Alien Invasion of the Trees

Bug-lover finds her niche as an undergraduate researcher at VCU

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In the worst seasons, you can hear caterpillar droppings pouring from the naked trees.

“The rain of poop,” as entomologist Derek Johnson, Ph.D., describes it, is the most visceral sign of the gypsy moth, the invasive pest that has been Johnson’s specialty for ten years.

The mystery is this: while the moths have spread across much of the United States, destroying forest, they are dwindling on Virginia’s coastal plain east of Richmond. If scientists can understand what is hindering the moths in this area – be it food scarcity, parasites, predators or weather – they may be able to suppress the moth from stripping America’s woods.

It’s a mystery Stephanie Roddy, an enthusiastic undergraduate researcher at Virginia Commonwealth University, would like to solve. Roddy has joined her mentors, Johnson, an assistant professor of population and invasion biology at VCU, and entomologist Karen Kester, Ph.D., an associate professor of biology, in the search for answers.
Through a fellowship from the university’s Undergraduate Research Opportunities Program (UROP), Roddy has been studying gypsy moth pupae at six locations across Virginia. Roddy, together with Johnson, has tracked how the moths are affected by predators, parasites and weather factors, offering answers to the question of what is, and what is not, killing the moths.

Roddy’s efforts represent one of many student research success stories at VCU, which celebrates its Third Annual Research Week from April 19 to April 27. (A complete list of Research Week events can be found here.)

“I have always liked insects,” says Roddy, now a senior biology major in the VCU College of Humanities and Sciences.

“I grew up in the woods. We always had stink bugs and ladybugs and cicadas and (other insects). So I always liked playing with them and looking at them,” she said, before adding, with a smile, “and frightening people with them.”

After being raised in Charlottesville, among mountains, farmland and forests, Roddy brought her rural love of bugs to Richmond, where she got involved in entomology and found her way to the gypsy moth problem.

Gypsy moth trouble
The gypsy moth was brought to Boston by an amateur French entomologist in 1869, and it quickly spread.

“The larvae escaped and, within a few years, neighbors were complaining that there were caterpillars everywhere,” Johnson said.

More than a century later, the moths’ range has spread as far south as North Carolina, north into Canada and west to Wisconsin. Every five to 10 years, the moth population in some parts of the U.S. reaches densities high enough to strip all the leaves off the trees, making forests look like winter in spring.

If the same tree is stripped twice in consecutive years by gypsy moths, it may die. The moth invasion poses a major ecological and economic threat to American woods and forestry, prompting the United States Department of Agriculture (USDA) to fund Johnson’s work looking into the invasion.

Entomologist in the making
Roddy first became involved with insects at VCU when she took BIOL 309, an entomology course that Kester taught.

Roddy was the top student in Kester’s class and was recommended for the UROP fellowship and directed to Johnson’s work by her professor.

Kester, who specializes in parasitic wasps, was interested in the gypsy moth mystery because she believed a particular wasp species, called Brachymeria, might be the culprit. Her hypothesis: that the tiny wasps, which she described as “a cute little thing with fat yellow legs,” may be killing the gypsy moths.

Wasps that parasitize caterpillars of various species, including moths and the tobacco hornworm, are like the outer-space monsters in the movie “Alien”: they lay eggs inside their caterpillar hosts. In fact, the writers of that movie based their aliens on insects like these, called endoparasitoids, and consulted entomologists to learn how they work, Kester said.

The parasitic wasp Brachymeria lays its eggs inside moth pupae, which is the stage between larva and adult. Then the wasp develops inside the caterpillar, feeds on the host insect’s blood and ultimately starves it.

Not quite as dramatic as an alien bursting from an astronaut’s chest – but still amazing.

That’s what Kester thought might be happening to the gypsy moths in eastern Virginia. Other hypotheses, Johnson and Roddy added, were that the gypsy moth pupae were being killed by vertebrate predators, such as rodents, or invertebrate ones, such as ants, or by differences in weather patterns or urban sprawl and forest fragmentation. The project set out to narrow down these options.

In the field
Roddy conducted her study between May and July 2012. She focused on six Virginia sites, ranging from York River State Park in the east, which is on Virginia’s coastal plain, to Washington National Forest in the mountains near Clifton Forge. She used gypsy moths in the pupae stage shipped to VCU by the United States Department of Agriculture (USDA).

Each week, she and Johnson spent Tuesday through Thursday in the field, visiting two sites per day, placing 120 gypsy moth pupae at each site. Half of the moths were exposed to vertebrate predators — glued to burlap pieces so they would not move – and the other half were housed in a cage that would block out large predators, so they were only exposed to insect predators, such as ants. All the moths in theory were exposed to the parasitic wasps, which would be found inside the larvae if they invaded.

The team picked 10 trees at each plot. Each tree was its own experiment, with four separate conditions: caged moths in the tree; caged moths on the ground beneath it; uncaged moths in the tree; uncaged moths on the ground. Each week for five weeks, they would return to the site to see how many moths in each condition were eaten, and put out fresh pupae to be collected the next week.

Mondays were prep days, Fridays were for sorting caterpillars in the lab; Tuesday through Thursday were in the field. They spent every evening before a field day gluing 240 gypsy moth pupae to burlap squares. The process took about two hours. On days when they were working in the Washington National Forest mountains, the site farthest from VCU, they would start their days as early as 6 a.m., and end by 7 p.m.

The work was grueling, but the science was rewarding.

Roddy’s findings surprised both of her entomologist advisers. First, no parasitoids were found in any of the moths, out of approximately 3,000 pupae — a disappointment for Kester’s alien-wasp theory. Second, there was no difference in the number of moths killed by large predators in the west or eastern sites, while markedly fewer moths were killed by insects in the east. In Roddy’s experiment, gypsy moths had higher survival rates on the eastern coastal plain, which was opposite to their original hypothesis that predation would be higher on the coastal plain, causing the decline in wasps there.

What this suggests, Roddy and Johnson explained, is that neither parasitoids nor predators explain the decline of gypsy moths in eastern Virginia.

So what is killing the pests?

It may be that we are: humans, with our strip malls. Johnson’s next hypothesis is that the difference between western Virginia, where the moths thrive, and the east, where they are disappearing, is urban sprawl and agriculture, or “forest fragmentation patterns.” Johnson is now applying for funding from the National Science Foundation and the USDA to investigate this possibility.

Next steps 
What about the bug lover from Charlottesville? 

Roddy, who will present her results at a USDA conference on the gypsy moth and other invasive species, has been selected to participate in the 2013 Undergraduate Research Internship Program at the Northwest Entomological Research Center in Eugene, Ore. As a paid intern, she will participate in a three-month project on the effects of herbicidal suppression on forest arthropod communities in the Cascade Mountain region.

“The UROP provided a meaningful research experience and the opportunity for Stephanie to test her career goal. She plans to gain more field research experience before going on for graduate work,” said Kester.

“I want to do research – what in, I’m not sure yet,” Roddy said. “If I could stay in school my whole life and keep learning things, I’d be happy. So I think research is right for me.” 

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