You have likely heard in the recent past that honey bee (Apis mellifera) and wild bee populations across North America and other parts of the world are in decline. A number of factors are responsible for these declines, including the use of insecticides, parasites like the Varroa mite (Varroa destructor), pathogens, and a lack of suitable habitat.
Research shows insecticides like neonicotinoids and pyrethroids, which can be found in over-the-counter products, can negatively affect bees. However, these effects can be mitigated when pesticide sprays are timed to avoid key periods when bees pollinate crops. Thus, it is important to be aware when bees or other pollinating insects like flies or butterflies are foraging for pollen and nectar. While cereal crops do not provide nectar rewards for bees and are rarely visited, weedy species that produce flowers can attract bees. Canola and mustard crops, as well as some legumes, are also attractive to bees.
There are several important precautions that can be taken if insecticides need to be applied to crop plants or weeds. The following list of precautions comes from Washington State University Extension Fact Sheet FS147E Pollination and Protecting Bees and Other Pollinators (pdf):