Preparing for High Spongy Moth Densities

After accidental introduction to New England in the late 1800s, Lymantria dispar, or spongy moth (formerly "gypsy moth"), has become a significant insect pest affecting trees and shrubs. The insect's range is predominantly in the Northeast, but isolated infestations and moths have been identified in other areas of the United States, and its current invasion front spans from North Carolina to Minnesota. Here in Pennsylvania, behind the invasion front, L. dispar populations persist at a variety of densities but periodically erupt every 5-10 years. This article contains basic identification and life cycle information and an introduction to a few key management strategies to consider when population densities are high.

About the Insect

Fuzzy, felt-like, tan egg masses laid by females during the prior adult mating season contain hundreds of eggs per mass and are often found attached to tree trunks, in sheltered branch locations, and on many other outdoor structures. Larvae hatch from early April to late May, usually climbing the same tree where their egg mass is located to feed or blowing on silk strands to neighboring stands of trees. Caterpillars feed on foliage and molt several times as they mature. Mature caterpillars are distinctive in appearance, with five pairs of blue spots six pairs of red spots running down the body. In late June or early July, caterpillars pupate, emerging as winged adults a few weeks later. More detailed life cycle information can be found in a related Penn State Extension publication.

Depending on where you are in Pennsylvania, you may confuse L. dispar with a few other insects. Here are a few quick tips for differentiating them:

• Forest tent caterpillar has keyhole-shaped spots down its back unlike the yellow lines and blue and red spots down the length of a mature L. dispar larva’s body
• Eastern tent caterpillar has a solid white line down its back unlike the yellow lines and blue and red spots down the length of a mature L. dispar larva’s body
• Fall webworm caterpillars tend to appear later in summer than L. dispar larvae, and they are lighter and more yellow in color, often congregated in masses of grayish webs.
• Fall cankerworm larvae have a smoother, more “inchworm-like" appearance than the very hairy-looking L. dispar caterpillars.
• Periodical cicadas emerge from the soil as winged nymphs, maturing to flying adults at a time when L. dispar will be in the larval stage.
• Spotted lanternfly egg masses are laid in similar locations, but those masses look grey-brown, like smooth or cracked mud depending on the season, unlike the fuzzy, tan L. dispar egg masses. Nymphs and adults are distinctive and easy to identify.

Plant Hosts and Damage

It is during the 7-week larval life stage in spring and early summer when L. dispar caterpillars feed on the foliage of plant hosts. If you live in the Northeast, you likely associate L. dispar with damage to oak trees. Certainly, oaks are a plentiful and preferred host in this region, suffering significant impacts when L. dispar populations are high. However, larvae feed on a wide range of plant hosts. Other preferred hosts include apple, alder, basswood, hawthorn, poplars, and willows. Less preferred hosts include elm, black gum, hickories, maples, and sassafras. When larval populations are high, feeding is also observed on beech, hemlock, white cedar, pines, and spruce. Rarely are L. dispar larvae observed feeding on ash, balsam fir, butternut, black walnut, catalpa, red cedar, dogwood, holly, locust, sycamore, or tulip poplar.

Defoliation due to caterpillar feeding occurs in spring and early summer, tapering off in July. This defoliation stress is the primary damage mechanism associated with L. dispar pressure. Risk of tree death as a result of this feeding pressure depends on how severely a tree is defoliated (what percentage of leaf cover in the canopy is eaten) and what other stressors have previously or will subsequently affect the tree within a few years of the defoliation event.

Management – Woodlots and Larger Forested Areas

Effective L. dispar control predominantly targets caterpillars (the larval life stage), not the flying moths. Depending on the setting and scale, a variety of options exist.

Option: Insecticides

The most effective active interventions for L. dispar at a large scale and in closed forest canopies involve aerial spray applications of chemical and biological insecticides in high-risk areas. The two control agents commonly used in aerial applications are

1. Bacillus thuringiensis subspecies kurstaki (Btk), a bacterium that kills lepidopteran caterpillars upon ingestion, and
2. tebufenozide, which causes L. dispar caterpillars to prematurely molt and stops them from feeding.

