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.
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.
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