Tuesday, November 19, 2013

Grub Treatments with Nicotinoid Insecticides may Affect Bees

Dr. Dave Smitley
Department of Entomology
Michigan State University

There is a growing concern among entomologists and other agricultural scientists about the undesirable impacts of imidacloprid and other nicotinoid insecticides on the health of honey bee colonies, and native bees.  Nicotinoid insecticides are widely used on agricultural crops to control destructive insects.  They are also used on golf courses and home lawns to control white grubs and other turfgrass pests.  A recent study conducted in Kentucky by Dr. Dan Potter provides some useful information for how to avoid negative effects on bees that forage in treated lawns (http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0066375).  The results of this study can be summarized as follows:

1. Turf without any flowering weeds is not likely to be harmful to bees even immediately after a nicotinoid insecticide application because it is unlikely that bees will be present on the turfgrass.  Avoiding spray drift to surrounding flowering plants is advised.

2.  Applications of clothianidin (a nicotinoid insecticide used extensively on turfgrass) to turfgrass with clover in flower was harmful to bumble bee colonies foraging on the clover.   However, if the lawn was mowed just before or immediately after clothianidin was sprayed, so that there were no sprayed flowers present, there was no harmful effect on the bumble bee colonies.  Also, no harmful systemic effect was observed when the clover began to bloom again after clothianidin was applied, as long as the new flowers were not present when the lawn was sprayed.  In the discussion of these results Dr. Potter notes that more research is needed to address the long-term effects on bees throughout the growing season after the uptake and translocation of nicotinoid insecticides by clover and other flowering weeds.
White clover flowering in a low maintenance turf.
3. Chlorantraniliprole, a new turf insecticide with a different mode of action, did not have any harmful effects on the bumble bee colonies under any conditions. 

Considering recent research on the impact of imidacloprid on honey bees, and the results of the recent study summarized above, I recommend that if a lawn with weeds in flower needs to be treated for grubs, that the homeowner or lawn care professional mow the lawn immediately before spraying a nicotinoid insecticide, so that flowers are not present in the lawn at the time of application.  Please note this is approach is also in compliance with the precautionary statements on the pesticide labels which specify not to apply clothianidin, or other neonicotinoids, to blooming nectar-producing plants if bees are visiting the treatment area.  An alternative may be to use chlorantraniliprole which did not appear to be as harmful to bees in the Kentucky study.  Also, homeowners and lawn care professionals should follow three simple steps of MSU Smart Gardening to grow turf with a dense root system that is tolerant of grubs, so that no insecticide is needed:

1. Mow lawns at the highest cutting-height setting on your lawn mower
2. Use at least 2 lbs N per 1000 square feet per year
3. Water lawns during dry periods

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