Assessing a pile of dead bees: what happened?

After mosquito spraying in his neighborhood, Jim Metrailer of Little Rock, Arkansas, discovered a pile of dead bees. He wondered if his bees got into the spray itself or if they ate poisoned nectar. He also wondered if his bees had succumbed for another reason entirely.

Many choices

The answer to the first question is complex. Hundreds of different insecticides are available, but those for mosquito spraying are somewhat limited in popularity. From what I’ve read, many of the commonly used ones are broken down by sunlight, quickly becoming harmless once they are exposed to bright sun.

It’s been many years since I studied pesticides as an agronomy student, and the formulations have all been replaced or enhanced. I simply don’t know the specifics and I don’t want to spend the time to re-learn them. If you want to know the details, you need to find the name of the chemicals used in your area and look them up.

Systemics vs contact poisons

Poisoned nectar usually isn’t a problem unless a systemic pesticide was used. A systemic moves to all parts of a plant through the vascular system, reaching the leaves, stems, roots, fruits, nectar, and even the pollen. This is why seeds coated with the neonicotinoids are such a problem for bees.

Generally, it works like this: First the seed is factory-coated with the chemical. After planting, the pesticide coating washes into the soil. When the roots start to grow, they pick up the chemical along with water and nutrients, then transport it through the interior of the plant. No matter which part a bug eats, it gets a dose of poison.

Since mosquitoes don’t eat plants, exterminators use contact poisons on them—something that is absorbed through the exoskeleton when it lands on the insect. You see contact poisons in things like household bug sprays that kill almost instantly. Often they affect the central nervous system of the victim.

Jim’s bees

Depending on when the spray was applied and what kind was used, Jim’s bees could have been exposed in several different ways. If the pesticide was applied during flying times, they could have been sprayed directly. If the bees traveled to a plant soon after application, they could have come into contact with it on leaves or petals before it broke down. Or, if a systemic was used on certain nectar plants, the bees could have collected poisoned nectar that had nothing to do with mosquito spraying.

Only one of Jim’s two colonies was affected, so they were probably foraging in different areas. One got into it and one didn’t, which is not unusual.

Altruistic bees can accumulate

Without further investigation or testing, it is difficult to say exactly why a pile of dead bees accumulated. Honey bees exhibit altruistic behavior, meaning a sick or dying bee will often fly out of the hive and die in order to protect the rest of the colony from the same fate. They may leave the hive and fall immediately to the ground or sickened bees may be carried out by others. In either case, quite a pile can build up.

Besides insecticides, bees may pile up from a number of different brood diseases, nosema disease, and even viral diseases carried by varroa. However, the rate of accumulation can tell you a lot.

My own pile of dead bees

Last year, I had two colonies succumb to pesticide on a spring day. The reason for my certainty is simple. I was getting ready to go on a trip. On the afternoon before I was due to leave, I did thorough hive inspections where I looked at all frames in all hives. I usually check things before I depart because I don’t want to leave my non-beekeeping husband with a bee problem in my absence.

In this case the colonies looked great. They were active, packed with brood, pollen, and making lots of honey. They were not feisty at all, but were as busy as bees. I was relieved to find no problems. The next morning, two colonies were toast, all piled neatly in front of their respective hives.

When I reported this here in a post, a number of people said it maybe wasn’t pesticide but probably a disease. To this I say, no no no. Why? The rate of accumulation was too great.

How fast they pile makes a difference

A colony that was poisoned can die all at once. Most diseases start slowly and build up, so you are likely to get a pile that starts small and then grows. But with poison, a pile can appear within hours. The rate of accumulation is important diagnostic evidence. Even without looking for extended tongues, deformed bees, or signs of disease, the rate gives you a clue.

In my case, I examined every frame and found no sign of anything amiss twelve hours earlier. And since nothing else happened in the interim—no floods, forest fires, suffocation, intense heat or cold—they almost assuredly died of pesticide. Another important clue is simultaneity. How many colonies totally collapse from disease in exactly the same twelve-hour time frame?

Sometimes we don’t know

Of course, if you haven’t paid close attention to your bees, it is harder to say. For example, if I found the piles after I returned a week later, then more investigation would be required. But in this case, I knew they were healthy the evening before, so something big happened during those hours.

Thanks, Jim, for a great question. It is a subject we should think about when assessing any colony problem: look at the evidence carefully before making a diagnosis. Don’t rely on gossip you heard at a bee club. Sometimes the conventional wisdom just doesn’t make sense.

Honey Bee Suite

Jim found this pile of dead bees in front of a hive soon after pesticide spraying nearby. Fortunately, this colony recovered from the incident.

Jim found this pile of dead bees in front of a hive soon after pesticide spraying nearby. Fortunately, this colony recovered from the incident. © Jim Metrailer.


  • Good evening Rusty, that photo of dead bees does indeed look like a pesticide kill, as it looks like a “carpet of dead bees” not a pile. I learned this from the U.K. National Honey Show video with Kirsty Stainton lecturing on the foulbroods and CBPV, Chronic Bee Paralysis Virus, which 3 of my hives (out of 17) had last summer/autumn. She shows the difference on the lecture video between the virus pile and a pesticide carpet of dead bees. Thanks again for all the great research you do. Deb

    • Deb,

      Thank you for that insight. I’ve never heard that before but I can picture those two kinds of accumulation in my mind because I’ve seen them both. What a handy tip!

  • When we say “fortumately this colony recovered from the incident” I’m glad to hear it, but, it makes me wonder how the pesticide manufacturers react to that. Gives them a green light. They don’t see how that pile of bees could have made a nuc, or bumper crop. They see the word “recovered” and all is well. Just my 2 cents. I had 173 colonies last October, and started this spring with SIX. Ugh. No “proof” it was pesticides, but I’ve been keeping bees for nine years, and never had a loss like this. Thanks for your posts, and letting me vent.

  • Unfortunately this is a sight we see too often. There should be strict laws that spray is applied during evening hours when it’s still dark out and hopefully the stuff will dry before the bees get up. It’s terrible too that the insects that help us with mosquitoes the most also get killed off in the ditches when they spray. And the damage to other wildlife in the area, birds, ditch turtles and toads, little chipmunks. Every time I see a truck go down a road in the middle of the day spraying the ditches I get really angry. I feel like taking the spray wand and spraying the person applying the poisons. ha ha ! We also have to be careful that the poison does not get ‘stored’ in the bee bread, then one ends up with another kill of the brood and the emerging bees. What’s a beekeeper to do? Beekeeping is definitely NOT easy. It appears that just about everything is against the little workers.

    • Debbie,

      You are correct. I come really close to giving up when I see and hear all these things. It seems most people want to kill the planet, and they are doing a good job of it.

  • I lost three hives to a neighbor’s non-selective spraying a couple years ago. It made me question if I wanted to continue beekeeping. Instead of quitting, I wrote an article for Bee Culture on Zika and Bee Kills ( I’ve since realized that to reduce the dependence on non-selective spraying for mosquitoes, home owners need to be aware of alternatives to calling the local mosquito control company. This time, I’ve written a paper, “How to Control Mosquitos Without Killing Honey Bees and Other Beneficial Insects”, and I am distributing it any way I can. You can download the PDF file at Give a printed copy of it and a bottle of honey to your neighbors.

    • Thanks, Tom. I printed a copy to read later today. I’m glad to see people taking action instead of giving up. Sometimes it’s hard to go that route.

      • Another possibility for a pile of dead bees is usurpation. I looked into it recently when the landowner where I keep several hives swore he saw a colony swarm into another, occupied, hive. I thought he was nuts, but after a little digging on the web, I found a number of articles about the phenomenon, and some researchers believe it’s increasing in frequency. Some link it to Africanization. New to me. Seems for all we know, we know so little.

  • Thanks for sharing that!

    One of the articles I read said that there isn’t always fighting but that negotiations take place between two colonies prior to one taking up residence with another. Like one has a lot of bees and no room, and one has more room and less bees. Scouts from the one negotiate with guards at the other and the colony with too many bees sends a swarm over for a win-win. This behavior was reported by a researcher in Michigan, I believe. A northern guy anyway. Said it wasn’t all that unusual. I think you’d have to observe bees for a very long time to see it as not unusual. 🙂

  • Hi Rusty,

    I am a second-attempt first year beekeeper in the Lincoln, Nebraska area. I had posted here a couple times a few years ago. I have learned a lot since then (I failed in my first attempt) and this year went really well. I have two hives that have done amazingly well this year. I have run medium 8 frames for both my brood boxes and, due to how well they have done, even a few supers this summer. I performed monthly sugar roll tests and the mite load was 1/2 to 1% all year until two weeks ago where it jumped significantly. I didn’t finish the count but it was closer to 5 or 6%. I had been running 2 brood boxes and using an excluder but removed it and left a third box on top that was mostly full of honey and will remain a brood box next year. I began a treatment with apivar per the instructions given to me by my local bee supply store.

