Common name: fall webworm
Host plants: wide range of about 90 species, including: fruit trees, walnut, maple, willow, alder, arbutus, cottonwood, and sweetgum.
Adult size: wingspan: 1.4 – 1.7 in (35 – 42 mm)
Generations per year: one, in the Pacific Northwest
Description & Life Cycle: Adult fall webworm moths are pure white with a hairy body. The wingspan is 1.4 to 1.7 inches. (There is another race of fall webworm that lives below 40° N latitude in North America. It is about the same size, and white, but the forewing is covered in dark spots.)
Northern race, (sometimes called orange race or black-headed race), adults start emerging in mid- to late May after over-wintering in the pupal stage. Like most moths, the fall webworm is nocturnal and attracted to lights. Females lay 400 – 1000 eggs in a single layer on the underside of a leaf on her chosen host plant. They are iridescent green in colour and lightly covered with scales from the female’s abdomen.
The larvae hatch in a week to ten days and immediately begin to feed on the leaves, spinning their web as they eat. The larvae of the northern race will increase the size of the web as they grow in size, producing a dense, multi-layered tent. They leave the web when they reach the fifth instar stage, whereas the larvae of the southern, red-headed race remains within their web until they are ready to pupate.
Reaching full size in four to six weeks, the mature larvae drop to the ground to pupate in leaf litter, or in the soil, where they will over-winter. They will also pupate in crevices in the bark of the tree. The pupa is dark brown in colour and about five eighths of an inch long.
There is generally just one generation per year in the northern race.
Special notes: Most years the numbers of fall webworms are not too severe, so they rarely do any serious damage to the trees…except for their unsightly webs and the defoliation of the leaves.
However, as with most insects, the fall webworm numbers can increase to drastic proportions for one or two years running before returning to more moderate numbers once again. During these heavier infestations, there will also generally be an increase in predatory insects and birds who do a good job of going after this pest.
Gardeners can assist in controlling this pest by cutting out the webs and bagging for the garbage.
Posted on July 6, 2016