Q: I have many Penstemon parryi seedlings that have been growing through the summer. Within the last two weeks they have begun to develop “burned” scale patches on the leaves that are accompanied by black “seedy” areas. The black beads could be the seeds for future plants, except that these haven’t ever bloomed. The black beads don’t appear to be adult insects, but could be eggs. Do you have any idea what’s going on?
A: Very small caterpillars, the chalcedon checkerspot (Euphydryas chalcedona) most likely, are eating your Penstemon plants and the black “seeds” are actually caterpillar poop or frass as we call it when it comes out of insects.
The plants will likely be OK regardless and you are supporting a healthy butterfly population but if you see active caterpillars chewing your plants to the ground you can spray the leaves with Bacillus thuringiensis, aka Bt. This organic compound will make the plant poisonous to the caterpillars that eat it and will not harm other organisms on the plant. There are multiple strains of Bt, so if you choose to buy some, make sure you are getting the strain designed to kill caterpillars.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to plwarren@cals.arizona.edu.
