Pest description and damage Webworms are larvae of webspinning sawflies. The larvae cause characteristic damage on spruce and some pines. The older needles on the tree are consumed, and the webworms construct a large web at the feeding site, which becomes covered with frass. The new growth on the tree may extend beyond the defoliated stems of the older growth.
Biology and life history The insect overwinters as a larva or pupa in the soil. Adults emerge in mid-spring. The females lay eggs on old needles of the tree. The larvae hatch and soon begin feeding, spinning loose webs at the base of the needles. They feed on the old growth, pulling the cut needles into the web. The larvae feed for up to 3 weeks. When they are mature, usually in June, they drop to the ground and form earthen cells 3 inches below the surface. There is one generation per year.
In Austria, nematodes applied to the soil gave good control if the soil pH was kept at 5.0-7.0 with lime applications.
See Table 2 in:
Chemical Control of Landscape Pests
For more information
See "Sawfly" in: