The pine webworm (Tetralopha robustella) occurs throughout the eastern half of the United States wherever pines grow. This insect derives its common name from the habit of the larvae; they weave clusters of pine needles together into a silken nest. This webbing also protects the larvae from many natural enemies. In Texas, pine webworm frequently feeds on loblolly, shortleaf, slash and longleaf pines, often causing’ problems in young pine plantations where seedlings may die from complete defoliation. Pines grown for Christmas trees have a lower market value and are sometimes difficult to sell when they contain unsightly webworm nests. Infestations on ornamental pines around homes detract from the beauty of the trees.
From one to three generations develop per year in East Texas. The moths appear in the late spring, but are seldom noticed due to their small size (about 1 inch wingspan) and nondescript color pattern. After mating the female moth lays from one to more than twenty eggs in single rows along the length of pine needles.
Upon hatching, groups of up to 75 young larvae wander among the needles spinning silken threads. Each larvae then bores into a needle and mines it. Once they have grown too large to feed inside individual needles, colonies of larvae feed among loosely webbed clumps of foliage, filling the webbing with brown, oblong fecal pellets. This mass of webbing and fecal pellets encloses the needles and may be 2 inches to 5 inches long. Full grown larvae are 3/4 inch (15-18 mm) long, and are yellowish-brown with a dark brown longitudinal stripe on each side of the body. When fully developed, larvae leave the tree and construct a silk cocoon in the soil where they remain through the winter.
Natural enemies of the pine webworm help keep populations at tolerable levels. These include several wasp parasites, a fly parasite, and some birds that tear open the nests to feed on larvae. Often pine webworm larvae complete feeding and vacate their nests before the damage is noticed. It is too late to apply any type of control measures if this has happened. If a homeowner finds webworm nests that contain larvae, the nests can be pruned from the tree and destroyed. If pruning is not feasible, effective chemical control can be achieved by spraying the foliage and nests with Dylox, methoxychlor or Sevin. If reinfestation occurs, a second application in mid- to late summer may be necessary.
Chemicals must be currently registered for use on pine webworm by the US Environmental Protection Agency and the Texas Department of Agriculture. Before using any pesticide, read and carefully follow all application directions, cautionary statements, and other information appearing on the label.
Typical nest made by the larva of a pine webworm: