   Fall Webworms - October 11, 2006 Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Fall webworms (Hyphantria cunea) have been feeding on cottonwood, black walnut, alder, and other deciduous trees in the Verde Valley area since mid-summer. They have just become more noticeable in September because it’s the second generation. Fall webworm infestations bear some resemblance to tent caterpillar (Malacosoma spp.) infestation in that they create an live within silken webs at the ends of hardwood branches and feed on the foliage. Even though their feeding appears to damage the plant, their long-term impact is not usually severe. Fall webworms are native to North America and Mexico and can produce one or two generations per year. Adult moths emerge from overwintering pupa in the spring. After mating, females lay masses of eggs (400 to 500) on the undersides of suitable host tree leaves. The larvae feed in colonies in web enclosed foliage. They begin by constructing small webs near the ends of branches and gradually increase the size of the web as the need for food increases. Mature caterpillars are yellowish with black and brown markings, and have many tufts of long hair. Feeding continues for four to six weeks. Mature larvae crawl down the tree and pupate in the litter on the ground. Pupation normally occurs in June with the adults of the second generation appearing in late June to early July. These adults mate and the female lays eggs on the underside of the leaves. Second generation larvae are usually present from late July through late September. As larvae mature, they crawl down the tree and spend the winter as pupa in the leaf litter under the tree. The first generation is often small and usually goes unnoticed. The second generation is much larger and more noticeable. Fall webworms are found more often on trees that are not surrounded by other trees. High populations of fall webworm can completely defoliate host plants but do not kill them. However, on pecan trees, nut production and quality can be reduced if webworms are not controlled. Pruning and destroying the infested portions of branches is a common control practice while webs are still small. Also, a stick or pole with a nail inserted crosswise can be used to snag individual webs. Twisting the pole after insertion will cause the web to wrap around the pole where it can be removed and bagged or burned. When webs become too large, controlling webworms by pruning may ruin the aesthetic shape of the woody ornamental plant. Various species of natural enemies help to manage this native insect. Birds and many insect predators and parasitoids (often wasps or flies) attack the larval stage. Eggs may also be destroyed by predators and insect parasitoids. Over 80 species of parasites and predators have been identified in North America. Social wasps (yellow jackets and paper nest wasps), birds, predatory stink bugs and parasitic flies and wasps are the most important. Contact insecticide sprays should be withheld until it is certain that predators and parasites are not present in sufficient numbers to control the webworms. If insecticides are warranted, either Bacillus thuringiensis (Bt) or chemical sprays can be used. Bt is quite effective against fall webworms if it is applied when the larvae are small. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested. Chemical sprays can be effective for protecting fruit and nut crops. Locate nests early and merely wet the nest and cover nearby foliage. Carbaryl is labeled for fall webworm. As the larvae walk on the nest surface or incorporate new foliage, they will contact the insecticide. Second applications may be needed if additional generations occur. Some systemic insecticides (acephate) are labeled for use on fall webworms, but they are not labeled for use on most edible crops. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |