Photo by Dennis Swartzell, Mountain States Wholesale Nursery Grapevine

Q: Something is shredding my grapevine leaves. What’s causing this, and what should I do?
A: Examine both sides of the leaves for caterpillars that have distinctive black and yellow bands, advises Lee Ann Aronson, University of Arizona Maricopa County Cooperative Extension Master Gardener. Known as western grape leaf skeletonizers, these caterpillars line up and feed in groups, stripping leaves until only their veins, or “skeletons,” are left. The adult stage is a blue-black moth, about one-half inch long, that lays masses of yellow eggs on grape leaves in early May.

As soon as the caterpillars hatch, they begin eating. Each morning, Aronson says she turns over the leaves on her grapevines and removes the leaves with eggs and caterpillars. This is considered the least invasive control method. She recommends wearing gloves and a dust mask because the larvae may be covered with stinging hairs that can become airborne and inhaled.

Another control method is to sprinkle both sides of the leaves with Bt (Bacillus thuringiensis), which is a naturally occurring biological insecticide. Effectively applying Bt to the undersides of the leaves is not easy, but it is important because the grape leaf skeletonizer feeds mainly on the back sides of leaves. After consuming treated foliage, a caterpillar will stop feeding within a few hours and die within a few days. Apply Bt when caterpillars, or the damage on the leaves, first appear. Read and follow all package directions. Repeat at seven- to 10-day intervals, if necessary.

“Although these caterpillars can completely strip a grapevine, my vines have always rebounded later with new growth and without affecting fruit production,” says Aronson. “If you control their population early with hand-picking or Bt, your vines should be fine.”

Q: My corn plants are falling over, yet they appear to be healthy. What’s happening?
A: Corn, hay, wheat and other shallow-rooted grain crops have a tendency to fall or lie down, which is called “lodging,” explains Maricopa County Cooperative Extension Master Gardener Olin Miller. Tall varieties of sweet corn may do this more often than shorter varieties. When the corn is beginning to form silks and tassels, it is essential that the stalks be erect, by staking if necessary, so that the pollen from the tassels can be distributed on the silks. To minimize lodging, it helps to irrigate corn in the early morning. Irrigating just before or during strong afternoon winds will soften the soil and increase the chance of corn blowing over.

Q: Is it possible to grow peonies in the low desert?
A: Since winter chilling is required during the dormancy of this plant, peonies do not perform well in the low desert, according to University of Arizona Pima County Cooperative Extension Master Gardener Linda Drew. Peonies also need a more acidic soil that is richer in organic matter than what the low desert offers.