I’m taking an Integrated Pest Management class (online) and learned a disturbing thing about Colorado Potato beetles. The woman that walks our puppy asked about controlling them. Here’s the question I posed and the professor’s response.
This is the original post from Beth:
Sorry to jump ahead, but I had a question. I'm a Maryland Master Gardener living in southern Maryland. I was just asked about controlling Colorado Potato beetles. I had used Bacillus thuringiensis var san diego and Bacillus thuringiensis var tenebrionis in the past. I see that those have been replaced by Spinosad. The concern is that Spinosad is very toxic to bees and the gardener is a beekeeper. I understand that there is minimal risk if the potato plants are sprayed when the bees are inactive and plants are allowed to dry but are there any other cautions I should give? Does anyone manufacture Bt for potatoes anymore?
Here is my reply:
There is now considerable resistance to Bt in multiple, wide-spread Colorado Potato Beetle (CPB) populations. Therefore, the chance of success against the CPB is probably low even if you could find Bt. FYI – cross-resistance between the neonicotinoid insecticides (imidacloprid, etc) and spinosyns (e.g. Spinosad, Spinetoram) is being seen for CPB in commercial potato production and this could carry-over to garden potato production as well.
Spinosad to suppress CPB can be hard on pollinators if your application timing is off. However, you can minimize impact by spraying when bees aren't foraging. (Not always easy to do in practice.) Direct application tests proved spinosyns (e.g. Spinosad, Spinetoram) are highly toxic to bees but the dried residue was rated as having negligible toxicity (but it must be dry!). Although it is weakly translaminar (moves across leaf surfaces), it does not appear to be systemic (moving within the plant) and unsprayed leaves have no protection against CPB. Put another way, while there might be residual toxin in sprayed leaves, it doesn't seem to be transferred to other locations in the plant. Spinosad is a reduced risk product with a very short pre-harvest interval and a short post-application re-entry interval.
Bottom line: Spinosad is the current best choice. Apply when bees are not foraging (e.g. in the evening) and weather conditions favor rapid drying of the spray residue.
Mark E. Ascerno, Professor and Co-Chair
IPM3 Training Consortium
Department of Entomology
College of Food, Agricultural and Natural Resource Sciences
University of Minnesota
St. Paul, MN 55108