Q. Can you help identify these guys on my grapes?
A: These are the western grape leaf skeletonizers (Harrisina brillians). They are moth caterpillars and they can do quite a bit of damage to grape plants if allowed. Fortunately, there are some natural enemies out there that sometimes keep them in check. Bacillus thuringiensis (Bt) is a good organic insecticide for these caterpillars but it works best when they are small. Simply apply to the leaves where they are feeding as directed on the label. Also be careful when handling these caterpillars because they possess stinging hairs on their bodies that can cause some irritation on your skin.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to firstname.lastname@example.org