Gypsy Moth

Effects and Impacts

[photo:] Cluster of trees killed following gypsy moth defoliation, Central PA. Defoliation Causes Ecological Effects and Economic Impacts

The effects and impacts of gypsy moth (GM) defoliation have been studied extensively by Forest Service Northern Research Station (NRS) scientists, who inherited and continued the government’s institutional research infrastructure in the 1930s.  Major research programs to understand, treat, and control the GM were headed up by NRS scientists first located on the Yale University School of Forestry campus and then at a new laboratory build in Hamden, CT, and also at Morgantown, WV. Much of their knowledge is summarized in a paper by Twery (1991) Effects of defoliation by gypsy moth.

During a heavy infestation, GM caterpillars may devour much, if not all, foliage from trees. This defoliation is an additional stress (along with drought, soil compaction, etc.) that can affect tree physiology and may ultimately result in death. When GM densities reach high levels, large quantities of foliage are consumed and partial or total defoliation of the forest canopy may occur. Gypsy moth outbreaks often extend for hundreds of square miles and the defoliated areas may be extensive. However, areas behind the invasion front later recover to some extent and suffer limited outbreaks only occasionally. The defoliation and tree death caused by GM can have severe ecological and economic effects, especially along the invasion front.

Ecological Effects
Forest defoliation is a major ecological disturbance that has both positive and negative effects. Healthy hardwood trees will usually refoliate, that is, produce a second flush of leaves, but some trees are weakened and become susceptible to secondary agents. For oaks, the most important secondary pests are shoestring root rot fungus (Armillara spp.) and the two-lined chestnut borer (Agrilus bilineatus). Both of these agents attack weakened oaks and are usually the direct causes of mortality. Extensive mortality usually occurs following two or more consecutive years of defoliation, although mortality can occur following just one year of defoliation if other predisposing conditions exist, particularly drought. The mortality associated with GM outbreaks can profoundly affect the composition of forests and affect successional trends. However, oak mortality from GM does have certain positive effects. Extra sunlight reaching the forest floor allows forbs, shrubs, and new seedlings to grow and thrive. This also alters habitats for birds, small mammals, and invertebrates.

Economic Impacts
Oaks are important timber trees in much of the Northeast, and GM infestation can result in tree death, growth losses, and changes in wood quality, which all have direct secondary effects on timber values. Standing timber degrades quickly after tree death, causing losses of up to 25% within 5 years. Because as much as 50 to 90% of the forest is killed in some areas, these losses can be extremely disruptive to timber markets. The quality of the wood for pulp and papermaking is not degraded and the most dramatic effect is the immediate downgrading of potential veneer logs to sawtimber as the tree dies, resulting in value losses of 50% or more. Also affected is the potential growth in value of future crops. Stands that are understocked due to mortality following defoliation cannot utilize the site's growing potential fully, and fully stocked stands with trees that grow more slowly because of defoliation both contribute to the decreased value growth.

Twery, M.J.  1991. Effects of defoliation by gypsy moth. In: Gottschalk, K.W.; Twery, M.J.; Smith, S.I, eds. 1991. Proceedings, USDA Interagency Gypsy Moth Research Review, 1990.  GTR-NE-146. Radnor, PA: USDA Forest Service, Northeastern Forest Experiment Station. 152p.

Last Modified: September 10, 2015