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Caterpillar damage not as bad as it looks

This seems to be the year for tent caterpillars in Spokane. Never heard of them before? If you have seen weird webbed tents wrapped around leafy tree branches, you’ve seen their handiwork.

The good news is that while the caterpillars will defoliate a branch quickly, making it look terrible, the tree usually recovers and grows new leaves by midsummer.

We have two common types of tent caterpillars in the Northwest: the western and the forest. The western caterpillar is 2 to 3 inches long with a hairy, dull yellow-brown body lined with rows of blue and orange spots. The forest caterpillar is about 2 inches long, blue with black and white markings.

The caterpillars do all their damage in their larval stage. Their most common targets are alder, ash, birch, cottonwood and willow as well as fruit trees and roses. Larvae emerge from egg masses laid the previous year as the leaves begin breaking bud. The emerging larvae move to feeding sites and begin building their nest. Western caterpillars will build a tent around a leafy branch while forest caterpillars will build more of a matlike structure. Both usually will be on the south side of the plant so the caterpillars can warm themselves early in the day.

The caterpillars then go through at least four molts growing larger – and hungrier – each time. They start out feeding as a large group but over time as scouts locate new food sources, they split into smaller groups.

About mid-June, the larvae begin to pupate, with adult moths emerging in early July. The adult moths are stocky with brown to lighter brown markings. The female lays her eggs in masses around tree limbs or under building eaves and covers them with a foam that keeps the eggs hydrated and protects them from predators. The cold-tolerant eggs emerge the following spring to begin another cycle.

Tent caterpillars are more of a nuisance than a real threat here. Their nests and the bare stems they create make trees look ugly but the trees usually recover easily. Infestations come in cycles depending on the levels of predator insects.

Integrated pest management is the best way to manage tent caterpillars. Start by looking for egg masses in the early spring and remove them. Cut out branches that hold nests, if they are within easy reach, and dunk the whole nest in a bucket of soapy water to drown the caterpillars. Don’t burn the nests especially if they are still in the tree. Playing with fire in our dry summers isn’t a good idea, and the plants can be severely damaged or killed in the process.

Chemical control is rarely needed because the caterpillar damage is mostly cosmetic and their presence usually varies enough year to year that they relatively are easy to live with. If it is necessary to spray, use sprays containing the bacteria Bacillus thuringiensis (Bt) or spinosad in the late evening. Both are organic, attacking the caterpillar’s internal systems and leaving little effect on other insects or wildlife.

Pat Muntscan be reached at

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