LIVINGSTON - A menacing type of crawler has attacked Polk County in recent weeks, harming trees, spinning webs and creating a nuisance.
Walnut caterpillars have kept the phones ringing at the AgriLife Extension Office, where County Extension Agent Mark Currie says the pests have increased in part because of the wet weather.
“They are leaf eating,” Currie said. “They do not bite or sting, but they do make a huge mess, so I wouldn’t park under them. They are going to strip foliage off of trees. Their primary targets are walnut or pecan trees.”
Walnut caterpillars (those without webs) are reddish brown to black with white markings and long white hairs. Large larvae are conspicuously fuzzy and may grow up to 2 inches long. Larvae characteristically arch their heads and tails in a defensive posture when disturbed.
Adults emerge from pupae, which overwinter in the soil at the base of a host tree. The female moth deposits about 300 eggs on the underside of a leaf. Caterpillars (larvae) hatch from the eggs in about nine days, living together in a group. The caterpillars often move in a group to the tree trunk to molt from one stage (instar) to the next, leaving a patch of fur-like hair and cast skins. When they finish feeding, they drop to the ground and pupate in the soil. They do not spin a cocoon, but form a naked pupal case.
Populations of this species are variable from year to year and there may be several years between outbreak levels. In Texas, at least two generations of the insect develop each year. The second generation is usually larger in number and causes more damage.
The Walnut caterpillars have chewing mouthparts. Adults have siphoning mouths. Host plants can include hickory, walnut, oak, willow, honey locust and certain woody shrubs. Young larvae feed only on soft tissue, leaving a skeletonized leaf behind, while older larvae feed on the entire leaf, including the petiole. The last few stages, or instars, do the majority of the feeding damage.
Damage may be localized to just a branch to two because they feed together. Isolated trees are more subject to attack than forest or orchard trees. They can rapidly defoliate ornamental and orchard trees if not controlled. In some past years, almost all of the native pecan trees in certain areas of the state were defoliated by walnut caterpillars.
Currie said the Walnut Caterpillars would not kill trees on their own. They can be a contributing factor, but Currie said mature trees that die after having the unwanted guests were probably going to die anyway.
“It happens every year. The difference in this year is just the larger number.”
Protecting trees and plants that have not fully developed, according to Currie, could help. For those that wish to eliminate some of the caterpillars and webworms, insecticides such as Sevin are suggested.
“I would probably try to put it on the small trees. On mature trees, there’s no way I’m going to spray them. I can spray and kill what’s on the lower portion or lower branches. On sidewalks you can get them. But, don’t think you are going to eliminate them. When it runs its life cycle, they will go away.” Currie said the life cycle should end around the time it strips the trees. They do not become a moth or butterfly, nor will they bite or sting, Currie said.
In Texas, at least two generations of the insect develop each year. The second generation is usually larger in number and causes more damage. In some past years, almost all of the native pecan trees in certain areas of the state were defoliated by walnut caterpillars.
For control, use products containing carbaryl (Sevin), malathion, diazinon, acephate (Orthene), methoxychlor, trichlorfon (Dylox, Proxol) or Bacillus thuringiensis. Thoroughly spray leaves, twigs, limbs and tree trunks. Large infested trees may require treatment by commercial applicators with high-pressure sprayers.