What does it mean for a pesticide to be “bee safe”? Most people can picture a honeybee colony killed by highly toxic pesticides — piles of dead bees killed all at once. We know that the pesticides causing this type of acute death are not safe, and their labels generally reflect their toxicity.
However, these dramatic bee kills are actually quite rare; often pesticide damage to honeybee colonies is more gradual and caused by less toxic chemicals. Many of the products considered “bee safe” can actually be worse for colony loss than chemicals that are labeled as highly toxic to bees, because they initiate a cascade of events that kills an entire colony over time.
When honeybee colonies die from exposure to sub-lethal chemicals, it is not a dramatic, instantaneous death, but a slow dwindling over time. This dwindling death is a direct result of the super-organism structure of the colony.
A healthy colony is composed of a diverse work force, with each age group fulfilling a distinct role. Pesticide stress disrupts the health of the hive by altering the demographics and social hierarchies that make the hive function as one super-organism.
Six Stages of Colony Death
In the image below, we can see how stressors like pesticides can kill a honeybee colony over time, even when they are not highly toxic to individual bees.
1. Stress. Most pesticide danger is from sub-lethal effects, when a pesticide does not immediately cause death, but severely disrupts a bee’s health and ability to function properly. Examples of sub-lethal effects include behavioral changes, difficulty navigating, increased disease susceptibility, decreased sperm counts, etc. These sub-lethal effects cause bees to become stressed.
2. Infection. A stressed colony is at high risk of developing disease. When a colony experiences disease they can enter an infection loop, creating more disease in the hive. When honeybees are sick they will fly off, rather than die within the hive, causing hive population to be reduced.
3. Chilling. Honeybee larvae and pupae (known as brood) need to be kept warm (90˚F-95˚F) as they develop. As the hive population is reduced, fewer bees are available keep the brood warm. If hive temperature is too low, the larvae may not properly develop, further reducing the hive population.
4. Early foraging. Foragers are the oldest bees in the colony, and they express a chemical known as ethyl oleate (EO) that restricts younger bees from becoming foragers too early, allowing each bee to do the jobs appropriate for their age. As the foragers are lost to stress and disease, the reduction of EO prompts younger bees to start foraging. Early foraging causes bees that should be keeping the brood warm and fed (nurse bees) to abandon their posts and head to the field. The brood becomes chilled, stressed, and unhealthy, which means that the newly emerged bees will not be strong enough to support the colony.
5. Starvation. The loss of nurse bees leaves the larvae without adequate care, such as food and warmth, and the young bees starve. As the colony loses foragers, less food is brought back to the hive. As a colony starves, workers begin to withhold food from any older foragers that remain. They then begin to withhold food from larvae, and will instead eat the larvae to retain precious protein resources.
6. Collapse. Although bees remain in the hive, it is no longer a functioning super-organism. These sub-lethal effects from pesticide exposure set off a cascade of negative feedback loops, eventually leading to colony collapse.
Why Sub-lethal Formulations Are More Lethal to Hives
When a pesticide is labeled as “toxic to pollinators,” it is because it is known to kill an individual bee outright. In some cases, these highly toxic pesticides may not kill a colony, because the exposed bee will die in the field and not make it back to the hive.
Bees that are exposed to sub-lethal pesticides will survive the exposure and return to the hive, carrying these pesticides with them on their bodies, or in collected nectar and pollen.
As bees bring pesticides from the field into the hive, the number of bees exposed dramatically increases, even exposing bees and larvae that never left the safety of the hive.
Pesticides can remain in pollen, wax, and honey for a long time, resulting in chronic exposure. For this reason, pesticides with sub-lethal effects can actually be much deadlier for colonies than highly toxic pesticides.
Just because a product is not labeled as “toxic to pollinators” does not mean the product is “bee safe.”
How to Reduce Bee Pesticide Exposure on Your Farm
Because we cannot tell by a label if a pesticide will harm a honeybee colony, it is best to try to reduce overall exposure of pesticides. Every grower, even those that don’t directly rely on pollination, can greatly impact bee health when making pesticide applications. Pesticides can drift onto nearby flowers, or contaminate water through runoff, exposing bees.
Here are some tips to help reduce the risk of pesticide exposure for bees on your farm:
• Don’t solely rely on the label for all information regarding risk to pollinators. Just because a product is not labeled as “toxic to pollinators” does not mean the product is “bee safe!”
• Make applications at times when bees are inactive, such as after sunset, before sunrise, or when the temperature is below 50˚F. This will allow for residues to dry, reducing bee exposure and the likelihood that residues will be carried back to the hive.
• Do not apply foliar pesticide applications during crop bloom. Do not apply systemic pesticides prior to the flowering period of your crop, as these products can accumulate in the flower’s pollen and nectar resources.
• Use drift reduction practices and always spray when the wind is low. Remove any flowering weeds from crop plantings that may be contaminated during applications.
• When filling spray tanks, be sure not to leave behind contaminated puddles. Bees regularly drink from shallow water sources and can be exposed to high levels of pesticide from messy tank-filling. Ensure that all products are stored properly.
• If renting honeybee hives for crop pollination, don’t place the hives directly in crop rows. Place hives in a sheltered location behind a tree line or hillside where they will be protected from sprays and other stress.