From oak leaf gall mites to tuliptree aphids, a number of native pests attack trees across Maryland and Pennsylvania. However, because our native trees have evolved alongside these threats, many have developed adaptations that help the trees to survive attacks, or at least minimize the damage caused.
Invasive species, on the other hand, are often much more destructive. Because our trees are largely unequipped to fight off these pests, many invasive pests cause widespread destruction.
The spongy moth is a great example of this. Originally native to Europe and Asia, this innocuous-looking insect is currently wreaking havoc on many of our trees. Fortunately, there are ways to support and protect your trees.
We’ll explain everything you need to know to give your trees a good chance of thriving below.
Spongy Moths: The Basics
Before you can protect your trees from spongy moths, you need to understand a little about their biology, range, and life cycle.
Basic Biology of the Spongy Moth
Known to scientists as Lymantria dispar, the spongy moth is a small, relatively normal-looking moth. Males are generally brown, while females are larger and primarily white or cream-colored. Males fly well, but the females of the North American population do not, despite having functional-looking wings.
But it’s not the adult moth that causes problems for our local trees — it’s the larval caterpillar stage that feeds on tree foliage. The caterpillars typically hatch in April or May and feed heavily through late spring and early summer.
They then pupate, with adults emerging during the summer to mate and deposit egg masses.
The Spongy Moth’s History & Spread
Unlike some other infamous invasives, which arrived on U.S. shores relatively recently, the spongy moth has been here since 1869. In fact, it was deliberately brought to the country by an artist and amateur entomologist. Unfortunately, the moths escaped, leading to our current problems with the pest.
The insects were first documented defoliating trees in the artist’s hometown of Medford, Massachusetts.
By the middle of the 20th century, they’d spread throughout the Northeast, from Maine to Eastern New York. By the 1970s, they were already causing significant forest damage in Pennsylvania, and Maryland experienced its first serious infestation in the 1980s.
Unfortunately, the insects continued to spread and are now established throughout much of the northeastern United States.
Boom-and-Bust: Spongy Moth Outbreaks
Spongy moth populations tend to wax and wane over time, as they respond to a variety of environmental changes. In many years, their populations remain low, leading to relatively little damage. They may feed on tree foliage, but they generally don’t cause significant harm.
However, when conditions are favorable to their survival, their populations reach outbreak proportions. During these seasons, caterpillars may emerge from egg masses in large numbers, leading to significant defoliation on their host trees.
Preferred Hosts: Which Trees Does Spongy Moth Attack?
Oaks are generally the preferred host for these caterpillars, so the following species are at elevated risk:
- White oak (Quercus alba)
- Northern red oak (Quercus rubra)
- Black oak (Quercus velutina)
- Chestnut oak (Quercus montana)
- Scarlet oak (Quercus coccinea)
- Pin oak (Quercus palustris)
- Bear oak (Quercus ilicifolia)
However, they will also feed on other tree species, including:
- Gray birch (Betula populifolia)
- Paper birch (Betula papyrifera)
- Sweet birch (Betula lenta)
- River birch (Betula nigra)
- Quaking aspen (Populus tremuloides)
- Bigtooth aspen (Populus grandidentata)
- Eastern cottonwood (Populus deltoides)
- American basswood (Tilia americana)
- Black willow (Salix nigra)
- Sweetgum (Liquidambar styraciflua)
This list is certainly not exhaustive, as spongy moths will feed on many other trees. In fact, they’ll even feed on some of our local pines and hemlocks during outbreak years.
Protecting Your Trees from Spongy Moth: Prevention & Treatments
Another thing that distinguishes the spongy moth from some of the other invasive tree pests in our area relates to treatment. Unlike many other pests and diseases that can leave you watching helplessly as your tree declines, there are a number of things you can do to help support, protect, and treat trees threatened by spongy moth.
A few of the treatments we use to combat spongy moths include:
- Btk: Btk is an abbreviation for Bacillus thuringiensis var. kurstaki, a biological insecticide that works best against young spongy moth caterpillars while they are still small.
