Spring Tree Care: Identifying Common Tree Insects in Spring, Part 4

This is the fourth part of a series on spring tree insects. This article examines emerald ash borer, and forest tent caterpillar.


In spring, deciduous trees break dormancy, and resume the growing process. During this period, many insects become active, emerging from their overwintering sites to plague their hosts. The following describes some of the insects that may be commonly observed in spring, and how they can impact susceptible trees.

Emerald Ash Borer (Agrilus planipennis)

Emerald ash borer is an exotic, invasive wood-boring beetle native to eastern Asia. It is believed to have been introduced to North America in the 1990’s through wood packing material imported from Asia. The insect has since become widespread throughout the United States and Canada, and is responsible for decimating large ash tree populations, with tens of millions of ash trees confirmed to have been affected.


Emerald ash borer feeds primarily on ash trees, including green, white, black, and blue ash. Infestations have also been reported in white fringetree. Emerald ash borer reproduces in trees of all sizes: from saplings to fully mature trees.

Symptoms of Infestation

As larvae feed on the inner bark of trees, they produce serpentine feeding galleries that disrupt the tree’s conductive system. This impedes the tree’s ability to transport and distribute water and nutrients, resulting in foliar dieback and declining tree health. The severity of the injury tends to culminate in the death of the tree within three to five years.


  • A visual survey should be conducted to determine if trees are displaying symptoms of emerald ash borer.
  • If emerald ash borer is discovered to be present, there are a number of traps that can be used to contend with adults. One type of trap is made up of colors that are attractive to emerald ash borer. These can be hung in trees in an effort to monitor emerald ash borer behavior. Other traps can have volatile pheremones applied to them in order to lure out adults. These are most effective with males.
  • Trees can be girdled and used to trap egg laying females in spring. Once eggs have been deposited into the bark, the tree is then debarked in fall so that the larvae may be detected.
  • If larvae are discovered in a tree, the area can be placed under quarantine in order to prevent further infestations from occurring. Although complete extermination is not often viable, additional control methods can then be taken to reduce the larval population, and hinder their dispersal.
  • Infected trees can be removed and replaced with non-ash species to deter population growth.
  • A large ground-nesting wasp called cerceris fumipennisto can be employed to detect emerald ash borer in trees. The wasps do not impact the emerald ash borer populations, but they enable arborists to more effectively determine their presence. This process is referred to as biological surveillance.
  • Systemic insecticides can be incorporated into the tree through direct injection or soil drenching. These remain effective for one to three years depending on the product, but if applied over a period of years, tree injections can compromise the tree’s health. If a tree is already infested, insecticides may deter further population growth, but they will not prevent emerald ash borer from causing additional internal damage. Insecticides are not feasible for large areas experiencing widespread infestation.

Forest Tent Caterpillar (Malacosoma disstria)

Forest tent caterpillar is a significant defoliator of various trees and shrubs. It is indigenous to North America. Outbreaks of forest tent caterpillar occur periodically, often at 10 to 12 year intervals. During outbreaks, infested plants may be completely defoliated. Infestations that persist for more than three years can result in plant mortality.


Forest tent caterpillar infests a wide range of hosts, which vary by region. In the Northeastern United States, forest tent caterpillar infests apple, ash, aspen, basswood, birch, cherry, elm, and oak. In the Central and Midwestern United States, forest tent caterpillar favors oak and quaking aspen. In the Southern United States, plum, sweetgum, swamp blackgum, and water tupelo are common hosts. In the Western United States and Canada, trembling aspen is the preferred host. Other tree species that are frequently infested by forest tent caterpillar include cottonwood, red alder, and willow. When populations are dense, forest tent caterpillar may feed on wild and ornamental shrubs such as azalea and rose, as well as on numerous cultivated fruits and vegetables.

Symptoms of Infestation
In winter and early spring, masses of eggs can be observed on the branches and trunk of infested plants. Once the eggs have hatched, the flowers and leaves of infested plants will be rapidly consumed. If the larvae hatch prior to leaf expansion, they will mine the buds. On sugar maples, sap production may be significantly reduced in the year following infestation. Plants that have been weakened by forest tent caterpillar may be invaded by other pests, and disease pathogens. When defoliated, many plants will produce a second flush of growth. This second flush of growth will often appear stunted. Repeated defoliation can result in widespread foliar dieback, as well as plant mortality.


  • Exposure to freezing temperatures can cripple larval populations.
  • When plants are defoliated before the larvae mature, the lack of available foliage may cause them to starve.
  • Forest tent caterpillar has a multitude of natural enemies that help to limit its populations. Several species of flies and wasps parasitize the eggs, larvae, and pupae. Two large gray flies, Sarcophaga aldrichi and Sarcophaga houghi are often responsible for the collapse of entire colonies. The adult flies deposit larvae on the cocoons. The larvae penetrate the silk of the cocoon and the epidermis of the pupae, effectively killing them. Itopletis conquisitor is an important ichneumonid wasp parasitoid of the pupal stage. Predatory beetles, ants, true buds, and spiders feed on the larvae and pupae. Frogs, mice, skunks, and over 60 species of birds prey on the larvae and pupae.
  • Forest tent caterpillar is susceptible to a bevy of bacterial, fungal, protozoan, and viral diseases, which can cripple populations.
  • Early instar larvae can be plucked from the branches that they feed on. This method is most effective when populations are low.
  • Infested branches can be pruned out and disposed of. Pruning is most ideal in winter, when the egg masses are visible.
  • Several insecticides and a microbial insecticide are registered for use in controlling the insect. Applications should begin in spring, when the larvae first appear. Subsequent applications may be performed at 7 to 14 day intervals, as required.

Photo courtesy of U.S. Department of Agriculture CC-by-2.0