Malus: Tent Caterpillars

Problem
In spring, silk nets appear in the crotches of trees or on the ends of branches. The leaves are chewed and the tree may be completely defoliated. Groups of bluish or black, hairy caterpillars with yellow or white stripes and blue or white spots are feeding in or around the nets.

Analysis
(Malacosoma species)

Tent caterpillars feed on many ornamental and fruit trees in the garden. The insects are found in nearly all parts of the United States. In summer, tent caterpillars lay masses of 150 to 300 eggs in bands around twigs. The eggs hatch in early spring when leaves are beginning to unfold; the young caterpillars immediately begin to construct the nets. On warm, sunny days, they devour the surrounding foliage and may strip trees in just a few days. The caterpillars feed for 4 to 6 weeks and then pupate. In mid- to late summer, brownish or reddish moths emerge and lay the overwintering eggs.

Solution
Cut out and destroy large nets; or spray with an insecticide containing carbaryl or malathion, or the bacterial insecticide Bacillus thuringiensis (Bt) when nets are first noticed. A bacterial insecticide is most effective against small caterpillars so it is best to spray before nets are large. Use high-pressure spray equipment to penetrate the netting. To prevent damage next year, destroy the brown egg masses that encircle the twigs during the winter.