Caterpillars Provide New Clues on Impact of Warmer Temperatures

November 01, 2012

Superstorm Sandy put climate change back in the national spotlight in late October, but Associate Professor of Biology John Lill needed only look in his backyard to see evidence of climate change. A specialist in the area of plant-insect interactions, Lill saw a species of caterpillars weeks before they were expected, an observation that sparked new questions on how a change within one species’ life cycle affects the other insects and plants that move within its circle.   

“The bulk of research to date on climate change and biological manifestations and effects has mostly looked at individual responses,” said Lill. “Plant species flowering a week early. Birds showing up for migratory pathways two weeks earlier. The independent effects of warmer winters and earlier springs have been documented, have been in vogue, for at least 10 years.”

But Lill said not all organisms are responding in the same way. “Some have speeded up, some are unaffected, some are slowed,” he explained. “Because individual organisms are not all responding the same way, yet they interact with one another, you can get temporal mismatches.”

Those chronological mismatches—and what they portend—are the subject of Lill’s investigation.

“I appreciate how insects can take advantage of good conditions and accelerate their life cycles. I’m fascinated by it,” Lill said. “But it’s deeply alarming to see something that has been going on for millennia shifting so dramatically. It’s definitely worrisome.”

What about evolution, nature’s way of fixing problems? “Normally, evolution is the response,” Lill said. “All organisms are under strong evolutionary pressure. But the problem is the speed with which this is happening—perhaps too fast for them to adapt.”

The Caterpillar Factor

PhD candidate Mariana Abarca shares Lill’s interest in plant-insect interactions, and it was his research that drew her to GW’s Department of Biological Sciences two years ago. Abarca’s research, in conjunction with Lill’s, focuses on the effect of climate change on what many consider a harbinger of spring: Eastern Tent Caterpillars, which lay their eggs on cherry and apple trees.

The tens of thousands of caterpillar species in the United States play an important ecological role, eating tree foliage, affecting tree growth, and providing food for birds and other insects.

“What we’ve been noticing is that the caterpillars have been hatching earlier and earlier,” Lill said. The shift could have a wide-reaching domino affect on the other organisms in the caterpillars’ community, including trees, other insects, songbirds, and even mammals.

Tent caterpillars hatch from their eggs in early spring and use the sun to elevate their temperatures so they can complete development in late spring or early summer. Too much heat, however, has a detrimental effect. When fully grown, the caterpillars spin cocoons, emerge as moths, mate, and lay eggs—which remain on the tree branches until the next spring when the process starts all over again.

“They have to hang out there in the trees during the hot summer,” Abarca explained. “If the temperatures keep rising, I don’t know if they’ll be able to manage with the increasingly hot summers. The mild winters could also be bad, stressing them.”

And if the caterpillars respond out of synch, it’s more than just the caterpillars that are affected.

For example, if climate change sparks larger caterpillar populations, then deforestation could be a problem. Deforestation of fruit trees can affect local agriculture—and all the organisms whose own life cycles are linked to cherry and apple trees. Also, Eastern Tent caterpillar excrement contains cyanide, which is poisonous to some animals. The more caterpillars, the more toxic material.

Abarca is examining databases of temperatures throughout the East Coast to analyze the natural spring conditions that different caterpillar populations experience and to detect the sites where asynchrony between hatching and plant bud bursts is more likely to occur. In tandem, she is doing field work: watching caterpillars, both in controlled settings at GW and in the wild at Maryland’s Seneca Creek State Park, an hour north of Washington, D.C.

If the caterpillars hatch too early, the cherry trees may not have the early foliage the insects needs for nourishment. If they hatch too late, the tree leaves may have turned toxic. “I want to know if they manage to adapt to climate change or not,” Abarca said.

She’s also on the lookout for cues that trigger caterpillar hatching and traits that would allow caterpillars to cope with climate change in the absence of evolutionary response. In particular, she’s interested in whether the caterpillars might be responding to an aroma emitted by cherry tree leaves when trees begin to leaf out. Such a cue could help the insects remain synchronized with their host plants even as temperatures continue to warm.

By the time Abarca completes her PhD, she’ll have five years of data, including information on generations of the caterpillar families—offering short-term information on evolutionary tendencies. “If the parents hatch on time, will they have offspring that hatch on time, too?” Abarca asked. “That’s what I’ll watch.”

The Global Picture

Researchers such as Lill, who are focused on shifting patterns among U.S. plants and insects, work at a disadvantage: The United States has not conducted comprehensive long-term monitoring of biological responses to weather. That means much of Lill’s work is in the field.

“Other countries have good historical datasets. In Britain and Japan they have long-term data that people have been collecting for centuries,” Lill said. “The United States just doesn’t have a tradition of doing that.”

U.S. researchers rely on eclectic sources and plenty of fieldwork. Lill has even referred to author Henry David Thoreau’s botanical observations. “Thoreau noted flowering times of spring wildflowers in New England and by tracking those exact same populations a century or more later, researchers found that flowering times had shifted by as much as three weeks for some species.”  

Thanks to a new initiative, the National Ecological Observatory Network, or NEON, the U.S. government hopes to begin addressing the dearth of records. Work began this summer to develop an open access database offering 30 years of ecological data linked to climate change, land-use change, and invasive species. The virtual catalog of biodiversity is a project of the National Science Foundation, based on work from 62 field data sites across the United States. In addition to site-based sampling and experiments, NEON will collect airborne remote-sensing information that will help measure changes in entire ecosystems. NEON is expected to be fully operational by 2017.

Lill, who long ago thought he might go to medical school, received one of the country’s first graduate degrees in conservation biology from the University of Maryland—a new field in 1992. At the time, climate change wasn’t even discussed.

Now it dominates science conferences and meetings, and more students are pursuing climate change as a focus area for ecological and evolutionary research.

“My two most recent graduate students are both studying climate change,” Lill said. “And the good news is that there’s more funding available—from an array of sources—for this kind of research.”