Winter Tree Care: Identifying Common Tree Insects in Winter, Part 5

This is the fifth part of a series on winter tree insects. This article examines emerald ash borer and galls.


Through the winter months, plants conserve their energy, often enduring inclement weather in anticipation of spring. Due to the frigid temperatures, many insects enter a state of dormancy, overwintering on or within their hosts. While some insects are visible during winter, others conceal themselves in bark crevices or beneath the soil surface. The following examines some of the most common insects to infest trees during winter, and how they may be observed.

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 was officially discovered in southeastern Michigan near Detroit in June 2002. Emerald ash borer 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.

Impact on Trees:

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.

Detection & Management:

  • A visual survey should be conducted by a professional arborist 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 the 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 is discovered, the area can be placed under quarantine in order to prevent further infestation. 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 are often removed and replaced with non-ash species to deter population growth.
  • Government agencies within the United States and Canada have utilized a large ground-nesting wasp called cerceris fumipennisto 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.


Galls are abnormal swelling growths that occur on the leaves, twigs, branches, roots, and flowers of plants. A gall’s characteristics are determined by its causal organism. They vary in shape, appearing as balls, knobs, lumps, or warts. Galls display an array of colors, ranging between red, green, yellow, brown, and black. There are 1500 species of gall producers, the majority of which are comprised of insects such as aphids, midges, wasps, and mites. Some galls are the result of bacterial or fungal infections; others are produced by microscopic worms called nematodes. These types of galls can be difficult to distinguish from those created by insects. Many gall producers are host specific.

Galls that develop on leaves are generally an aesthetic problem. Plants that are affected by galls may develop an unsightly appearance, but are otherwise seldom injured. Gall formations can sometimes deform foliage on younger trees. Oaks are one of the most susceptible tree species. They are host to over 500 different wasps, aphids, mites, and midges that cause galls on leaves, branches, and twigs.

Detecting Galls on Plants

Gall formation is recognizable on most plants. Galls appear as abnormal swelling growths that vary in size, shape, and color. Galls that enlarge on smaller plants can interrupt the vascular system, hampering the flow of water and nutrients. This often results in plants becoming stunted, and developing a malformed appearance. Some galls are less apparent, occurring on twigs, small branches, and roots. Over time, they may weaken or kill portions of a plant.


  • Cultural practices can be employed to prevent gall formation. Rake and destroy fallen leaves from plants that have been affected by galls. Prune and dispose of gall-infested twigs and branches.
  • Prevent plants from becoming stressed by ensuring they are sufficiently watered, especially during dry periods.
  • Fertilization in spring or summer will help maintain plant vigor.
  • Applying a layer of organic mulch around the base of trees and shrubs can improve soil quality, and reduce soil moisture loss. It will also help moderate soil temperature.
  • Chemical sprays may be warranted on trees that are in poor health. Sprays will reduce the number of galls and gall-inducing insects, but may also kill beneficial insects. To prevent galls, applications must be made before galls form.

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