Last summer's caterpillar invasion (gypsy moth larvae) left many gardeners desperate for solutions. Unfortunately, the outlook for 2021 is no better. BTK refers to the active ingredient in this productâBacillus thuringiensis var. kurstaki. This is NOT a chemical pesticide, but a natural bacteria microorganism. As soon as caterpillars ingest it, they stop feeding and die within days. BTK can be sprayed on all treesâ fruit, shade and evergreen as well as vegetable plants. Birds, animals and people are not endangered, but please avoid spraying milkweed plants, as monarch butterfly larvae feed on this plant only. Mix 33 ml per 10 liters water and spray in late afternoon as direct sunlight breaks down the active ingredient.