Sunlight vs. Hemlock Woolly Adelgids

Increased Sunlight may Help Infested Young Hemlocks

Hemlock Seedling
Robert Jetton, one of the scientists who contributed to the study, infests a hemlock seedling with adelgids. Photo by Bud Mayfield, USFS.

Scientists have identified a potential new strategy for protecting hemlocks from the miniscule insect that plagues them. “High levels of sunlight help reduce hemlock woolly adelgid abundance on young seedlings,” says U.S. Forest Service project leader Bud Mayfield. “Follow-up experiments in the field are still needed, but the results suggest thinning or strategically creating gaps in the forest could help conserve hemlocks.”

Currently, pesticides and biocontrol with predator beetles are the most common strategies for protecting eastern and Carolina hemlocks from the non-native adelgids. Both methods can be effective, but they have drawbacks: pesticides are expensive and impractical for large areas, while predator beetles take a long time to become established. Meanwhile, healthy trees can succumb to adelgid infestations in less than 5 years.

“Combining chemical and biological controls with forest management practices that improve hemlock resilience could prove to be a long-term management solution,” says Mayfield. “Creating conditions in which hemlocks get more sunlight may be an effective component of large-scale conservation and restoration strategies.”

Mayfield was part of the research team that evaluated the effects of shade on potted hemlock seedlings infested with the hemlock woolly adelgid. Steven Brantley, who is currently a researcher at the Joseph W. Jones Ecological Research Center in Georgia, led the study, and a number of university and Forest Service scientists also contributed. The study was published in Forest Ecology and Management.

Hemlock woolly adelgid’s egg sacks cling to branches like deadly lint. When the nymphs emerge, they are just one thousandth of an inch long. Photo by Lorraine Graney, Bartlett Tree Experts,

The scientists intentionally infested 100 eastern hemlock seedlings with adelgids. Groups of seedlings were shaded from above with different levels of shade cloth, ranging from 90 percent shade to full sun. To measure how severe the infestation was, the scientists counted the hemlock woolly adelgid’s egg sacks.

“Our team also measured several short and long term indicators of carbon balance, as well as leaf nutrition,” says Mayfield. Hemlock woolly adelgids are known to affect carbon balance and cause water stress, but scientists do not yet know the exact mechanisms that cause tree death. However, many scientists suspect that the aphid-like insects suck out the carbohydrate-rich sap faster than trees can replace it, essentially starving the trees.

Mayfield and his colleagues found that when seedlings had 30 percent sunlight or more, they tended to have lower levels of hemlock woolly adelgid infestation, and when they had between 30 and 70 percent sunlight, they also had healthier carbon balance and better growth. Seedlings in full sun (100 percent light) had higher stress, even in the absence of hemlock woolly adelgids.

The study suggests that when hemlock woolly adelgids are present, deeply shaded environments are not the best place for hemlock trees to survive and grow. Silvicultural strategies that provide more light to hemlocks could be especially beneficial in places where predator beetles are already established, or in areas where the winters get cold enough to limit adelgid infestations.

“We are initiating some follow-up, tree-release studies in order to develop some field management recommendations,” says Mayfield. “We hope an integrated pest management approach will prove to be a successful, long-term strategy for conserving and restoring eastern hemlock.”

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For more information, email Bud Mayfield at

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