Both control agents only affect lepidopteran species of caterpillars feeding at the time of spraying; they do not impact other species of insects, birds, mammals, or arthropods.

Btk is most effective when applied in spring, usually May, for optimal timing with respect to caterpillar populations and leaf expansion. If you as a landowner or municipal leader are alerted to spongy moth issues after observing defoliation damage in midsummer and begin contemplating aerial spray options, know that at that point, it is unfortunately too late to take action. It may be beneficial instead to monitor damage, survey egg masses, and form a plan for next May if necessary. For more information on organizing a private aerial spray contract, including a list of potential contractors, refer to links on the PA DCNR Division of Forest Health L. dispar information page.

Option: No Intervention, Monitor Natural Controls, Plan Ahead

Suppression programs involving agents like Btk are designed to reduce damage and keep trees from becoming stressed and/or dying until other naturally occurring controls rise to levels that negatively affect L. dispar populations. Observation of the progression of these controls to guide multi-year management approaches can be useful.

Two notable controls exist in the landscape and act as a significant source of L. dispar mortality. Nuclear polyhedrosis virus (NPV) kills caterpillars over a short period of time, causing them to liquefy and disintegrate rapidly. Caterpillars killed by NPV typically hang limply from a tree in an upside down V-shape. Control from NPV particularly occurs when L. dispar densities are high and insects are stressed by high competition. The fungus Entomophaga maimaiga is another key pathogen that kills L. dispar even when L. dispar populations are low, but only if weather conditions are favorable. A cool, wet spring aids the establishment of the fungus. Parasitoids and predation may also play roles in control.

Management – Homeowners

Homeowners have a few more options to take action at smaller scale in which more intensive interventions may be more feasible. It is often a good idea to combine multiple approaches in one season to be more successful. In most cases the practical homeowner mitigation goal is not to achieve complete control but rather to keep defoliation events below 50% loss of foliage.

Option: Chemical or Biological Insecticides

A number of insecticides are registered and labeled for use in controlling L. dispar, but not all are available “over-the-counter" to homeowners. As with any pesticide application, if you are considering this approach, you should read all labels carefully. Look at active ingredients, understand the mode of operation of the active agent (systemic vs. foliar spray, etc.), consider any potential off-target effects, and weigh all pros and cons of any application to make an informed decision about what is best for your property. Properly applied insecticides can be very effective in limiting defoliation damage during heavy infestations, but understand that you will not achieve complete control or shorten the infestation period. As stated in the section above, insecticides are most effective during the prime caterpillar feeding period and before major defoliation occurs, around May; by midsummer when defoliation is most apparent, the window of time to act using the most commonly employed control agents is over.

Option: Tree Banding during Larval Stage

On a small scale (a few trees in a backyard, for example), some homeowners may wish to employ trap/burlap banding or sticky banding to capture caterpillars as they crawl daily up and down the stem. Trap or burlap banding involves a large width of burlap (about 18 inches) encircling the tree at about chest height. Tie the center of the burlap band tightly to the tree and allow the upper portion of the band to hang over the tie. The upper fold provides a shelter where caterpillars rest and where you can physically remove and destroy these caterpillars by smashing them or scraping them into soapy water. Burlap banding requires frequent checking (every few days), and burlap bands should be removed after July. Sticky banding similarly traps caterpillars on an adhesive material wrapped around the stem of a tree. Some people experience skin irritation when handling L. dispar; be careful and wear gloves when managing bands.

Banding techniques have limitations. Persistence is necessary to make an impact, with repeated attention through early July or when male L. dispar moths begin to fly. Even with persistence, some level of defoliation will probably still occur, and banding doesn’t make much of a dent where local L. dispar densities are very high or where you see a high degree of infestation on many nearby trees. In many cases, banding results in a feeling of accomplishment for homeowners who participate in and directly observe the destruction of larvae, but it offers little practical efficacy amid high L. dispar densities.

