The orangestriped oakworm (Anisota senatoria) is a commonly encountered late-season defoliator. As with many of its fellow species in the genus Anisota, larvae are gregarious and feed in clusters, causing localized, heavy defoliation within an otherwise full canopy. While their damage may be unsightly, many of these common caterpillar species that emerge in late summer to munch on tree leaves are generally not a significant threat to tree health. A new guide in the Journal of Integrated Pest Management takes a closer look at these late-season defoliators and offers tips for management. (Photo by Erich G. Vallery, USDA Forest Service – SRS-4552, Bugwood.org)

By Kelly Oten, Ph.D.

Kelly Oten, Ph.D.

This time of year in the southern United States, late-season hardwood defoliators can attract a lot of attention for their conspicuous damage. But, how harmful are they? Usually, a single defoliation event doesn’t cause long-term health impacts to a tree. This is particularly true for defoliators feeding in the late summer and early fall. In a paper published earlier this year in the Journal of Integrated Pest Management, two colleagues and I review the distribution, biology and management of four of the Southeast’s most common late-season defoliator genera: Anisota, Dryocampa, Datana, and Lochmaeus.

In contrast with more destructive tree pests such as wood-boring beetles, which feed within trees, defoliation occurs outside trees and is therefore more detectable by the most casual observer. Moreover, defoliation is common on highly visible trees in yards, parks, or urban areas—trees with which the public interfaces often and where there is a lower threshold for loss of aesthetic value.

In addition to their damage, several late-season defoliating insects are visually conspicuous. For example, Anisota, which contains the commonly encountered orangestriped oakworm (A. senatoria), are gregarious as larvae. They feed in clusters, causing localized, heavy defoliation within an otherwise full canopy. Further, high caterpillar populations result in high levels of waste. In non-forested settings, frass can accumulate on sidewalks, patios, and driveways. As an extension specialist in forest health, I have had calls about “small black stuff falling from my tree” before callers notice the caterpillars causing it. In forests, dropping frass may sound like raindrops falling, as was the case on a clear day during the 2017 outbreak of Lochmaeus manteo in North Carolina, as observed by co-author Robert Jetton, Ph.D. Of course, let’s not forget the eye-catching web-making of the fall webworm, reviewed in a 2021 paper in the Journal of Integrated Pest Management.

Anisota caterpillars are highly visible due to gregarious feeding and accumulation of frass underneath trees with high caterpillar populations. (Photos A-D by David Coyle, Ph.D.; photo E by Kelly Oten, Ph.D., originally published in Oten et al 2023, Journal of Integrated Pest Management).

Despite this, the impact of late-season defoliation to tree health is minor; therefore, management is seldom necessary. Natural enemies regulate pest populations and, even during high populations, impact to tree health is low. However, management may be warranted in some cases due to impact on aesthetics of high-value trees or following consecutive years of defoliation.

Mechanical control can be of great use and offers the least impact to natural enemies that keep pest populations low. Small populations of gregarious Anisota larvae can be picked off or the entire branch removed and destroyed. Similarly, Dryocampa eggs can be removed from the undersides of leaves.

In more severe cases or when pests cannot be reached for removal, insecticides can offer protection, as well. Foliar applications of Bacillus thuringiensis or use of systemic or contact insecticides can be useful for many lepidopterans, but applications should be timed and targeted appropriately for maximum efficacy. Typically, applications should occur when larvae are small and should limit impacts to non-target species. In some cases, applications can be directed. For example, Anisota larvae are most common in the lower third of the canopy, so monitoring and management should focus in that area.

So, while late-season hardwood defoliators might look bad, their damage is usually much ado about nothing.

Kelly Oten, Ph.D., is an assistant professor and extension specialist in forest health at North Carolina State University in Raleigh, North Carolina. Email: kelly_oten@ncsu.edu.