By TODD McLEISH/ecoRI News contributor
It’s almost gypsy moth caterpillar season again, a time of tree defoliation, a variety of other environmental impacts, and caterpillar droppings raining down upon us.
And now comes the news that last year’s infestation may have also affected water quality in the region and will likely do so again.
Gypsy moth caterpillars — along with winter moth caterpillars and forest tent caterpillars, but mostly gypsy moths — defoliated about 230,000 acres in Rhode Island last year, according to University of Rhode Island entomologist Heather Faubert, making it the worst defoliation since at least the early 1980s. More than half of the state’s 400,000 forested acres were impacted.
The defoliation also allowed sunlight into areas usually shaded by forest canopy, which local ecologists said allowed sun-loving invasive plants to spread into the forest, denied native birds and small mammals protection from predators, and made it difficult for frogs and salamanders living on the forest floor to remain cool and moist.
Coupled with last year’s drought, it also resulted in what botanist Keith Killingbeck called “a muted display” of fall foliage.
The water-quality implications from the caterpillars, reported last month by URI researcher Kelly Addy at a research conference at Brown University, were a coincidental result of a comparative study of how rainstorms affect stream-water quality in forested, urban and agricultural watersheds.
Addy said sensors in Cork Brook in North Scituate picked up a “signature” of gypsy moths that lasted for many months.
“When you lose canopy cover, you have more sunlight hitting the streams, which warms up the water, and warm water cannot hold as much oxygen, so dissolved oxygen levels go down,” she said.
Addy also noted that dissolved oxygen levels were further suppressed when large quantities of additional carbon — from caterpillar excrement, the caterpillars themselves and leaf fragments — dropped into the water from above.
“All that carbon fuels the organisms living in the water, causing them to flourish,” she said. “Suddenly, you have more biomass of life in the streams, which sounds good, but they are then consuming more oxygen, and dissolved oxygen levels decline even more.”
In Cork Brook, dissolved oxygen was measured at 8 milligrams per liter in summer 2014 and 2015, but just 5 milligrams per liter last summer.
“At that level, you can start getting oxygen distress in sensitive species,” Addy said.
The low levels of dissolved oxygen in Cork Brook remained through at least last fall, when the sensors were removed.
“If gypsy moths are not a big issue this spring, then the water will likely recover,” she said. “But if it happens repeatedly, then the streams won’t bounce back as easily, and each spring it may remain low.”
Unfortunately, gypsy moths are poised for another big year, with one caveat. “How bad it will be will depend somewhat on the weather,” Faubert said.
In years when it’s rainy in May, the moisture abets several fungal diseases that get passed back and forth between gypsy moth caterpillars, causing the population to crash.
“But even if almost all of our gypsy moth caterpillars die off from the diseases, they don’t die until they’re already large caterpillars, so they will have already eaten a lot of leaves,” she said. “So we’re in for a lot of gypsy moth damage, regardless of the weather.”
That means the likelihood of many more dead trees, since the botany rule of thumb suggests that three consecutive years of defoliation will usually kill most trees. And even one year of defoliation of spruce or hemlock trees can kill them, Faubert said.
The only good news is that Faubert found fewer winter moth eggs this spring than in the past two years, so winter moth caterpillars, which typically hatch in early to mid-April and feed on leaves and tree blossoms for about a month, may have a lesser impact on local trees this year than previously expected.
Rhode Island resident and author Todd McLeish runs a wildlife blog.