Tree Foliage Insect Pests - Timing, Impacts of Early Defoliation, and Control

Written by Jeff Osborne, Forest Stewardship Program Associate, James C. Finley Center for Private Forests at Penn State

     Tree pests eat or cause leaves to brown at different times in the growing season – various pests impact trees in early or late parts of the growing season. Spongy moth is an infamous early season defoliator, for example, with this year’s defoliation events occurring in May; locust leaf miner impacts are frequently seen in late season, with black locust leaves currently starting to brown from the miners’ feeding patterns. Below is a table of nine common tree defoliating pests (some native and some introduced) that cause noticeable foliage damage in the early or late growing season. Many of these pests do not warrant large-scale control methods in the forest, but control of landscape trees may be desired. We will discuss the relative damage and treatment options for the more damaging pests.

 Table 1. Select defoliating insects, their host, seasonality, and effects.

     Many early defoliators emerge from eggs laid the previous year, such as the spongy moth and bagworm. Forest managers assess oak forests for spongy moth egg masses in early fall. Penn State Extension created a guide for surveying forests for spongy moths and developing treatment plans if the egg masses exceed 500 egg masses per acre. The caterpillars hatch in mid-May and feed into late June before transitioning to non-feeding adults. They molt several times, and the last two instars create most of the defoliation. There is short window, about four weeks, where the caterpillars can be killed before causing major damage.
     Aerial spraying with pesticides by plane or helicopter is the general method of spongy moth control.  It can be difficult for owners of fewer than 50 acres of forest, in a fairly square shape, to treat oak stands since this is about the minimum area that is needed to effectively control the caterpillar by aerial spraying. Spray contractors may not want to treat areas less than 50 acres in size because a significant percentage of the stand can be damaged from caterpillars moving into the stand from the untreated areas outside the stand. Spray contractors have a limited amount of area they can spray based on the caterpillar’s life cycle, weather that allows effective spraying, and availability of pesticides. Landowners should contact spray contractors early in the year; some will expect to have contracts signed by March for spraying later that spring. For high-value trees, stem injection of insecticides may also be an option.

This picture shows spongy moth damage to a 4” tall white oak despite the author manually removing the caterpillars nearly daily for two weeks.

      Bagworms can ravage arborvitae. These caterpillars hatch and start feeding in May. They can defoliate entire trees or those adjacent in rows planted as privacy or wind screens. Bagworms quickly disguise and protect themselves by attaching plant debris to themselves using silk which later forms their namesake sac around them. As they grow, they feed on and damage increasing amounts of foliage. In late summer they transition to moths. The female moths do not fly. They then lay eggs inside their bags that will start the next generation in the following spring. They may defoliate newer foliage in the first year of infestation, and in the second year, as they are eating the new growth and buds, the two-plus-year-old foliage dies, leaving the tree brown and dead. Trees can be sprayed with insecticides during the beginning of caterpillar activity in May/June. You can also inspect landscape trees for bags, then remove and destroy them to reduce the hatch size.
     European pine sawfly and eastern tent caterpillar, both early season defoliators, are not usually a mortal threat to well-established trees because the sawflies eat second-year needles, and well-established and healthy hardwoods should be able to establish two sets of foliage in a year. These two pests are not generally treated in forest stands; but nursery stock or newly established trees should be monitored and protected from them. They can be manually removed or treated with insecticides on a large scale. These and other early defoliators deplete energy and nutrients the trees spent for foliage before the foliage pays for itself. This can kill small trees and established trees that encounter other serious stressors on top of the defoliation.
     Late season defoliators, such as oak sawfly, mimosa webworm, cherry scallop moth, fall webworm, and locust leaf miner (pictured below), often strike after the leaves have already helped the tree produce molecules for structural growth and stored energy throughout the growing season which can then be used the following spring. The late season defoliators can be unsightly in the lawn but are generally not treated in the forest. They can be treated with insecticides as well, however, are less often fatal to the trees.

Left: Locust leaf miner feeding on a black locust leaf. Right: Leaves on portion of a black locust beginning to brown. Taken on July 27^th in Clearfield County

You can find more comprehensive guides to diagnose tree pests and diseases through Penn State Extension. The Virginia Department of Forestry also has a great guide to diagnose tree pests and diseases