Bacillus thuringiensis (Bt) is a bacteria that occurs naturally in the soil. It is not harmful to humans but it is deadly to moth and butterfly larvae. Bt is approved for use on organically grown food and is an ingredient in numerous insecticides. Sold under trade names such as DiPel and Thuricide, the insecticide is usually applied to plants in liquid form. Because plants are certified to have been organically grown doesn’t insure they are safe for caterpillar consumption.

The function of Bt in nature is to keep the caterpillar numbers in check. Predators and numerous other pathogens also reduce the larvae population so it doesn’t explode. Pathogens are bacteria, virus, or other microorganisms that can cause disease.

Vegetable farmers often rely on Bt to kill caterpillars and have done so for decades. Without Bt, their cabbage, broccoli, cauliflower, parsley, dill, and fennel crops might be completely destroyed by caterpillars. Cabbage White larvae feed on plants in the cabbage family while Black Swallowtail caterpillars consume parsley, dill, and fennel.

Unless you grow your own host plants or purchase your plants directly from the grower, and the grower insures you that the plants have not been sprayed with Bt, there’s no way of knowing that the plants are safe for caterpillars to consume. “Organically grown” does not mean “caterpillar safe”

When caterpillars consume foliage that was sprayed with Bt they often:

  1. appear to be hungry, crawling onto fresh food, but not eating it
  2. die at a rapid rate during the 1st and 2nd instar
  3. lay on the floor of the cage in a semi-curled position (3rd – 5th instar larvae)
  4. are sluggish
  5. appear darker in color than normal
  6. appear flaccid (their skin seems to be hanging loosely)

When you notice that your caterpillars aren’t eating the fresh food provided, and this continues for two days, suspect that Bt might have been sprayed on the foliage you are feeding to them. Once the larvae have consumed foliage laced with Bt, they cannot be saved. The Bt bacteria becomes active and attacks the gut lining, punching holes through it. The caterpillar dies within a few days.

Note: Other insecticides, like those used to kill mosquitoes, often contain larvae-killing ingredients also. Even bleach residue, if consumed by caterpillars, can poison them.