EPA Releases Residual Time to 25% Bee Mortality (RT25) Data Reducing Pesticide Hazards for Bees

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):

  • Avoid spraying whenever bees are or might be flying. It can be difficult to see some bees on some types of flowers. Additional precautions should be used when using pesticides where flowers are present.
  • Try to spray before bloom or after the flowers are done blooming.
  • Look for other flowers that may be blooming nearby or on the ground below the tree or bush to be sprayed. Look around for other flowering bushes or trees where the pesticide may drift.
  • Try not to spray just before or during bloom, but if it is necessary, try to spray at night or very early in the morning, before the bees are flying. Once the spray has dried on the plant, it may be less toxic to bees.
  • Avoid spraying if there is heavy fog or dew, as this will keep the pesticide wet and will increase the chance of the bees receiving a toxic dose.
While we know that direct applications of certain pesticides to foraging honey bees can be detrimental, it is also important to note that certain insecticide residuals can also pose a threat to bees. The EPA recently released a study looking at the “residual time” i.e. amount of time that pesticide residues on plants are still considered lethal, that is required to kill 25% of a population. An explanation of the study and its result can be found at Pollinator Protection: Residual Time to 25% Bee Mortality (RT25) Data.

It is always important to carefully read and follow all pesticide label instructions and to be aware of pollinating insects and other beneficial arthropods that may be present during or after a pesticide application.


For questions or comments, contact Dale Whaley via email dwhaley@wsu.edu or contact Dave Crowder at dcrowder@wsu.edu.

Washington State University