Spongy Moth: Summer 2022 Update

Spongy moth populations rise and fall in periodic cycles in our region, and rapid and significant population increases ("eruptions" or "outbreaks") tend to occur about every 5-10 years. When populations are high, this insect can cause significant damage and losses for many plant species. In both 2021 and 2022, a high population of spongy moth has affected many parts of Pennsylvania, especially central, north-central, and northwestern Pennsylvania. This article addresses a few issues that may be at the forefront of your mind during these midsummer months, when damage has already occurred and control of caterpillars is no longer possible as they advance to a new life stage. Another article, "Preparing for High Spongy Moth Densities," offers more comprehensive information about the insect and what to do when populations are high.

Updating Language

You may know this insect as "gypsy moth;" however, a name change to "spongy moth" was adopted by the scientific community in July 2021 after a recommendation from the Entomological Society of America (ESA)'s "Better Common Names Project." The term "gypsy" in the former common name is a widely acknowledged ethnic slur that is dehumanizing and disrespectful to the Romani people. The new common name "spongy moth" references the sponge-like appearance of the insect's egg masses. Changes to common names and even scientific names have, do, and will happen as science and society advance. It's good to be aware of the former name to access previously published information and verify appropriate use of products whose labels may list the old common name of this insect, but most resources are likely to be updated, including labels on treatment products, going forward.

Defoliation and Risk

Spongy moth caterpillars are defoliators. They eat the leaves of host trees during their larval stage of life, which occurs over about 7 weeks in spring and early summer. When considering the potentially damaging effect of spongy moth feeding, the level, severity, and context of defoliation matters. Conifers that are completely defoliated are likely to die. For healthy hardwoods, in general, a single defoliation alone does not usually directly kill trees. Hardwoods can and do respond after being defoliated, but the defoliation event causes stress to the tree. With enough stress, either from successive defoliations over multiple years or from defoliation stress on top of other stressors (like drought, other insects or pathogens, soil compaction, etc.), trees may die.

At this point of the year, in July and onward, Pennsylvania forests have passed the moment of peak defoliation, and caterpillars are now pupating to mature into adult moths. In other words, the damage is done for this year. The visibility of this damage at the moment may have only recently alerted you to the spongy moth issue for the first time this year, but if that awareness motivates you to take action, know that the most effective strategies are undertaken in spring. The best thing to do now is monitor conditions and plan for next year.

What to Do Now:  Monitor Biocontrols and Trees

Suppression programs like the aerial spray program conducted by the PA Department of Conservation and Natural Resources, which sprayed control agents over 200,000 acres in Pennsylvania on state and federal lands early this spring, or similar spray activities undertaken by private landowners on private lands, are meant to reduce damage and defoliation stress on trees until other natural controls in the landscape begin to reduce spongy moth populations, eventually causing a collapse of the high population cycle. At this point in the year, you can begin looking for signs of these biocontrols in your area.

Surviving caterpillars are now pupating and emerging as adults. However, you may still observe dead caterpillars on or at the base of trees or on other surfaces that have succumbed to natural biocontrols present in the landscape. Nuclear polyhedrosis virus (NPV) kills caterpillars over a short period of time, causing them to liquefy and disintegrate rapidly, often hanging limply from a tree in an upside-down V-shape. Control from NPV particularly occurs when spongy moth densities are high and insects are stressed by high competition. The fungus Entomophaga maimaiga is another key pathogen that kills spongy moth, but only if weather conditions are favorable to development of the fungus in an area, with cool, wet conditions in spring and early summer being ideal.

Now is also a good time to consider the stress environment of your trees. When considering potential impacts on and risk of death for heavily defoliated trees, think about whether defoliation or other stress events occurred on those trees last year. Start paying attention to other stressful issues such as drought that may pop up in the upcoming months and add new stress to the trees. Defoliated trees in a weakened state may be more susceptible to other pest and pathogen issues later in the season or in subsequent years. Two of the most important secondary issues that may cause mortality in an oak tree weakened by spongy moth defoliation are shoestring root rot fungus (Armillara spp.) and the two-lined chestnut borer (Agrilus bilineatus).

What to Do This Winter:  Survey Egg Masses, Prep for Next Year

Spongy moth females begin depositing egg masses on trees in August. By October, you should be able to begin assessing how many egg masses are present in your forest, which will help you predict the likelihood of spongy moth causing high defoliation damage next spring. There are direct correlations between the density of egg masses per acre and the level of damage that is expected in that area. Counting egg masses in a systematic way with an accurate survey will help you predict what the risk will be in your area next year and decide whether or not pursuing a contract for aerial spraying of control agents may be worthwhile. This article and video describe how to conduct an accurate assessment. Because we are no longer in the first year of a high population cycle, subtracting the ratio of old egg masses from your counting activities (described in the linked resource) will be a particularly important step in your egg mass survey. As you consider signs of potential spongy moth population collapse, you should also pay attention to the sizes of egg masses; smaller egg masses (the size of a dime, for example) may be a sign of a stressed population of insects.

If you do decide to pursue aerial spray control measures in spring (typically involving one of two control agents:  Btk (Bacillus thuringiensis subsp. kurstaki) or tebufenozide, known by the brand name "Mimic®"), you should be proactive about setting up a contract with an aerial spray applicator to have everything in place for a timely control activity in May. The DCNR webpage on spongy moth information includes a guide to conducting aerial spray activities as well as a list of aerial applicators licensed to work in Pennsylvania.

Aerial spray controls may not be practical for landowners with small parcels of land or individual yard trees of concern; those individuals might seek the advice of an arborist for smaller-scale control options or consider single-stem mitigation methods like tree banding.

Take a Long-Term View

Heavy defoliations are visually dramatic, and it is understandable to feel very concerned when looking at a midsummer oak forest resembling midwinter conditions. Spongy moth damage can lead to significant mortality and loss of trees, but you should try to remain patient and calm. Watch your trees and see how they recover. Trees may put on new leaves later in the season or show signs of life next spring. Remember that spongy moth populations regularly rise and fall in cycles in our region. The place of control methods is to lower the intensity of stress caused by this insect until the high population cycle collapses; if these methods are warranted and available to you, pursue them confidently. If not, monitor conditions and take a long-term view. Stress events are frequent and complex in our forests, and even the worst-case scenarios of significant tree mortality should be contextualized as events in a dynamic and changing system, requiring us as landowners and managers to offer in return our engaged attention, responsive decisions, and active management.