Pesticide is a fearful thing when we’re raising butterflies and moths.
When we buy a plant or think our plants may have been treated with pesticides, we test the plant on one or two caterpillars before feeding it to all the caterpillars we are raising. But the question is WHEN do we know a plant is safe to feed to caterpillars? If the caterpillars are alive the next day, is the plant OK?
The answer is a simple and resounding NO. Some pesticides will kill immediately. Others take a day or two. Some take three days (such as Bt). Others will slowly kill them, by interfering with their mandibles and how much they can eat (lack of nutrition). Yet others won’t affect them at all until they begin to pupate and won’t be able to finish the pupation process.
If caterpillars are fed some pesticides just before they pupate, they will emerge fine, inflate and dry their wings, and begin to fly before falling to the ground, quivering and dying with their wings folded downward over their legs.
The only fully correct answer is that a caterpillar should be fed the plant and it considered unsafe until it emerges and lives a day as an adult. BUT …
In most cases, after a caterpillar that has been eating the plant has successfully and fully pupated, it is safe.
That isn’t the answer we hoped for but it is the answer we want, the one that will help prevent more deaths.