- Spinosad: Spinosad-based insecticides can control feeding caterpillars and may remain effective against somewhat larger larvae than Btk.
- Tebufenozide: This insect growth regulator interferes with the molting process and can provide relatively selective control of spongy moth.
- Egg mass removal: Accessible egg masses can be scraped into a container and destroyed before they hatch to reduce the number of caterpillars in the immediate area.
- Burlap banding: Burlap bands placed around trunks can provide hiding places for larger caterpillars, allowing them to be collected and destroyed regularly.
Signs & Symptoms of Spongy Moth Infestations: Things to Watch For
While there are several ways to protect your trees from spongy moth, it’s important to act quickly — that will give you the best chance of keeping your trees healthy and allowing them to recover. So, monitor your trees regularly and keep an eye out for the following:
- Caterpillars: Hairy caterpillars may be seen feeding on foliage or crawling along trunks and branches.
- Egg masses: Tan or buff-colored, fuzzy-looking egg masses may be attached to bark, branches, or nearby objects.
- Pupae: Dark reddish-brown pupae may be found in bark crevices, beneath loose bark, or in other protected locations.
- Adult moths: Brown male moths or larger, whitish females may be seen on or around affected trees.
- Frass: Caterpillar droppings may accumulate beneath heavily infested trees and can become especially noticeable during outbreaks.
- Chewed leaves: Leaves may develop small holes, ragged edges, or extensive feeding damage as caterpillars consume the foliage.
- Thinning canopy: Heavy feeding can cause the tree’s canopy to become noticeably sparse as the infestation progresses.
- Partial or complete defoliation: Large populations can strip substantial portions of the canopy, sometimes leaving preferred host trees almost completely bare.
- Second flush of foliage: Deciduous trees may produce a new set of leaves after severe early-season defoliation.
- Reduced vigor or dieback: Repeated severe defoliation can reduce growth, weaken trees, and contribute to branch dieback or decline, particularly when other stressors are present.
If you notice any of these signs or symptoms, be sure to contact Baker Tree Service for an onsite evaluation.
Identifying Spongy Moth Caterpillars
There are a number of caterpillars you may see on your trees, so it’s important to know how to identify spongy moth caterpillars. This can be tricky when the caterpillars first hatch, but they are relatively easy to identify as they mature. Just look for caterpillars with hairy bodies and five pairs of blue spots at the anterior end, followed by six pairs of red spots at the posterior end.
Get Your Trees the Support They Need
Spongy moths are certainly a threat to trees across Maryland and Pennsylvania, but there are things you can do to help protect your landscape. The key is securing expert arboricultural help at the first sign of trouble.
If you’re concerned about spongy moths and want to protect your trees, contact Baker Tree Service. We’ll have one of our ISA-certified arborists visit your property, evaluate your trees, and recommend the best approach.
Sources and References
- USDA Animal and Plant Health Inspection Service – “Spongy Moth”
Federal profile of the spongy moth (Lymantria dispar), covering its life cycle, host trees, defoliation impact, and the national Slow the Spread program and quarantine measures.
https://www.aphis.usda.gov/plant-pests-diseases/spongy-moth - Pennsylvania Department of Conservation and Natural Resources – “Spongy Moth”
State forestry resource on spongy moth biology, outbreak history in Pennsylvania, defoliation damage to oak and other hardwoods, and suppression and management options.
https://www.pa.gov/agencies/dcnr/conservation/forests-and-tree/insects-and-diseases/spongy-moth - Maryland Department of Agriculture – “Spongy Moth Program”
State agency overview of Maryland’s spongy moth survey, quarantine, and aerial suppression program to monitor populations and protect susceptible forests.
https://mda.maryland.gov/plants-pests/Pages/spongy_moth_program.aspx - University of Maryland Extension – “Spongy (Gypsy) Moths and Caterpillars on Trees”
Extension guide to identifying spongy moth caterpillars and egg masses, understanding their damage to landscape trees, and managing infestations at the homeowner level.
https://extension.umd.edu/resource/spongy-gypsy-moths-and-caterpillars-trees/