While the labels of agricultural chemicals usually list any potential hazards to honey bees, there is usually no information about the hazards to other bees. Application rates that only irritate honey bees may be more harmful to smaller bee species.
Pesticide formulations should be chosen with caution. In general, powders and microencapsulated pesticides tend to be the most harmful to bees. These small particles become trapped along with pollen in the hairs covering a bee’s body, and are brought back to the nest as food for the larva. Whenever possible, liquid formulations of equivalent ingredients should be chosen.
Obviously the least toxic chemistry available should always be considered whenever spraying is necessary. For example, pesticides like Bt offer effective control of many pest insects with less harm to most bees. Other chemistry issues to consider include the use of growth regulators for thinning tree fruit instead of products containing carbaryl, and the cautious use of systemic neonicitinoid products like imidacloprid, which may be sequestered in the nectar of flowering plants.
Finally, pesticide toxicity to bees can be reduced by minimizing exposure. Crops should not be sprayed while in bloom if possible and fields should be kept weed free to discourage pollinators from venturing into the crop when it is not in bloom. Night time spraying, when bees are not foraging, is often ideal. Periods of low temperatures may also be good for spraying since many bees are less active. However, the residual toxicity of many pesticides tends to last longer in cool temperatures, so exercise caution.
The following tables (download each PDF by clicking the link) list the relative toxicity of many common agricultural chemicals to bees. They are based on information on pesticide labels and information in these publications: Johansen et. al., Pollinator Protection: A Bee and Pesticide Handbook; Riedl et. al., How to Reduce Bee Poisoning From Pesticides; and Tew, Protecting Honey Bees from Pesticides. See the Bibliography, for more information on these publications. Chemical registrations and formulations change frequently—always check the label for guidelines.
Table D.1 Agricultural Chemicals Highly Toxic to Bees—Do Not Apply to Blooming Crops or Bees
Table D.2 Agricultural Chemicals Moderately Toxic to Bees—Only Apply During Late Evening