My Juniper Is Being Attacked by Caterpillars

Tiger Moths

• The two caterpillars that feed on junipers exhibit different feeding patterns and nest in different ways. Tiger moths lay eggs in junipers and other evergreens in mid- and late summer. Egg clusters hatch in early fall. The caterpillar colony first appears as a small web nest at the intersection of the tree's upper branches. Caterpillars of the silver-spotted tiger moth feed through the winter during favorable weather, causing greater damage to the tops of trees in spring when the caterpillars mature. Large nests of dense silk threads contain hundreds of black caterpillars about 2 inches long, and rows of orange and yellow markings on their sides. Stiff bristles covering the worms break off and cause dermatitis if touched. The moth's range covers the western US and southern Rocky Mountains.

Bagworms

• Bagworms range throughout the US and threaten more serious harm to junipers than do tiger moth caterpillars. Bagworms spread from tree to tree slowly, since only the male bagworm moth actually flies. Bagworm caterpillars could fall to the ground and find their way to new homes, but the worms usually stay on the host tree. Individual caterpillars weave partial cocoons around their lower bodies and camouflage themselves with the tree's needles. When worm populations stay small and cause little damage, the bags look like natural parts of the tree. The dark greenish caterpillars stay within the cocoon, emerging just far enough to feed. Cocoons of mature worms resemble slender Christmas ornaments that are 2 inches long and covered with layers of dead needles or bits of evergreen leaves. Small worms travel along needles and twigs while holding the cocoon upright like a tiny ice-cream cone studded with natural debris.

Bagworm Development

• Female bagworms never leave their protective cocoon and don't develop into winged moths. Male bagworm moths fertilize the females within the cocoons, and females lay about 300 eggs in the bags. Female bagworms don't survive the winter, but the cocoons shelter the eggs until spring. If bagworms establish a breeding population in a tree one year, the next year's population explodes in number. Infestations last for years, and denuded junipers often die.

Caterpillar Control

• Bacillus thuringiensis spray controls either type of caterpillar by infecting feeding worms with a fatal illness. Pyrethroids or carbaryl insecticides also knock down feeding populations quickly. Either approach works with the best effect if applied early in the life cycle of the insect when caterpillars are still small and have caused little damage. Treating new tiger moth nests in the fall prevents serious damage. In early spring, small upright needle clusters on twigs reveal the first hatch of bagworms. Spray the tree as soon as the feeding population appears. For complete control, clip the bagworm cocoons off the tree to prevent a new generation from hatching.