public transportation does, and if paving over land is generally good because it reduces primary productivity, this could be a tiny argument for driving. However, I would guess the marginal contribution of one more car on the road to paving more road land area is basically nil, except for your own driveway or something.  (back)
  5. In an interview I did with Vox on 23 Apr. 2014, I mentioned insect suffering due to entomophagy. On 30 Apr. 2014, Vox published an article in support of entomophagy. Coincidence? Maybe, but I'm afraid it may not have been. Probably many more people saw the pro-entomophagy article than the little paragraph in my interview against entomophagy. Also, since the baseline is usually for Western consumers not to eat insects, even if there's variance in people's reactions, this will tend only to increase insect consumption relative to the status quo.  (back)