Surveying Lymantria dispar Egg Masses – Why Bother?

In spring of 2020, nightly news shows “buzzed” about the arrival of the Asian giant “murder” hornet (Vespa mandarinia). Then, again, in spring and summer of 2021, news abounded regarding the Brood X periodical cicadas (Magicicada spp.). But in the world of Pennsylvania forest health, this year’s spotlight – and for years to come – is shining on Lymantria dispar (“gypsy moth”)*.

L. dispar is an invasive insect know for impacting forests via its feeding pattern: it eats the leaves of trees and shrubs, thus defoliating them. In 2021, high populations of L. dispar were observed across various regions of the state. Knowing this, natural resource professionals and landowners are preparing for a cycle of infestation and for monitoring and mitigation where necessary. Key to these efforts is an assessment of the current conditions of a given forest as well as the expected upcoming population densities in that forest, both of which can be achieved via egg mass surveys conducted during the early winter months.

Naturally, questions of what and why may come to the surface here: why does it matter that we know where and how large the next population of moths is going to be and what does current forest condition have to do with it? To answer this, we must consider the nature of L. dispar outbreaks and what that means for our forests. L. dispar is present in Pennsylvania each year – every spring, a population hatches from egg masses, feeds on leaves as a caterpillar, pupates, and emerges as adults to mate, lay eggs, and die. Concern for forest health arises when, from one year to the next, the size of that population significantly increases relative to “normal.” In Pennsylvania, L. dispar populations go through cycles of low and high density, wherein outbreaks occur every 5-10 years. These outbreaks consist of several years of high population densities, before declining again, and the cycle starts over. During these concurrent years of high L. dispar density, trees are repeatedly defoliated. This recurring stress can lead to various forest health concerns, like decline in tree vigor, decline in seed or acorn production, increased vulnerability to other pests and diseases, crown dieback and epicormic sprouting from branches or trunks, and potential for mortality. Consequently, timber volume, value, and wildlife habitat capacity are often reduced. These impacts are magnified in forests that are already experiencing stress, like those that are heavily browsed by deer or have poor soil quality or are simply old and in decline. Studies in New Hampshire found that stressed forest stands that have experienced a typical outbreak cycle – wherein annual growth declines each year of the outbreak – can take up to 7-8 years longer to recover to “normal” growth rates than healthy, resilient forest stands. Essentially, high densities of L. dispar place our forests at risk for significant damage and can hamper forest health and productivity for years to come, particularly those forests already under stress from other factors. So why does counting L. dispar egg masses and observing current forest conditions matter? Because if we care about the health and productivity of a resource that sustains us (a.k.a. forests), then we need to understand how intense the impact of L. dispar will be such that we can assess that risk and make decisions about proper mitigation strategies. Beyond that, the most effective mitigation for large scale, closed canopy forests is the aerial spraying of insecticides, which, in terms of costs, may not be worthwhile where population densities/potential impact are low; therefore, knowing how many L. dispar are likely to impact your land and surrounding area and assessing how well your forest could sustain such an impact is vital to determining the efficacy of such a larger scale activity. 

Recently, Penn State Extension developed a Guide to Gypsy Moth Egg Mass Surveying, utilizing protocols followed by the PA DCNR Bureau of Forestry Division of Forest Health. This guide employs a step-by-step process for counting and calculating egg mass density within a forest or parcel and provides guidance for decision making. This process involves taking a number of sample plots, dependent on the size of your property. These plots are circular, with a fixed radius, and all trees (and other objects, like rocks or fallen limbs) that fall within that radius will be surveyed for egg masses. Calculations are then necessary to estimate how many egg masses are present per acre and to account for how many masses are from prior years. With a final average of egg masses per acre, you can compare your count to thresholds of egg masses/acre developed by natural resource managers to help determine the necessity of aerial spray applications. For more details regarding this survey process and to plan for taking your own survey, check out the comprehensive guide and video.

As you are conducting your L. dispar survey, take notes about the composition and condition of the trees in your plot and make some observations of your forest at large. Is there a large component of oak species in your plots or woods? Oaks are a preferred host of L. dispar (among many favorable hardwoods and conifer hosts); therefore, the more oak present on the property, the greater the anticipated defoliation and damage. Do the tree crowns seem to be in good quality or are they thin, with large areas of dead branches or patchiness in foliage (note: this is harder to determine when leaves have fallen)? Have there been other kinds of stress that have impacted your forest’s structure and function, like droughts or flooding, disease or other insect infestations, or decreased regeneration potential due to invasive plants or deer browse? The greater the prevalence of current stressors, the higher the risk of damage to the forest by L. dispar. When was the last time the forest was harvested and what kind of harvest was conducted? What is the species, form, size, and quality of the trees that were left behind? The more recent and intense a timber harvest was, the greater the stress the forest is currently under, and the more susceptible it is to damage.

Remember that as you assess your forest and prepare to make decisions, there are natural resource professionals prepared to help. Contact your PA DCNR County Service Forester and share your egg mass counts with them or invite them to evaluate some of your plots with you. Chat with Penn State Extension Forestry and Wildlife Educators about surveying your property or any questions you may have regarding L. dispar. If you are interested in learning about aerial spray plans and contractors, consult the PA DCNR Bureau of Forestry Division of Forest Health L. dispar page and begin to make plans now – spraying can only be conducted at a critical point in L. dispar’s life cycle occurring in the spring.

As always, we at the Center for Private Forests at Penn State are also available to help answer questions, get you connected with resources, and encourage you in the stewardship of forests. And remember that just because L. dispar will remain in the spotlight for a while longer, doesn’t mean that its impact needs to be so significant as to be the lead story on the nightly news – act now to prepare for the care of your forest.

*The historically used common name for this insect is "gypsy moth." This release utilizes the insect's scientific name L. dispar, reflective of the recent discussions to rename the insect.

The Pennsylvania Forest Stewardship Program provides publications on a variety of topics related to woodland management. For a list of publications, call 800-235-9473 (toll free), send an email to PrivateForests@psu.edu, or write to Forest Stewardship Program, 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. 

Written by Abby Jamison, Forest Stewardship Program Associate, Center for Private Forests at Penn State