Insect Pests in May

Several insect pests of trees and shrubs make their appearance this month. The following are among the most common and widespread.

Eastern Tent Caterpillar. From late April to early June you may see apple, crab apple, wild plum, and related trees "decorated" with the silken tents of the eastern tent caterpillar. The larvae gather at a major branch fork and construct a dirty-gray silk web or tent. The caterpillars leave the tent to feed on tree buds and foliage on warm, sunny days. At night and on cool, cloudy days the caterpillars rest in the protective confines of the tent. As the caterpillars feed and grow, they enlarge the tents, making them more obvious in the landscape and along roadsides.

Damage by eastern tent caterpillars is more annoying than harmful. Defoliated trees usually recover with a new flush of leaves in June. The appearance of the web, more than the defoliation, is often the major concern.

The most practical control is usually to remove by hand the tents and caterpillars as soon as they are noticed. Remove the tents in very early morning or at night when the caterpillars are occupying the tents. The caterpillars and the silk are harmless to people. Discard the tent away from the tree or dispose in the trash or by burying. Insecticide sprays are rarely warranted, but Bacillus thuringiensis can be applied as a foliar spray if necessary.

Pine Needle Scale. The overwintered eggs of the pine needle scales hatch soon after the lilacs and as the Van houtte spireas bloom. In some years this can be in mid-May though it appears it will be late May in most regions of the state this year. Pine needle scale is a common sap-feeding pest on pines, spruces and firs. The scales appear as small, white, waxy bumps on the needles.

Because only the "crawlers" (newly emerged nymphs) are vulnerable to contact insecticides, it is important to spray the foliage just as the eggs are hatching. Crawlers can be controlled with sprays of horticulture oil (use the lower, summer rate), insecticidal soap, diazinon, Orthene, malathion, Dursban, or Sevin.

European Pine Sawfly. The needle-eating larvae of the European pine sawfly are common on various species of pine trees and shrubs. The larvae are grayish-green with two light stripes and one dark stripe on each side of the body. The legs and head are shiny black. Full-grown larvae, usually present by Memorial Day weekend, are about 1 inch long.

European pine sawfly larvae are gregarious and stay together in a cluster as they feed on the old needles. Because only old needles are eaten, defoliated trees are not killed. Control can be as simple as pruning off and discarding infested branches. Heavier infestations on larger trees may justify foliar sprays of Sevin, Orthene, or Isotox.

This article originally appeared in the May 16, 1997 issue, p. 73.


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