Option: Target Egg Masses

After L. dispar females lay egg masses, a longer window of time (mid-October through April) is available for homeowners to target this life stage. Egg masses can be destroyed through mechanical scraping, being careful not to damage the tree beneath. Scraped egg masses should be soaked in soapy water for a few days before disposing of them. Simply smashing egg masses will not effectively render the eggs nonviable, and any parts of the egg mass left on the tree after scraping may still contain viable eggs (remember, one egg mass can contain hundreds of eggs).

A horticultural oil that is labeled for L. dispar egg masses, often available at lawn and garden centers, also works to reduce hatch rates the following season. Application with any spray bottle will do (even a water gun you might designate and label solely for this purpose can work for hard-to-reach spots!) as long as you can accurately apply enough solution to saturate the egg mass. Always follow label instructions.

There are obvious access limitations to strategies targeting egg masses; many egg masses are placed out of reach for scraping and horticultural oil applications, particularly where many mature trees are present. Additionally, it is impractical to pursue where egg mass densities are extremely high and distributed over a large area. However, in a backyard setting and in conjunction with other interventions like banding, these more intensive strategies can certainly be effective in mitigating damage.

Management – Strategies to Skip

Attractant pheromones are used mainly in monitoring efforts, not control strategies. Mating disruption techniques using sex attractant pheromones are not often employed by homeowners and managers, and where they are, they tend to be effective only when L. dispar populations are very low.

L. dispar has been established in Pennsylvania for decades, and in that time, your friends, family members, and neighbors may have gotten creative with control strategies. However, Penn State Extension cautions strongly against home remedies for pest control (including using household items such as dish soap, vinegar, salt, boric acid, chili peppers, etc.). Many of these suggestions are untested, have the potential to harm humans, pets, and plants, do not come with precise directions, may not be effective, and their use in off-label activities can sometimes violate federal law. Approved pesticides and other control products have specific labels designating safe use conditions. Labels must always be followed.

Aftermath Monitoring – Secondary Stressors, Staying Calm

Trees that are already under stress are more susceptible to other, additional damaging agents. In these situations, the L. dispar defoliation is the primary stressor and subsequent issues act as secondary stressors. A tree that is at least moderately defoliated (30-50% loss of foliage) by L. dispar is likely to recover but may be at higher risk for negative impacts associated with other insects or pathogens. In particular, trees that experience heavy defoliation (loss of more than half of the foliage) are under higher levels of stress. These trees must expend reserves to re-foliate (produce new leaves) later in the summer. As a single issue, full recovery, even from a complete defoliation, is likely to occur with time. However, survival becomes more uncertain when multiple, successive years of defoliation occur, where defoliation occurs amid other stressors (recent drought, another insect pest in the same year, etc.), or where the effects of existing stressors ramp up in response to the primary stress from defoliation. If this is the first year you have observed L. dispar activity in your forest or yard, keep an eye on your trees. Signs of tree decline in future years may be more pronounced as multiple years of stress compound. After the first year of a newly significant outbreak is a good time to consider what management approaches might be practical on your land next year to minimize multi-year stressors. Note that conifers will not usually survive a complete defoliation, regardless of other contextual stressors.

Heavy defoliations are visually dramatic. June may look like the middle of winter where complete defoliation in a stand of suitable host trees has occurred. It is understandable for forest landowners to feel a rise of panic in this situation as they wonder whether their trees will recover. 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.

Be careful about timber buyers who offer to “salvage" defoliated trees. If warranted at all based on your goals and the forest’s condition, any potential salvage harvest (like any other harvest activity) should be carefully considered with the advice of a consulting forester who understands the ecology of your forest, helps you weigh pros and cons, places a harvest activity in the larger context of the goals you have for your land and the health of your forest, prescribes a sustainable cutting practice, and guides a competitive bid and contract process.