What is spongy moth?
Spongy moth, Lymantria dispar, is the insect long known as the gypsy moth. It was renamed in 2022. It has a close tie to this part of the state: it was brought to Medford, Massachusetts, in 1869 by an amateur scientist hoping to breed it for silk, escaped, and has spread across the eastern United States since.
The moth itself doesn’t feed. The damage comes from the caterpillars, which eat the leaves of hundreds of tree species. Oaks are their favorite, and Andover has a lot of oaks: red and white oaks in the woods of South Andover and Holt Hill, along the wooded lots of West Andover, and as big shade trees in older neighborhoods.
How does the yearly cycle work?
| Season | What’s happening | What you can do |
|---|---|---|
| Fall to early spring | Egg masses sit on bark, stone walls, firewood and outdoor furniture | Find and destroy egg masses |
| Late April to May | Caterpillars hatch and climb into the crown | Treatment works best now, while they’re small |
| May to late June | Caterpillars feed heavily; larger ones feed at night | Burlap bands, monitoring |
| Late June to early July | Caterpillars pupate in sheltered spots | Collect and destroy pupae |
| July | Moths emerge and females lay eggs | Note where egg masses are laid |
The egg masses are tan or buff, fuzzy and roughly the size of a quarter to a half-dollar, each holding hundreds of eggs. Young caterpillars are small and dark. Older ones grow to two inches or more and are easy to recognize: hairy, with five pairs of blue dots followed by six pairs of red dots along the back.
Why do outbreaks come and go?
Spongy moth populations stay low for years, then boom. The last major outbreak in Massachusetts peaked around 2016 and 2017, when a state aerial survey recorded about 923,000 acres of defoliation in 2017. Dry springs helped it along.
Two natural diseases usually end an outbreak. A fungus, Entomophaga maimaiga, spreads among caterpillars in wet springs, and a virus kills them when populations get crowded. Dead caterpillars hanging head-down from bark, or stiff ones stuck to the trunk, are a sign that one of these is at work. Because of these natural controls, the state does not run spraying programs for spongy moth, so protecting yard trees is up to property owners.
Will my oak survive being stripped?
Usually, yes, if it’s healthy. A deciduous tree stripped in June typically pushes out a second set of leaves by midsummer, using energy it stored in its roots and trunk. Most healthy oaks survive a single complete defoliation.
The risk climbs when defoliation repeats or stacks up with other stress:
- Two or more years of defoliation in a row drain a tree’s reserves.
- Drought in the same summer leaves it unable to rebuild.
- Secondary pests such as the two-lined chestnut borer and Armillaria root rot attack oaks that have been weakened, and are often what actually kills them a year or two after the caterpillars are gone.
- Evergreens are more vulnerable. White pine and hemlock can be killed by a single heavy defoliation, because they don’t put out a second set of needles the same year.
Oaks weakened by spongy moth also need the same protection from oak wilt as any other oak: no pruning or wounding from April through July.
What can homeowners do?
Destroy egg masses
From fall through early April, check trunks, the undersides of limbs, stone walls, woodpiles, swing sets and the underside of outdoor furniture. Scrape egg masses into a container of soapy water and leave them for a couple of days before throwing them out. Scraping them onto the ground doesn’t kill the eggs. Removing egg masses won’t stop an outbreak in the woods next door, but it can noticeably reduce the number of caterpillars on a single yard tree.
Use burlap bands
In late May, wrap a strip of burlap around the trunk at chest height, tie it in the middle and fold the top half down to make a flap. Older caterpillars hide under it during the day. Check it every afternoon and drop the caterpillars into soapy water. Avoid sticky tape directly on bark, which can trap birds and other wildlife and damage thin bark.
Treat when it counts
For valuable trees in an outbreak year, treatment protects the leaves. The usual option is a spray of Bacillus thuringiensis kurstaki (Btk), a bacterium that affects caterpillars that eat treated leaves. It works best on young caterpillars in May, and it has to reach the whole crown, which on a big oak means a professional sprayer. Trunk-injected treatments are another option for individual high-value trees. In Massachusetts, pesticide application for hire must be done by a licensed applicator.
Watch your skinThe caterpillars’ hairs cause an itchy rash in many people. Wear gloves and long sleeves when handling bands or scraping egg masses, and don’t let children handle the caterpillars.
How do you help a tree after defoliation?
The weeks after the caterpillars stop feeding matter as much as the treatment.
- Water deeply during dry spells that summer and the next. A slow soak over the root zone once a week does more than frequent light watering.
- Spread mulch two to three inches deep over as much of the root zone as practical, kept off the trunk.
- Hold off on heavy nitrogen fertilizer, which pushes leafy growth the tree can’t support. A soil test tells you whether anything is actually needed.
- Prune only deadwood, and only in the dormant season. Oaks should be pruned between November and March. Our guide to the best time to prune trees in Massachusetts explains the timing for each tree type.
- Watch for dieback the following year. Dead branch tips at the top of the crown, thin leaves or holes in the bark can point to borers or root rot.
Our plant health care program covers watering plans, soil testing and monitoring for exactly this kind of stress.
When is a stripped tree a hazard?
Most defoliated oaks recover. But an oak that fails to leaf out the following spring, or has large dead limbs over a house, driveway or play area, should be looked at. Dead oak wood is heavy and becomes brittle as it dries. If a tree has died or lost much of its crown, tree removal on your schedule is safer than waiting for a storm to bring it down.
To learn which oaks and other trees you have, see our guide to common trees of Andover. If your hemlocks look thin as well, our guide to hemlock woolly adelgid explains what to check.
Call our arborists to check whether the tree is recovering and whether treatment next spring makes sense.
Spongy moth FAQs
Is spongy moth the same as gypsy moth?
Yes. The common name was changed to spongy moth in 2022. The insect is the same.
When should I scrape spongy moth egg masses?
Any time from fall through early April, before the eggs hatch. Scrape them into soapy water rather than onto the ground.
Will my oak die after spongy moth eats all the leaves?
A healthy oak usually survives one defoliation and grows new leaves by midsummer. Repeated defoliation, drought and secondary pests are what kill trees.
When is the best time to treat for spongy moth?
In May, while the caterpillars are small. Treatments are much less effective on large caterpillars in June.
Does Massachusetts spray for spongy moth?
No. The state relies on natural controls, so protecting individual trees is up to property owners.
