Spongy moths (Lymantria dispar) were accidentally introduced into Massachusetts from Europe in the 1860s. Since then, they have been found in at least 20 states. They were first found in Wisconsin in the late 1960s in the eastern part of the state and have now spread across to every county.
Each summer during the months of June and July, spongy moth caterpillars defoliate thousands of acres of trees in both forests and urban areas. They feed on the leaves of more than 300 species of deciduous and evergreen trees, with their favorites being oak, aspen, birch, crabapple, willow, tamarack, and basswood. Spongy moth damage will not necessarily kill your tree, but heavily defoliated trees (more than 50% of the leaf area) are weakened and at an increased risk of decline and mortality by other insects and diseases. Additionally, the spongy moth caterpillars themselves can be a nuisance to humans, as their hairs can cause skin rashes and irritation.
Why are there so many spongy moths around in 2023?
In 2023, we are seeing the fourth year in a row of a significant outbreak of spongy moths. We have been experiencing hot and dry conditions this spring and summer, which are perfect for the spongy moth to thrive due to a lack of one of their natural enemies, Entomophaga maimaiga.
Mature trees can still rebound from spongy moth defoliation and replace the leaves lost to caterpillars. However, trees have to use a lot of energy to produce new leaves, and with ongoing drought conditions, they do not have much energy to spare. One thing you can do to help your tree endure damage by spongy moth caterpillars is to reduce its stress by keeping it well watered and postponing any pruning.
Your Management Options:
Depending on what time of year it is, the spongy moth is in different life stages and the best management approach will change. Life stages are shown in blue and management approaches are shown in yellow. Some of these options are only feasible for trained professionals. (UW-Extension)
This is what the spongy moth looks like in each of its life stages. It is the most harmful to trees and humans in the large caterpillar (larva) stage. This occurs in June-July and is when they feed on the leaves of trees! (UW-Extension)
The most effective methods to control the moth population are destroying egg masses. This can be done from August through April. Masses look like fuzzy, brownish patches and can be found on tree trunks, on the underside of branches, under picnic tables, along sheds, and many other flat surfaces. They can be scraped off surfaces with a putty knife and dropped into a container of soapy water for several days to kill them. Alternatively, egg masses can be sprayed with a horticultural oil such as Golden Pest Spray Oil that suffocates and kills the eggs.
For more detailed information on other management options of spongy moth such as barrier bands, collection bands, and insecticides:
The trees in Greenfield Park are suffering from spongy moth damage worse than any other area in the City of Fitchburg. Therefore, we are focusing our management efforts there. This fall, we will be spraying egg masses with a horticultural oil to destroy them and limit the population of spongy moths that will return next spring.
Pictures of spongy moth in Greenfield Park as of late July 2023. As you can see, the moths like to lay their egg masses on the underside of branches where they will be protected, as well as on the sides of buildings. By early August, the moths seen in the pictures will die, leaving just the egg masses. This is when we will start to spray them.
If you are unfamiliar with the common name of spongy moth for Lymantria dispar, that is because it replaced the prior name of this insect, gypsy moth, in 2022 by the Entomological Society of America as part of their Better Common Names Project.