The Western Grape Skeletonizer is one of the most irritating pests of the home grape grower, and one of the most interesting. This blue-black moth, about half-an-inch long, lays her yellow eggs in clusters on grape leaves in early May. After they hatch, the yellow, black, and blue striped larvae line up in a tidy rank on the leaf and march across it, eating the leaf tissue as they go and leaving behind a "skeletonized" leaf. It's like synchronized eating. Once they finish their communal feed, each caterpillar heads off to find its own meal on a nearby leaf; with sometimes dozens of caterpillars hatching on a single leaf, they can strip a large grape vine in very short order, so the key is to stop them while they're still in a group.
There are four steps to controlling the Grape Skeletonizer:
- Very soon you'll see the blue-black adult moths fluttering around your grape vines. You may even see the male moth doing an intricate little courtship dance on a grape leaf, hoping to impress nearby females. Nip this romance in the bud by swatting any adult that you see.
- After the romance has ended, look around your grapevine for small yellow eggs laid in clusters on the undersides of your grape leaves. When you see them, squash them.
- As the month progresses, stand back and cast a keen eye on your grape vine. What you're looking for is small silvery patches on the leaves; that's the first sign that the caterpillars have hatched and are beginning their march across the leaf. Get up close, and check the undersides. When you find a leaf with caterpillars on it, cut the leaf off the plant and destroy it and its residents. Keep looking; you missed a few.
- Once the caterpillars have each moved to their own leaf, you'll see damage across the vine, and it happens fast. At this stage, your best tool is to spray BT (Bacillus thuringiensis) on the vine, following the directions and being thorough. BT is completely organic and safe to use on edibles. The caterpillars will ingest the BT spores with the leaf matter and come down with a terminal stomachache.