Defoliation and Impacts
Peaks of Otter, 2001
Gypsy moth has defoliated nearly 90 million acres nationwide since records were first recorded in 1924. In Virginia, historical defoliation is nearly 5.5 million acres with a suppression cost of over $17 million. These costs do not include the economics associated with tree mortality, reduced tourism, and adverse recreational and residential impacts due to defoliation and the nuisance of large numbers of caterpillars in and around dwellings and public areas. There also are human health impacts such as allergic reactions to gypsy moth larvae and their excrement as well as environmental impacts resulting from changes in the forest canopy.
It is difficult to predict tree mortality from defoliation. Mortality in poor sites with stressed trees can be very high. For example, approximately 50% of susceptible trees died in parts of Shenandoah National Park when gypsy moth first swept through that area. Mortality rates of healthy trees on good sites will be far less. When trees are defoliated beyond about 60% they will refoliate. This causes an additional stress on the tree and makes it more susceptible to dieback and infection from secondary organisms, which can cause tree mortality. Healthy trees can tolerate one or two defoliation events, but stressed trees are more susceptible and may succumb after only one defoliation. In addition, stands with high percentages of oak are more susceptible to defoliation than are mixed stands.
This graph shows various negative impacts as a function of predicted defoliation and egg mass density. Remember, there is a large amount of variability in damage as a function of population density, and some environments (and people!) have higher tolerances than others.
Defoliation in SW Virginia, 2006 & 2007
Additional Resources
- Map of 2007 defoliation in Virginia
- Larger scale map of 2006 & 2007 Southwest Virginia defoliation
- Map of historical defoliation in Virginia
- Graph of historical defoliation in Virginia