    At the store’s advice I also added a spacer at the top and put on a Langstroth style top instead of the warre quilt box I have used.

    Here is my concern, I got back this morning from a business trip (was away for a week) and checked on the bees. both hives have some dead bees in front of them. The one I have considered the healthier hive has a reasonable amount, not a major change. The hive that has been the weaker of the two (but still very active and healthy, just compared to the other hive) has a HUGE pile of dead bees but the coming and going seems normal except the bees are preventing drones from entering and I saw a couple (I think) drones drug out of the hive. I can’t tell for sure if all the dead bees are drones but it is a LOT of dead bees. In reading, I am aware that pushing out drones is normal fall behavior but is actively killing drones normal behavior and is there a way to determine if all the dead bees were drones? It is a deep, thick pile. I am wondering if there is something else I should be doing.


    • Robert,

      First off, it’s impossible to tell from here. I can only guess. Forcing drones out is, of course, normal and the workers will be brutal if they must. I wouldn’t worry about that aspect. To know what the pile of dead bees is composed of, just take a handful and go through them one-by-one and separate them into two piles to see what you’ve got. It’s normal to see some increase in dead bees once the weather gets cold because instead of dying out in the field while foraging, they die inside the hive and must be carried out, and they they pile up.

      The mites surged because that is what happens in the fall. The number of new bees decreases rapidly, but the number of mites continues to increase, so you get many more mites per bee. This is just one of the reasons that timely fall mite control is so important. Make sure Apivar works in your area. There are large pockets of Apivar resistance, so you’ll want to make sure it did the job by doing another mite count.

      I don’t know why the store had you use a spacer for Apivar. Apivar strips go down between the frames, so that seems odd. I also don’t understand the reasoning for abandoning the quilt box in favor of a standard lid. It seems you should be adding a quilt rather than removing one. Did they give you a reason?

  • I am new to beekeeping, got my bees in May 2020. One hive died after the extreme freezing temperatures in Texas, February 2021. Once the weather warmed up right after the freeze, they did not come out until the 3rd day of warm weather. I was very sad bc I thought the cold had killed them, but they came out on 3rd day. It got cold again so very few were out until they just stopped flying. When I was finally able to inspect, I found a big cluster stuck to the beehive wall, the rest were on the hive floor, and another dead cluster on a frame. What happened?

    • Gracie,

      The cold temperature may have dealt the final blow, but something else weakened the colony first. How and when did you treat for mites?

  • Rusty, I kept bees in Ohio roughly 50 years ago. Now in Wisconsin and restarted. Hive beetles and mites have changed things. Got two singles last year, never tested for more levels but treated first with formic pro, then Apivar and finally oxalic acid drip. Temperature made treatments problematic and I always felt like I was a week or two too late. Finished with one hive going into winter. Had done two dribbles, one using sugar solution the second just water. Both got substantial mite drops. There was a consistent poor of ten to twenty bees dead in the snow roughly every week. I’d put on a quilt top, sugar board, and a small fondant patty. Finally got in to check a couple of weeks ago. A third of the sugar board was gone, roughly half of the fondant and there was no cluster. Dead bees were spread out across the sugar and the bottom board was a carpet of dead bees. My guess is the cluster got too small to maintain heat and everyone went on their own to eat, gain energy and maybe survive. Sound right? There was some evidence of DWV before I did the oxalic acid drips.

    • David,

      Yes, that does sound right. It also sounds like mites finished off the colony despite all the mite treatments. Once you actually see evidence of DWV, it is usually too late to do much about it. If you try again, concentrate on the timing of the mite treatments. Winter survival can be helped by treating in August, which is the time when bee populations are dropping for the winter but mite populations are still growing. In that month, when winter (diutinus) bees are being formed, mites can do the most damage to a winter colony. The rest of the year, you can treat when mite drops start to rise.

      Anyone like you who kept bees before varroa can appreciate how different the landscape of beekeeping is. It can be discouraging, but hang in there.

      • Thanks for the quick reply. Two packages and two nucs this year. Going to be on top of the mites!! Steep learning curve. Great info on your site. Thanks again

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