Do not spray highly attractive plants with insecticide before or during flowering
It is clear to most people that insecticides sprayed onto open flowers can be highly toxic to bees, even if they are sprayed early in the morning or at night when bees are not present.
April 13, 2016 - Author: David Smitley, MSU Department of Entomology; Diane Brown, Rebecca Finneran and Erwin Elsner, MSU Extension; Joy Landis, MSU IPM; Paula M. Shrewsbury, Univ. of Maryland Dept. of Entomology; and Daniel A. Herms, The Ohio State Univ. Dept. of Entomology
It is clear to most people that insecticides sprayed onto open flowers can be highly toxic to bees, even if they are sprayed early in the morning or at night when bees are not present. However, some may not realize insecticides sprayed in the two-week period before a tree flowers can also be toxic to bees. Insecticides that tend to volatilize, like chlorpyrifos, can vaporize off the leaf surface and contaminate flowers after they open. Although this level of contamination is very low, it may still affect bees because some insecticides, like the neonicotinoids, can affect bees at concentrations as low as 10 ppb (part per billion).
Also, some systemic insecticides like most of the neonicotinoids may be partially absorbed by sprayed leaves and move systemically in the plant. Only a very small amount of residue is absorbed into leaf tissue, not enough to provide control of insect pests, but it may be enough to cause sublethal effects to bees if it moves into the pollen or nectar. Recent studies on cherry trees indicate if they are sprayed with imidacloprid after the flowering period is over, the amount of imidacloprid found in nectar the following year (1 to 6 ppb) is not a serious threat to pollinators.