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Rising temperatures may cause insects to eat more plants, Harvard study says

Part A of the photo shows damage made outside before collection, and part B shows damage made inside after collection.
Harvard University Herbarium
This shagbark hickory leaf from Harvard’s collections shows damage from a bug before it was collected (Inset a). Other damage shown occurred after the plant was collected.

Insects may be chomping on plants more because of climate change, according to new research from Harvard that took the innovative approach of examining more than a century’s worth of pressed plant specimens to look for telltale signs of insect damage.

Researchers examined four different plants that were collected in New England and found that specimens collected in the early 2000s were 23 percent more likely to be damaged by hungry insects than those collected in the late 1800s.

The increased damage was a response to warmer winters and the earlier arrival of spring temperatures, researchers said in the study, which was published in September in the journal Ecology.

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“The overwhelming pattern is that across these four different plant species, with different life histories, insect damage is increasing over time,” said Emily Meineke, lead author of the study. “In New England, it appears that warming in winter is an important factor driving insect herbivory [plant-eating] damage overall.”

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Meineke is a postdoctoral researcher in Harvard’s department of organismic and evolutionary biology. Researchers from the University of Vermont and the University of British Columbia also worked on the study.

In the Northeast, average temperatures have risen 1.44 degrees Fahrenheit from the early to late 20th century, while winter temperatures — a key determinant of insect survival — have increased by 1.62 degrees Fahrenheit, more than any other season, researchers said.

The study raises the prospect of further insect damage to plants, as average temperatures in the Northeast are projected to rise more than 3.6 degrees Fahrenheit above pre-industrial levels by 2040, researchers said.

Researchers examined specimens of shagbark hickory, swamp white oak, showy tick trefoil, and wild lowbush blueberry that were collected and pressed in the Harvard University Herbaria, or plant collections, from 1896 to 2008.

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One challenge researchers faced was that some of the specimens were damaged by insects even after they were collected, while they were in the collections. But Meineke said it was possible to distinguish damage inflicted before the collection of specimens and after.

Researchers sought to quantify insect damage by laying grids over the leaves, randomly selecting five grid cells, and recording whether the leaves were eaten in those cells, Meineke said in a statement.

The researchers noted in the study that the trend of increased insect damage to plants might not hold in the cities because “urbanization may disrupt local effects of winter warming on herbivory by excluding certain herbivores.”

But they said, “To the extent that patterns across the past century predict the future, we may expect herbivory in nonurban areas to continue increasing with warming winters in the northeastern US. This ecological trend could increase damage to plants that are of ecological, economic, and/or cultural importance.”

“Our results contribute to a growing body of research demonstrating how climate change and urbanization might disrupt species interactions,” the study said.

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Curtis Deutsch, a professor of oceanography at the University of Washington, said he was intrigued by Meineke’s approach — and it was consistent with his own research.

Deutsch grew different plants in a closed, temperature-controlled chamber and placed insects inside. He found that the warmer it was, the more insect metabolic rates increased, causing them to expend more energy and require more food. Their reproductive rates also increased.

“When you warm things up, you get a double whammy,” Deutsch said Tuesday in a telephone interview.

“All these different methods, including Emily’s, point to the same result,” he added, “it’s quite powerful.”

Meineke’s study suggested that crop and forest plants in the Northeast need to be frequently monitored in the future as the climate continues to warm.

“Knowing that insect damage on these plants is increasing is useful because we might be able to come up with management strategies before it reaches economic levels,” Meineke said. “I think this study is the tip of the iceberg. Now that we know these plants have more damage than they did 100 years ago, we can try to understand what that actually means for plants.”

While the study served to highlight the consequences of global warming, Meineke said it also highlighted the value of herbaria and how they could be used to provide a look into past climate and understand future climate change.

“In the last 30 years or so, studies have been trickling out showing how these specimens have immense value in showing and understanding global change,” Meineke said. “They were not collected for this purpose, but they show the imprints of things we did not monitor in the past.”

Katie Camero can be reached at katie.camero@globe.com. Follow her on Twitter @camerokt_