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Creatures and Critters

Moth Mayhem

Narrator: These trees are victims of the gypsy moth caterpillar. In the worst years, this voracious insect has defoliated tens of thousands of acres of trees in this popular Virginia national park. The gypsy moth has also chomped its way through forests in Maryland and other nearby states, since it first spread from New England into the Mid-Atlantic in the early 1970’s. Forest managers have long been concerned about the gypsy moths’ impact on forests, because defoliation can eventually kill valuable trees and change forest habitat.

Narrator: But Eshleman has recently made a surprising discovery. He is leading a team of researchers from the University of Maryland’s Appalachian Lab, a part of the Center for Environmental Science, and from the University of Virginia. They have found that gypsy moth activity is also degrading the streams that flow through these defoliated watersheds, polluting waters that ultimately end up in the Chesapeake Bay.

Eshleman: In the years following a insect defoliation by something like the gypsy moth caterpillar, what we found is that nitrogen concentrations in stream water rise very dramatically. Over the course of a particular year, the actual amount of nitrogen that is discharged from one of these watersheds may be fifty to as much as a hundred times higher than the amount of nitrogen that would have been discharged in the absence the disturbance.

Narrator: Well before Eshleman’s discovery, scientists were concerned about rising levels of nitrogen in the streams and rivers flowing into the Chesapeake Bay. Most of the nitrogen comes from traditional sources—fertilizer applied to farmlands or airborne pollutants formed during the burning of fossil fuels.

Don Boesch: The increase in nitrogen coming down into the Bay has been now shown to be one of the most serious problems affecting the Bay overall. What this has done, of course, is to cause excessive growths of algae in the water column, which has reduced the clarity of the water, caused the submerged aquatic vegetation to disappear, and depleted the oxygen down in the bottom of the Bay, all with undesirable effects.

Narrator: But how would moth-eaten oaks leak nitrogen into waterways? Eshleman hypothesizes that the problem stems from the caterpillar droppings. He thinks that as the caterpillars devour tree leaves, and deposit their droppings, they dump the equivalent of a huge load of nitrogen-rich manure on the forest floor.

Eshleman: One idea is that the droppings that occur during these periods of defoliation can actually be considered as fertilizer. That is, they’re very high in things like nitrogen and carbon.

Narrator: But because the trees have lost their leaves, they’re unable to reabsorb the nutrients and use them for growth, the way a healthy forest would.

 

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