It’s almost gypsy moth caterpillar
season again, a time of tree defoliation, caterpillar droppings raining down
upon us, and a variety of other environmental impacts. 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, who coordinates the Plant Protection Clinic, making it the worst defoliation
since
at least the early 1980s. More
than half of the state’s 400,000 forested acres were affected.
The defoliation also allowed sunlight into areas
usually shaded by the 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 that 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 explained.
Addy said that dissolved oxygen levels were
further suppressed when large quantities of additional carbon – from
caterpillar excrement, the caterpillars themselves, and fragments of leaves –
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 the summer of 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,” said Faubert.
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.
This article first appeared on EcoRI.org on May 4, 2017.
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