Insidious Insecticides (or Why are my caterpillars dying?)

Several years ago, our older son collected eggs from a wild female Vashti sphinx moth and was raising the caterpillars on Snowberry. One day, he needed more food quickly and -- without asking -- collected Snowberry from a neighbor's yard. Most of the caterpillars were dead within twelve hours. Only three survived. When we sent him back to the neighbor's house to apologize for taking leaves without permission, he found out that they sprayed their yard yearly "for spiders and ants."

Pesticides are so ubiquitous in our society that people don't even think about the dangers to "good insects." (Our neighbor has since stopped the annual spraying. They didn't realize they were killing butterflies and other beneficial insects, too.) When we get emails and messages about caterpillars mysteriously dying, over 90% of the time, it's related to a pesticide issue. 

Examples of stories we've heard:

  • leaves were collected in a yard where yearly spraying occurs or neighbors' pesticides blew in
  • household pet(s) had been treated for fleas with topical medication or flea collar(s)
  • leaves were sourced from a plant recently purchased at a plant nursery which uses pesticides (most nurseries do, or source plants that contain them)
  • caterpillars were raised in a home treated for ants or other insects
  • leaves were collected after mosquito spraying had been done in the general area

How to tell if your caterpillars have been exposed to pesticides:

  • Egg hatch rates will be very low or non-existent; any surviving caterpillars will be weak and die quickly.
  • Smaller caterpillars will go from eating to dead within 3-12 hours.  Sometimes faster.
  • Larger caterpillars will typically start vomiting and fall to the bottom of the rearing cage. 
  • It's important to note that pesticide death is usually fairly quick. This is not a slow process where one or two die, then a week later another one or two die. 

Ways to decrease pesticide-exposure risk: 

  • Before collecting leaves, verify with the landowner or municipality (if gathering on public land) that pesticides are not applied, and if they were, it's been a minimum of a year since the last application.
  • If using leaves from nursery-raised plants, wait one year after purchase before feeding leaves to caterpillars. These pesticides are systemic within the plant and cannot be washed away. 
  • Do not have your yard or home "professionally treated" for invertebrate pests. Do not use systemic or broadcast pesticides in your yard at all. (Wasp traps, ant baits, etc. are fine as the pesticide is delivered to only the target pest.)
  • If you must treat household pets for fleas, use an oral medication.
  • Be mindful of mosquito-spraying operations in your area. Wash leaves well before offering to caterpillars, or better yet, collect leaves where no spraying occurred.