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Allan Taylor
A long-standing member of the IOS and fomer editor of Oak...
Panayoti Kelaidis | Dec 17, 2022
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Plant Focus

Quercus macdougallii
A rare oak endemic to the Sierra Juárez in Oaxaca

Death by Defoliation: How an Invasive Caterpillar Kills Oaks—and How Some Survive

A recent multi-year outbreak of an invasive moth killed oak trees in thousands of acres across southern New England, USA. But interspersed among the wreckage were thousands of trees that survived. A new study published recently in Functional Ecology sheds light on why. Research by scientists from Harvard, UMass Amherst, Boston University, and MIT reveals that a tree’s carbohydrate reserves are crucial to surviving an onslaught of hungry caterpillars.

The biology of trees makes them resilient to even the most severe stressors. “Oak trees are planners, in a way,” says Meghan Blumstein, NSF Post-doctoral Research Fellow at MIT and a co-author of the study. “Some of the food they make during the growing season is used immediately for energy and some is stored in the stems and roots for a rainy day. With stored carbs, they are able to immediately create a new flush of leaves after an insect outbreak.”

Lymantria dispar (caterpillar shown here) is one of the most destructive invasive insects in North America. Defoliation by this insect can kill oak trees by draining the trees’ energy reserves. Photo credit: Nathan Oalican.
Lymantria dispar  is one of the most destructive invasive insects in North America: defoliation by this insect can kill oak trees by draining the trees’ energy reserves © Nathan Oalican 

But trees are not invincible, and the new study reveals the specific threshold of reserves necessary for them to survive: 1.5 percent carbohydrates in their dried wood—or about 20–25% of their normal storage capacity. The repeated emergence of Lymantria dispar (an insect formerly known as “gypsy moth”) from 2016 to 2018 challenged trees’ resilience by defoliating them year after year.

“The trees that died were the trees that were out of reserves,” says lead author Audrey Barker Plotkin, a Senior Scientist at the Harvard Forest. But the location of the trees mattered, too. The research team found that trees growing along forest edges tended to have more reserves, even at the same level of defoliation, making them more resilient than interior forest trees. The research team posits that forest edge trees may have simply experienced less severe defoliation in the years before 2018. And, because edge trees get lots of light, they may also be able to rebound without drawing down their reserves as much as their interior forest counterparts.

Meghan Blumstein collects a root sample from an oak tree in February 2019. Photo credit: Audrey Barker Plotkin
Meghan Blumstein collects a root sample from an oak tree in February 2019  © Audrey Barker Plotkin

The new study provides direct evidence, which had until now been lacking, that trees can indeed starve to death when insects invade. This more nuanced understanding will help improve forest resilience models as new pests and a shifting climate continue to drive change in the region.

You can read a detailed account of the project, including tribulations such as poison-ivy rash and marathon lab sessions, in a blog post by Audrey Barker Plotkin here.