Home & Garden

It’s caterpillar season in the Valley. Here’s how you can put them in their place

Tomato hornworm
Tomato hornworm Sacramento Bee file

June begins caterpillar season here in the central San Joaquin Valley. Budworms chew holes into the flower buds of petunias and geraniums and then devour the flower petals inside. Tomato hornworms defoliate tomato plants overnight. Redhumped caterpillars leave redbuds and liquid amber trees leafless and Western grape skeletonizers chew out the softer center parts of grape leaves leaving only the harder veins. Bt or Bacillus thuringiensis and spinosad provide very effective lesser-toxic control for all types of caterpillars and grubs in the garden, but recognizing the type of damage and the pest that’s causing it comes before treatment.

Elinor Teague

Budworms commonly attack the developing flowers on petunias and geraniums. They are the caterpillar stage of a small moth that lays her eggs in the soil to overwinter and hatch in spring when tender buds and leaves are most plentiful. Temperatures during our short, mild winters aren’t cold enough to kill the moth eggs or pupae, so if you’ve had problems with budworms in previous years, expect more this year.

The budworms are about 1 1/2-inch long, yellowish or greenish with a brown head. Their damage is easier to spot than the budworm. Look for small, eighth-inch holes in buds and leaves, black droppings on leaves, and empty flower buds. Spray Bt or bacillus thuringiensis at the first signs of damage, following label directions for application timing and amounts.

Tomato hornworms are big, green with a scary-looking black horn on their posterior and have voracious appetites. Patrol your tomato plants at dusk and dawn when the hornworms are actively feeding and pick them off plants; they’re too big to hide well. The “ big foot” method to kill hornworms works well if you’re not squeamish; otherwise drop the caterpillars into a bucket of soapy water. Tomato hornworms have two generations per season; one in spring, the other in mid-summer. Treat plants with Bt as soon as you see the caterpillars, their black frass on leaves or stripped leaves on plants.

Older redhumped caterpillars have a red head and a hump. Although they feed on many tree species, the home gardener is most likely to see them swarming over redbud or liquid amber leaves. The first generation hatches from pupae in April/May; the second hatches in late June/July. The eggs and young larvae of redhumped caterpillars have many natural enemies including parasitic wasps, green lacewings, spiders and damsel bugs, that generally keep populations low. It’s important to avoid spraying broad spectrum chemical insecticides on any caterpillars to avoid harming beneficial insects. If hiring a pest control company to spray tall, heavily infested trees, ask that they use Bt or a spinosad product. On small trees, spray the undersides of leaves with Bt as soon as the caterpillars or their damage are identified.

Western grape skeletonizers have been less of a pest on grape leaves due to the release of parasites that attack the larvae and the introduction of a virus that causes disease in the skeletonizer eggs and larvae. There are three generations per year of skeletonizers in the Valley; the older caterpillars cause the most damage. They are brightly striped with poisonous long black spines that can cause skin welts. Monitor backyard grapes during the growing season, checking for telltale damage of leaves with only the skeleton veins remaining.

Elinor Teague: etgrow@comcast.net