Q: The leaves of my grapevines are being chewed up by a funny-looking caterpillar that I’ve never noticed before. I’m embarrassed for anyone to see how bad the vines look. How should I fight this problem or is it too late?
A: I’m pretty sure that what you are seeing is the larval stage of the western grape leaf skeletonizer. Late in the season, such as it is now, it is almost 1 inch long, yellowish-green and striped black, and has two blue stripes between the blacks. It is a common pest of grapes, table and wine.
The life cycle starts in spring with a small dark bluish moth that emerges from a cocoon that is usually found under loose bark or in ground litter beneath the vines. The moth lays its eggs in clusters on the underside of the grape leaf. The eggs hatch and the larvae begin to feed on the leaves. The larvae go through five stages of development, feeding on the leaves and growing larger as they develop.
The first two stages are very small and cream-colored, and the third is an inconspicuous brown, but the fourth and fifth stages have the distinctive coloring that is most commonly noticed. The larvae have tufts of black spines along those black bands that can cause painful welts if your skin touches them. When they are finally mature, the fifth stage larvae spins a whitish cocoon in which it pupates. It emerges in spring as the dark-colored egg-laying moth that starts the life-cycle again.
As the season and larval feeding progresses, the grape leaves may become thin and brown or even nothing more than a skeleton of veins. If the larvae run out of leaves for food, they may begin feeding on the grapes. If the defoliation occurs before the grapes are harvested, the fruit may become damaged by sunburn. When defoliation occurs late in the season, after harvest, the vines may become weakened due to reduced carbohydrate reserve storage.
In commercial vineyards, the grapeleaf skeletonizer is controlled by the granulosis virus. This particular biological control affects the gut of the larvae and inhibits food assimilation, ultimately killing the larvae. Control in home gardens is best accomplished by the well-timed application of Bacillus thuringiensis (Bt), also a biological control.
Bt is only effective against young larvae so you should plan to make the first application next spring, right after flowering. You should probably make a second application several weeks later to catch any late-hatching larvae that the first…