Forest Snapshot July 2025

Come see the Extension Forestry and Wildlife Team and other exhibits at the 2025 Ag Progress Days, Pennsylvania's largest outdoor agricultural exposition, to be held August 12-14. Throughout Ag Progress Days, experts conduct tours highlighting research on various aspects of agriculture, including a Goods from the Woods and Demonstration Woodlot Tour hosted by forestry and wildlife experts. For more information, visit the Ag Progress Days website.

Forest Health   

Fall cankerworm (Alsophila pometaria) is a native spring defoliator of various hardwood trees, with a preference for oak trees. They are considered inchworms and move with the characteristic inchworm motion. Interestingly, cankerworm caterpillars tend to be green in color when in low population densities and a darker color when in high densities. Although fall cankerworm caterpillars feed in the spring, the adults emerge in the fall, which is where it gets its name. In general, fall cankerworm do not pose a serious threat to the health of our native forests; however, their populations can periodically reach outbreak levels, as we've seen in spring 2024 and 2025. When outbreaks occur in combination with other forest stressors or subsequent years of stress, tree mortality and dieback can occur. Historically, fall cankerworm outbreak populations crash within a year or two as natural controls, such as parasitic wasps and fungi, limit populations. During years of increased fall cankerworm populations, a variety of bird species and other wildlife can be observed at higher numbers in these areas, as the cankerworm serves as a great food source.

For 2025, fall cankerworm continues to be observed in the southwestern part of the state, although not as widespread as in 2024. Collapse of the outbreak has been observed in many areas in the southwest; however, pockets of high populations remain in Somerset, Westmoreland, and Cambria counties, especially along Chestnut Ridge and the Mt. Davis area. Isolated fall cankerworm populations have been observed in Bedford County as well as a few other southcentral Pennsylvania counties. Many of these populations remain isolated and are not as widespread as the outbreak that occurred in the southwest in 2024.  Many of the trees that experienced defoliation in these new areas of fall cankerworm infestations were able to re-leaf, thanks to the abundance of precipitation we've received thus far this spring, allowing the trees to better grow and develop new leaves. At this time, we are not able to predict fall cankerworm populations in 2026, however, concerned citizens should be on the lookout for adult fall cankerworm moths this fall to get an idea of what cankerworm populations may be in the spring. The females are wingless and flightless and may be easier to find as they begin to crawl up the trunk of the tree, shortly after the first frosts of fall. More information can be found in the Fall Cankerworm factsheet.

Native Species

Many trees and shrubs will be setting fruit in August and September, like the silky dogwood (Swida amomum, formerly Cornus amomum). Silky dogwood is a native shrub that can be found in a wide range of sites, from dry to wet locations with part to full shade. Stems of silky dogwood readily root when they meet the ground, forming thickets and acting as great erosion control along waterways and along rain gardens. Clusters of white flowers appear in mid-summer, supporting many species of insects, and post-pollination, gives way to dense clusters of blue fruits that are a favorite food source of songbirds. Keep an eye out for this native shrub as you explore Pennsylvania forests this season.

Wildlife and Insects

Coming into August, hummingbirds are finding new sources of nectar as early and mid-summer blooms fade. The flowers of jewelweed (spotted touch-me-not) are shaped well for hummingbirds to access their nectar. In doing so, hummingbirds also serve as one of the main pollinators of jewelweed.

Soon, adult Monarch butterflies will be taking wing, while their caterpillars are still feeding on the leaves of milkweed plants. The caterpillars feed exclusively on native milkweed species. Along streams, the white mayfly frequently basks on rocks or bridge structures. They are among the last mayflies to emerge.