About those fall webworms
The spring gardening season in New Mexico often starts with the emergence of the western tent caterpillar ( Malacosoma californicum). These caterpillars build messy silk nests and eat young leaves on trees. The New Mexico gardening season ends with the appearance of fall webworms ( Hyphantria cunea), caterpillars that also build messy silk nests but eat the mature leaves on trees. The adult webwormmoth is white and may have black or brown spots on the forewings and reddish-orange patches on its front legs. Their wing span is about 1¼ to 1½ inches. In most of New Mexico, the moths probably have only one reproductive cycle per year. Adults emerge from their pupa case in late June or early July. They mate and the female lays an egg mass of 300 to 900 eggs on the undersides of leaves. The eggs hatch in 7 to 10 days.
The newly hatched caterpillars build a small silk nest that encloses leaves at the end of branches. Initially they “skeletonize” the leaves, eating the tissue but not the veins. Larger caterpillars eat entire leaves. The webworms enlarge their tent as they grow, eventually enclosing entire branches. The caterpillars have black or red heads. Black-headed caterpillars have pale yellow or pale green bodies, while red-headed caterpillars have darker, yellow-tan bodies. The larvae start out quite small and grow to a final length of 1-1¼ inches. The nests become quite messy as they fill up with shed skins, insect droppings, and leaf fragments. Caterpillars continue to feed into September. Eventually, the fully grown caterpillars leave the nest and form a pupal case inside a silk cocoon in a protected place in the soil or in leaf litter, or on the sides of trees, rocks, or buildings.
The fall webworm can feed on hundreds of different deciduous trees and shrubs. It has been recorded on 636 different species, including cottonwoods, walnut, alder, poplars, many fruit and nut trees, and shrubs. In general, they will eat any tree in New Mexico except pines, firs, spruces and junipers.
Although the nests are extremely unattractive, the caterpillars cause little damage to the tree. The mature leaves that they eat are no longer necessary for the tree’s food reserves. Webworm infestations tend to decease over time due to parasites, predators and bacterial and viral diseases. The caterpillars are an important source of food for several species of songbirds, bats, and small mammals.
Nests can be removed mechanically. Pull down the webs with a long stick or rake and destroy the webworms by drowning them in spray water or sealing themin a garbage bag. Tearing a hole in the nest will allow natural predators to get at the webworms. Cutting off the branches with the nest may damage the tree. Do not use fire to destroy the nest. This not only damages the tree but could start a wildfire. It is possible to use pesticides such as Sevin or some Bt formulations; however, these pesticides are most effective when applied in the early summer before the nests get large. If you decide to use pesticides, it is essential to make sure that they are registered for fall webworm control and to followlabel instructions.
Terry McGuire was professor of genetics at Rutgers University for 36 years. He was also a Senior Fellow of theNational Center for Science and Civic Engagement, helping educators connect science to civic issues. He moved to Santa Fe in 2014. He is a Master Gardener and will soon be a Master Composter.