by Brian Tomasik
First published: 2 Jan. 2018; last update: 2 Jan. 2018


While I generally avoid killing healthy insects, I often find injured or dying insects in my house that I try to euthanize. This page describes the two methods I use to kill bugs with as little pain as possible: (1) thorough crushing against paper and (2) freezing (which is perhaps less advisable). I welcome more research and opinions on euthanasia techniques.

It's not completely clear to me if euthanizing dying bugs is more humane than leaving them to die on their own. For example, I worry that when crushing bugs into little pieces, there's a small chance the animal's tiny brain might remain somewhat intact in one of those pieces? I'd like to find out more about this concern.


It's a difficult question how to handle invertebrates like flies, ants, and beetles in one's house in a way that minimizes total invertebrate suffering (Tomasik "Speculations ..."). I generally err on the side of not killing healthy bugs because my euthanasia methods are probably not pain-free, and I worry about increasing total deaths per unit time by killing bugs prematurely (Tomasik "How ...").

However, it does seem that bug euthanasia is sometimes warranted. For example, if you've accidentally stepped on and half-crushed an insect, you should probably fully crush it immediately rather than leaving it to die slowly. Likewise, I suspect it's better to euthanize slowly dying bugs, such as flies that enter your house in the fall and eventually stumble around from exhaustion. It's an important question where the cutoff is at which euthanasia becomes the better option, and opinions are likely to differ on this point.

This article describes two euthanasia methods that I use. There has actually been research on insect euthanasia (Wikipedia "Insect euthanasia"), but unfortunately, most of those methods are targeted at laboratory researchers who have access to chemicals or other special equipment and expertise. Since almost all bugs that humans kill are killed outside laboratories, there's a need for more research on euthanasia methods that laypeople can use.

Why I worry about incomplete crushing

In my experience, many people assume that quickly swatting a fly or mosquito against an arm or against the wall produces an instantaneous, painless death. While this might sometimes be true, I fear that casually crushed bugs may not be completely killed right away. The flattened insect may be immobilized, but are you sure that the relevant neural connections for producing pain have been disconnected or otherwise disabled? Might they still be firing, at least partially? The brain of an insect is plausibly smaller than one of the chunks of ground-up body parts that result from crushing an insect. Is it possible that the brain remains at least partly intact in one of those insect body-part chunks?

Insects are robust creatures. For example, Choi (2007) explains: "Headless roaches are capable of living for weeks." Part of the reason why is that "The hardy vermin breathe through spiracles, or little holes in each body segment. Plus, the roach brain does not control this breathing and blood does not carry oxygen throughout the body. Rather, the spiracles pipe air directly to tissues through a set of tubes called tracheae." Moreover, insect nervous systems are somewhat decentralized (Choi 2007):

Insects have clumps of ganglia—nerve tissue agglomerations—distributed within each body segment capable of performing the basic nervous functions responsible for reflexes, "so without the brain, the body can still function in terms of very simple reactions," Tipping says. "They could stand, react to touch and move."

In discussing how to fight the giant ants in the movie Them!, LaBarbera (2013) explains:

Insects have a much more distributed nervous system than vertebrates, so the head is a pretty poor target. So is the body. Because of their highly efficient respiratory system [...], circulatory demands are much lower in insects than in vertebrates; insects can get away with a fairly low-pressure circulatory system. A hole in the exoskeleton, even a 30-caliber hole, just isn't going to bleed much, and the hole will be quickly sealed because insect blood has excellent clotting characteristics.

Following rainstorms, earthworms often inundate sidewalks near soil-rich locations. These worms may be crushed by inattentive pedestrians. In 2009, I asked entomologist Jeffrey Lockwood for his informal guesses about the fate of worms who are partly crushed by people's feet. Lockwood replied: "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" (qtd. in Tomasik "Worms ...").

So if insects are to be crushed, I think they ought to be completely crushed into tiny pieces. I would be curious for a neuroscientifically literate entomologist to advise about how valid the above concerns are and whether complete crushing can fully destroy or at least inactivate the relevant nervous connectivity. Also, how long does it take after an insect is thoroughly crushed for neurons to stop firing?

Method 1: Immediate, thorough crushing

When I find an injured bug requiring euthanasia, I gently pick it up and bring it to the part of my house where I keep sheets of rough scrap paper and a flat piece of wood. I lay the bug in the middle of the paper, bang the flat wood on it as hard as possible, and then continue a succession of banging and scraping motions. I scrape the bug out over the paper in order to fully flatten it, using the rough surface of the paper to trap small bug particles. I continue scraping and banging on whatever parts of the bug's body seem incompletely disintegrated.

This process is illustrated in the following video: "Sped Up Footage of a Dying Fly". Make sure to wash your hands after this procedure.

If I don't have my wood block nearby, I can also crush small bugs (like fruit flies or aphids) with my fingernail against a piece of paper or paperboard (such as the rough side of a cereal box). Then I use my fingernail to scrape up the crushed body and crush it again until all parts are flattened.

