Seeing change in our forests

Leslie Horner

UNIVERSITY PARK — Despite the sheer number (both individuals and species) of insects in eastern forests, they go unnoticed by many as they carry on with their life cycles, hidden from our cursory sight under the leaf litter or in the tree canopy or beneath the bark of a tree. Every once in a while, though, changes in environmental conditions will cause insect populations to boom. These booms, or outbreaks, in insect population are more commonly noticed–either due to sheer number of insects or by their effects they have.

While there are many native insects that escape our attention until such a population outbreak occurs, there are other species of insects that have gotten attention because of the havoc they are wreaking in the forests of Pennsylvania and other eastern states. Emerald ash borer and hemlock wooly adelgid are among several non-native pests that are on the radar of many woodland owners and managers as they have caused widespread death of ash trees and hemlocks. Spotted lanternfly, too, has also caused great concern for its rapid spread the past two years and the devastating effects its continued spread will have on trees and agricultural crops. Although native insects like gypsy moth, cherry scallop shell moth, and fall cankerworm can also cause significant forest damage during population outbreaks, a natural control of some kind (predators, for example) that brings the population back into balance is more likely within an insect’s native range.

Changes in environmental conditions–especially changes in temperature and precipitation–play a big role in the dynamics of insect population outbreaks. Forest researchers and managers have long been paying attention to changing environmental conditions, since they can have both short-term and long-term effects in forests, whether related to insect pests specifically or to overall forest health. As more and more woodland owners become aware of the damage from insect pests to some common tree species in our forests, many wonder how these pest insects are affected by weather extremes like a polar vortex, or an unusually warm winter, or a drought.

Will gradually changing environmental conditions and short-term weather extremes help or hinder the spread of the non-native pest insects in our forests? We know that some weather extremes can help in controlling non-native insect pests in the short term. For example, single digit temperatures can kill as much as 90% or more of hemlock wooly adelgids in a given location. Individuals that are especially cold-hardy, however, will survive and reproduce. Emerald ash borers, whose larvae survive the winter underneath the bark of an ash tree, are very tolerant to extreme cold–surviving at temperatures even 22 degrees below zero. Spotted lanternfly is also adapted to cold temperatures, and actually lays eggs during the winter.

Weather extremes and changing environmental conditions can directly or indirectly affect insect populations. In terms of direct effect, temperatures that vary significantly from the normal seasonal average may affect the development rates of insects, or cause a change in the timing of reproduction, and cause direct mortality of some insects. A long period of unseasonably warm days in winter, for example, can cause some species to break their winter dormancy, which in turn might lead to untimely reproduction or cause mortality in adults and/or immature insect forms when the temperatures snap back to normal. When we think of specific examples like some species of parasitic wasps that prey on emerald ash borers, it becomes clear how what seems like a small impact (i.e.–death of some parasitic wasps during unusual or extreme weather) can cause ripple effects in the web of life in the forest.

Changing environmental conditions can also have indirect effects on both insect and host tree species. For example, a trend of warmer winters can help facilitate a species extending an insect species’ range north. This has been evidenced by the southern pine beetle now moving further into the northeast. Similarly, trees can also extend to the edges of their range, facilitated by changing environmental conditions. When they are at the edge limits they are even more susceptible to damage resulting from insect outbreaks.

Forest researchers and managers have long been paying attention to changes in environmental conditions, documenting both short-term and long-term effects in forests. Effects are being observed in individual woodland stands as well as at the landscape. Defoliation, dieback, mortality of host trees resulting from insect outbreaks has impacts on forest structure and composition, often aiding the spread on invasive plants. Insect outbreaks can also leave trees more vulnerable to tree diseases.

With the complexity of the forest ecosystem and the wildcard factor of so many non-native, invasive insects and plants it seems daunting to think about the changes we may continue to see in our woodlands. For forest researchers, the focus of their work will continue to explore these complex relationships between environmental conditions, insects, and woodland plants and trees. For forest managers and woodland owners, there are tangible steps we can take to support forest health and function. We can maintain diversity of tree species and diversity of ages of trees. We can monitor our woodlands to watch for non-native insects and plants, and take action to control their spread. We can talk with our fellow woodland owners and neighbors about what we’ve learned in taking care of our own woodlands.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of free publications, call 800 235 9473 (toll free), send an email to, or write to Forest Stewardship Program, Natural Resources Extension, The Pennsylvania State University, 416 Forest Resources Building, University Park, PA 16802. The Pennsylvania DCNR Bureau of Forestry, USDA Forest Service, Penn State Extension, and the Center for Private Forests at Penn State, in Partnership through Penn State’s Department of Ecosystem Science and Management, sponsor the Forest Stewardship Program in Pennsylvania.


Leslie Horner is Forest Stewardship Program Associate, Center for Private Forests at Penn State.