Many Vermonters around the state are encountering spongy moth (Lymantria dispar dispar) caterpillars causing defoliation of their trees. This invasive insect arrived in the United States over 100 years ago and has been expanding its range ever since. Spongy moth can be a significant defoliator (leaf eater) of trees and shrubs, and although they prefer oak trees, high populations will cause them to eat many types of leaves, including maple and pine. Spongy moth caterpillars can create a nuisance for homeowners, from the sights of caterpillars climbing the sides of residences and falling frass to the sounds of chewing on leaves.
Vermont has not seen a large outbreak of spongy moth since 1991, although it has been present at low levels for decades. At the time of the last major outbreak, a fungus called Entomophaga maimaiga became prevalent in the area and significantly decreased the spongy moth moth population. This fungal pathogen is most abundant after periods of wet weather. Due to the recent droughts and dry weather we have experienced over the last few years, populations of this fungus have been limited and allowed the spongy moth numbers to increase.
Although spongy moth caterpillars are damaging, otherwise healthy trees can often survive a few years of successive defoliation. Defoliation and drought conditions can combine to stress tree health and vigor. Vermont may see another year or two of high levels of spongy moth unless the state sees some rainy seasons to increase the population of the spongy moth-killing fungus.
Based on the current stage of the outbreak, chemical controls are not recommended at this time. However, there are practical, non-chemical steps homeowners can take to limit damage and decrease next year’s population:
- Keep your plants well-watered throughout the summer to reduce stress.
- If there are only a few trees, caterpillars can be squished or pruned out of the trees and then submerged in soapy water.
- Egg masses can be removed from trees between August and May to reduce infestations in forthcoming years. Use a scraper to carefully remove the masses and submerge them into a container with soapy water or alcohol so that they can be destroyed (not onto the ground where they may still hatch).
- A band of burlap can be installed around the trunk of the tree this year and monitored and used as a trap for caterpillars. It can also encourage egg masses to be laid on the burlap. After the eggs are laid this fall, you can remove the burlap and destroy the egg masses. This can help reduce populations next year. Start by wrapping an 18-inch wide strip around the tree at chest height. Tie a string around the center and fold the top portion down to form a skirt, with the string acting as a belt. Pick off the caterpillars daily and dispose of them by submerging them into a container with soapy water or alcohol.
- In most cases, it is too late to use insecticides for control of spongy moth. As the caterpillars develop and become larger, insecticides are less effective at reducing populations. In future years, however, early-season treatment can be effective. The most commonly recommended products contain the bacteria, Bacillus thuringiensis kurstaki (Btk). Btk is applied to foliage where spongy moth larvae will consume it and are then killed. This strain of bacteria is specific to moth larvae, and its toxic properties are activated when it interacts with particular enzymes in the caterpillar’s digestive tract. Btk is most effective when applied between May and early June, when spongy moths are still in the larval stage and are are small (1/2 to 3/4 inch). Two spray applications are usually required for effective control. Application of Btk after mid-June is not an effective way to manage this pest.
For more information on spongy moth identification and mitigation steps, click here.