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| Walnut caterpillar, moth | Walnut caterpillar, adult | Walnut caterpillar, larvae |
Walnut caterpillars feed together in large numbers on pecan leaves, but do not construct silken webs like fall webworms. Larvae eat leaves, leaving only the mid-ribs and leaf stems, and large infestations can defoliate entire trees. This insect is found throughout Texas east of the Pecos River. Although economic infestations are uncommon, severe and widespread outbreaks of walnut caterpillar have occasionally occurred in Texas.
Biology: Walnut caterpillar moths emerge in the spring and deposit eggs in masses of 500 or more on the undersides of leaves. The egg masses are round, about the size of a half-dollar and are not covered with hairs or scales. Eggs hatch in about 10 days and larvae feed for about 25 days. Young larvae are reddish-brown with yellow lines running the length of the body. Full-grown larvae are about 2 inches long, black with grayish lines and are covered with long, soft, gray hairs. Larvae congregate in large masses on the trunk and scaffold branches to shed their skins before crawling back to complete their feeding on leaves. Most of the foliage is consumed by these final stage larvae and defoliation can occur very quickly. Mature larvae crawl to the soil to pupate. A generation is completed in about 6 to 8 weeks and there are two to three generations each year (see Extension publication L-1835, "The Walnut Caterpillar").
Control: Since the walnut caterpillar does not construct tents or webs, infestations often go unnoticed until leaf damage becomes obvious. Look for egg masses or leaf feeding to detect infestations early. Egg masses can be detected at night by shining a flashlight on the undersides of leaves and looking for white spots about the size of a half dollar. Caterpillars cause 80 percent of their damage during the last 3 to 4 days of feeding. Smaller larvae are easier to kill with insecticides than larger larvae, and control of this stage prevents serious damage. Insecticide treatment may be necessary if large infestations threaten to defoliate trees.