Black Walnut defoliation attributed to native insect

There is no expectation of widespread mortality in the early spread

(Photos courtesy of Mike Hillstrom, Wisconsin DNR)

MADISON, Wis. -- Some early questions have been answered in a mysterious case of defoliation of black walnut trees. The insect responsible has been identified as a native tortricid moth.

Mike Hillstrom, a Forest Health Specialist with the Wisconsin Department of Natural Resources (DNR), first heard reports of black walnut stands in Wisconsin and Minnesota experiencing some defoliation and webbing in late July 2020. Three counties in southwestern Wisconsin have since had cases, and additional cases were also reported in Iowa this year.

“It’s not something that’s on a huge scale, as far as acres defoliated, but being simultaneous across multiple states, it has a little broader impact than what we have seen with smaller outbreaks of other insects. It is worth investigating and figuring out what is going on,” Hillstrom said.

The investigation is still in the stages. DNA testing of the larvae indicates that the insect responsible is a native tortricid moth, Gretchena amatana. The moth itself has not yet been identified to confirm the DNA results, but they have confidence that the insect is at least in the same genus.

Other species in the genus feed on Juglandaceae (plants in the walnut family), and tortricid caterpillars are also known as leafrollers, small caterpillars that feed inside protective nests they make from leaves rolled together and tied with silk. This may explain the webbing found on the defoliated trees.

The sudden outbreak is unusual because both the black walnut and the tortricid moth are native species. Systems involving native insects and trees tend to have evolved to handle the damage. The fact that the damage seems to occur later in the season is also a good sign because it is generally easier for a tree to recover when it is not producing new leaves.

Healthy trees can usually handle a season of defoliation. It becomes more of a concern when multiple stressors (such as defoliation and drought) or multiple years of defoliation stack up. For now, there is no expectation for widespread black walnut mortality, according to Hillstrom.

Hillstrom and a researcher at UW-Madison recently submitted a grant request to the Forest Service for funds to continue studying the issue. Hillstrom hopes they can locate the moth next spring and confirm the species identification.

Besides staying informed, landowners are encouraged to check their walnut trees and report any sightings to their local DNR forester. Mid- to late-July will likely be the best time for landowners to spot the damage.

Tree Trunk
(Photos courtesy of Mike Hillstrom, Wisconsin DNR)

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