If you're outside on a road or sidewalk, you can crush bugs by sliding your shoe over them back and forth to flatten them out on the pavement. In my experience this doesn't crush the bugs as well as using paper, because the pavement is more rough and uneven than paper is, so the bug chunks left over tend to be bigger. Plus, if the bottom of your shoe is not completely flat, some parts of the bug may remain uncrushed in your shoe's grooves. Given these concerns, I typically only do crushing on pavement for large invertebrates, like injured earthworms, or grasshoppers who have collided with cars. Since crushing involves tissue damage over the whole body, the process may be intensely painful for a split second, which makes me question whether it's actually more humane than leaving the bug to die. If you do leave a bug to die, it's best to move it off of the sidewalk so that other people won't continue to step on it.

I think crushing is probably the best method of euthanasia for bugs with soft, easily destroyed bodies—including flies, moths, and carpet beetles. However, I find that it doesn't work as well for other bugs. For example, ants and ladybirds have hard exoskeletons, making it difficult to fully crush their bodies. Meanwhile, caterpillars and fly larvae are rubbery and so can also be hard to thoroughly crush. For these insects, freezing (see next section) might be a better option. On the other hand, since freezing takes a while to work, if you have a bug that's acutely injured and writhing in pain, squishing is probably warranted in order to kill it quickly.

If you have a large number of invertebrates and are too lazy to carefully crush them one by one, then freezing (discussed next) might be a reasonable alternative. For example, if you find a mass of fly larvae writhing in a food-scrap container, freezing the container for a while to prevent the larvae population from growing further may be a good idea.

Method 2: Freezing (probably less advisable)

Some entomophagy companies freeze insects to slaughter them. Some researchers do the same. Groening et al. (2017), after experimenting on bees, killed them by freezing: "At the end of each replicate, bees were sacrificed by freezing".

The "BIAZA Recommendations for Ethical Euthanasia of Invertebrates (Version1.1-Apr2013)"a says of freezing (p. 3) that "research has suggested that this is probably one of the least ethical options (Bower et al, 1999; Pizzi et al, 2002; Pizzi 2012)." Freezing (p. 9) "Does not provide muscle relaxation or analgesia effects. Considered to be inhumane without prior anaesthesia." However, this statement is directed at scientists who have access to fancier euthanasia equipment. The BIAZA paper also says (p. 8):

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).

I freeze insects by capturing them in a Ziploc bag and then putting that in my freezer. Based on the above BIAZA quote, I probably should refrigerate them first, but sadly, I usually don't.... Besides laziness, one reason for this is that I put a lot of bugs in just a few bags, and I'm nervous about handling insects more than necessary by transferring them from a "fridge bag" to a "freezer bag". I usually just put them in the freezer bag directly. Once a bag is in the freezer, I don't want to take it out again for more than a few seconds in case the bugs already in it will unfreeze and wake up.

If an insect you're trying to capture in a Ziploc bag can crawl, you can try to get it to crawl into the bag, or coax it to crawl onto a shred of paper that you can then deposit into the bag. If the insect is small and immobilized, you can try to blow it into the bag. Picking up a bug with your fingers is risky because you might crush parts of the bug while doing so. Before capturing insects, I try to blow some air in the bag so that the bag will puff outwards. That way, the insect won't get crushed between the two halves of the bag. If the insects are already in a container—e.g., fruit-fly larvae growing in a container of food scraps—you can refridgerate and then freeze that directly.

I don't know how long it takes insects to die in a freezer, especially a home freezer that's not necessarily maximally cold. As a precaution, I leave insects in the freezer for at least months, because I see no strong reason to take them out earlier. If you drop new insects into the same bag as old ones, then they don't take up much space. Invertebrates can often survive frosts or even a few days of snow in the fall/winterb (although perhaps with the help of burrowing or other escape measures), which makes me question whether freezing for mere hours or days is sufficient. In general, some insects are extremely good at surviving cold winters (Wikipedia "Insect winter ecology"), which I take to imply that you should keep insects in the freezer for a very long time, unless you know that a particular species dies quickly from freezing.

Freezing is not a humane method of euthanasia in larger animals, including cold-blooded ones. For example, Barten (1994/2014) explains: "Freezing has been used as a humane method to kill small reptiles under one pound in weight. Although low temperatures do result in a state of torpor, the formation of ice crystals in the tissue is quite painful. Freezing should only be done to anesthetized animals." Likewise, there's debate on the humaneness of freezing in invertebrates. For these reasons, I prefer crushing bugs when possible, and I'm not even sure if freezing is better than simply leaving a bug to die in some other way, such as from dehydration or starvation.


I'm interested in the possibility of euthanizing insects using some kind of fine shredder. Some farmed insects are killed by shredding.

I looked on Amazon at food shredders, trying to find one that had blades as close to the bottom as possible so that it would be more likely to shred insects, which aren't very tall. Cuisinart SG-10 Electric Spice-and-Nut Grinder seemed to have relatively low blades, so I tried ordering it. When it arrived, I saw that the blades were still probably too high to do a great job shredding, but I tried it out on some already long-dead insects that were lying around (ladybirds, houseflies, and a wasp). Even though the blades were high, these bugs were cut up into several pieces. However, because the dead bugs were quite dry, I'm not sure how much of this shredding was due to their inherent brittleness combined with the motion of the whirring grinder. Probably wet insects would have been harder to cut up. Also, the bug pieces weren't quite as small as those I create when crushing using wood and paper (as described in "Method 1" above). So for now I'll stick to wood and paper, but the shredding idea deserves more exploration.

I also tried adding a bit of water to the nut grinder so that the insects could float closer to the blades. I'm not sure if this helped (maybe a bit?), but the resulting chunks were still sometimes bigger than I was comfortable with.

If shredding can be made to work, it has advantages over crushing by hand, including being easier, more sanitary, less messy, etc.

Wanted: A humane bug-killing device

In the long run, someone should work on building a consumer device that can humanely kill bugs. The problem doesn't seem that hard, especially if it's possible to shred bugs into really small pieces. Imagine a future where people can buy on Amazon for $50 a simple box-shaped device that you can drop bugs into in order to humanely kill them.

Here's an extremely rough cost-effectiveness calculation for an effort to build a humane bug-killing device and sell it on a large scale. Suppose that one person who has an opportunity cost of $200,000/year spends 5 years developing a humane bug-killing device and building a startup company around the product or trying to pitch it to larger manufacturers. Such an effort would almost certainly fail, but suppose that it has a probability P of succeeding so well that the humane bug-killer becomes a common household device throughout the developed world.

In particular, suppose that if the device succeeds wildly, 1 billion people would use it, each giving, say, 50 bugs per year a much more humane death, for the next, say, 50 years. The expected number of painful bug deaths averted per dollar would then be

P * (1 billion people) * (50 bug deaths per person per year) * (50 years) / (5 * $200,000) = P * (2.5 million) bug deaths per dollar.

What is P? I don't really know, and someone with more expertise in the success rates of consumer-product innovations could comment here. It seems like the prior probability for any given new product to become so common that most Western households have it is quite low, because most Western households only have so many products, while the number of products people develop is vast. I would guess that P should be no greater than, say, 10-4, and maybe 10-5 is a reasonable or even optimistic parameter estimate. Using P = 10-5, the cost-effectiveness becomes 25 expected painful bug deaths prevented per dollar.

What is this in terms of invertebrate-years of suffering? In Tomasik ("Cost-Effectiveness ...") I suggest that, very roughly, "An insect-year of suffering is equivalent to the pain of dying for on the order of ~10 adult insects". So 25 painful bug deaths prevented would be 2.5 invertebrate-years of suffering prevented. On the other hand, many of the bugs that people kill in their homes, such as houseflies, ants, beetles, etc. are bigger and more cognitively sophisticated than small bugs like mites and springtails that dominate in counts of invertebrate-years of suffering in natural habitats. If we give a typical bug that humans kill in their homes, say, 10 times more moral weight per individual than a typical invertebrate in the wild, then we could bump the cost-effectiveness up to 25 invertebrate-years of suffering prevented per dollar. Since these numbers are so incredibly crude and made-up, in order to avoid false precision, we can represent this as just ~102(??) invertebrate-years per dollar.

Caveats on the calculation

In the above calculation, I didn't count any costs that a humane-bug-killer company would face after initial startup because private investment could probably cover it. I'm also not counting further time investment by the founder if the company is successful, because the company is so unlikely to be successful in a big way that this time cost is negligible in the expected-value calculation. Also, if the company succeeds, it would yield profits, which would offset these costs. Once again, this kind of big success is so unlikely that it doesn't factor significantly into the calculation.

I'm ignoring higher-probability scenarios where the product is less wildly successful because intuitively they seem less significant. For example, a 10-5 probability of reaching 1 billion people is an expected 104 people, while, say, a 10-2 chance of reaching 10,000 people is only an expected 102 people.

My calculation ignores possible memetic benefits of spreading a humane-bug-killing device for people's overall concern about insect welfare.

Perhaps technology for humane bug-killing could have much larger impact applied to the entomophagy industry than marketed to individual households.

Finally, it's plausible that information on the possible success of this kind of project could be gained with much less than 5 years of a skilled device-developer's time. The cost-effectiveness of further basic exploration of this idea may be higher than 102 invertebrate-years per dollar.


  1. This document is sadly not available online. I got a copy by contacting BIAZA using the email address found on its website.  (back)
  2. For example, in Tomasik ("Invertebrates ..."), I show a number of active invertebrates outdoors on November 15, even though "the temperature had already been below freezing a few times, including once when it snowed several weeks earlier" (0m30s).  (back)