The Spongy Moth Caterpillar Nightmare
I’m guessing I’m not alone in being haunted by the question of whether these caterpillars will plague us again this year. Is there any way to predict?
2 years of heavy defoliation in the Mount Washington Valley
The pitter-patter sound heard everywhere in the Valley last spring wasn’t the rain we needed – it was Spongy Moth caterpillar droppings. It was disgusting. 2022 was the second year of the outbreak, and 50,000 acres were defoliated. Human error is the cause – specifically the accidental release of a non-native insect in Massachusetts in 1869.
North Conway was really hard hit. Here’s an aerial photo.
How do they do so much damage?
They seem to grow bigger right before your eyes as they eat your trees. According to the UNH Cooperative Extension, Carroll County Field Specialist Wendy Scribner, they start out at 1/16th of an inch long and can grow to 3 inches. They can consume an astonishing 11 square feet of foliage per day between May and June. Here’s what they look like up close.
Left: Tim Tigner, Virginia Department of Forestry, Top & Bottom Right: Dode Gladders, UNH
Some affected trees regrow leaves, but in 2022 ours never fully leafed out after being defoliated. They had better regrowth in 2021.
No natural enemies
Since the moths are non-native, no natural enemies evolved with them in North America. Some local breeding birds like Yellow-billed Cuckoos and Black-billed Cuckoos will eat them, but most will not because they cannot digest the large hairy caterpillars. Some wasps, stinkbugs and mammals also eat the caterpillars. A number of biological controls have been introduced which have limited the outbreaks. Unfortunately, none of this is enough to make a significant dent in the population.
How serious is this problem?
The answer lies in what happens over the next two months. Two years of heavy defoliation (which we have had) kills some affected trees. An additional year will have a truly dramatic impact, changing the composition of our forests as the affected trees die off and the trees less favored by the moths remain. The cascading effect of that on numerous species would be enormous.
Predictions about this year
Experts say there are a couple of ways to predict this year’s outbreak. One way is to walk about 500 feet into a wooded area counting fresh egg masses (see picture at left). Do the same on the way back and average the two totals. If your average count is 12, it will likely be a light year. If 50, a heavy year.
I walked, counted and averaged, and found no fresh egg masses, but plenty of old ones. Even if I missed a few, the number would still be less than 12. That is good news.
Another significant factor is whether there were lots of moths in late July last year. Surprisingly, I didn’t see that many moths considering such a heavy caterpillar infestation. So that is also good.
A wet spring, which we are having, also helps reduce their numbers. A fungus introduced as a biological control works more effectively in controlling these caterpillars when there is plenty of moisture. So that’s one positive from our month of rain.
It seems hopeful that we will not have another year of heavy infestation, but only time will tell.
Left: Photo of Spongy Moth Egg Mass by Ryan Hodnett, Wikicommons
What can you do?
If you see these caterpillars when they are very small, you can use an organic spray containing a bacterial pesticide that will kill young caterpillars. It worked for us last year on our newly planted, small trees, but it is not a realistic solution for mature oaks and other hardwoods due to the cost of spraying such a large area directly. We just need to keep our fingers crossed and hope the cycle is coming to an end.