I read recently that the State of Wisconsin, like many other northern states, would be spraying for gypsy moths during the end of April and into May. Curious as to how this spraying would affect other insects, I wrote to WDNR representative Colleen Robinson Klug. With her permission, here is her response:
The formulation of Btk that is used for gypsy moth suppression and slow the spread programs only effects caterpillars in the Lepidoptera order. So, no other insects, mammals, birds, bees fish, etc…
For Lepidoptera, it only harms the caterpillar life stage, so if the insect is Oalready in the butterfly life stage, it will not harm it.
And for Lepidoptera caterpillars, it only harms them if they ingest it. Basically, the bacteria in Btk reacts with digestive properties specifically in the guts of Lepidoptera, and that reaction kills the caterpillar. So, caterpillar species that are not hatched from their egg masses and actively feeding on something that Btk is on (like tree leaves), will also not be harmed. Gypsy moth is a fairly early caterpillar in spring and Btk breaks down in several days in sunlight, so most other caterpillars are not feeding on Btk treated food.
Where there are threatened or endangered butterflies or moths present, or even good habitat for them, such as Karner Blue Butterfly, the gypsy moth spray program is required to use a different treatment called “Gypchek.” This is a virus absolutely specific to only gypsy moth. We cannot use this treatment everywhere though because it is much, much more expensive and difficult to create.
If you’d like to try controlling a gypsy moth infestation on your own, check out the UW-Extension website on gypsy moths.