A difficult few years for oak trees in the region could be coming to an end, as higher populations of defoliating oak leafroller and oak leaftier moths may have peaked with the wetter spring conditions.
An outbreak of the native oak moths was again present this spring across northern Wisconsin, although Paul Cigan, DNR forest health specialist, said the population of these moths has likely reached its peak.
“I think the fact that we’re on year three, and outbreaks of caterpillars don’t usually last more than three years, and No. 2, we’ve had cooler, more ideal conditions for both tree defenses and tree recovery,” Cigan said.
The partially defoliated, thin and yellow-tinted crowns left behind by the caterpillars of these two moth species have been visible across Barron, Sawyer and Washburn counties, along with northeastern counties, Cigan said. The defoliage that can been seen throughout Barron County and is more scattered, but there are heavy concentrations of impacted trees along the Highway 53 corridor down to around Rice Lake.
“Generally it’s scattered pretty reliably throughout the county,” Cigan said.
The outbreak first started in 2022 and appears to have peaked. It’s the first outbreak of the insects in the past few decades, Cigan said, and one reason for the outbreak would be drought cycles as the insects benefit from dry and warm conditions.
“Wetter conditions are going to really help both moderate the population but also probably more importantly, they are going to help trees recover from the last few years of the drought that they’ve been suffering from,” Cigan said.
The defoliation has mostly occurred on northern pin oak and northern red oak, while less so on bur oak. White oak hasn’t really been impacted, Cigan said.
In addition to the oak leafroller and oak leaftier moths, also active has been the two-lined chestnut borer, another native insect that takes advantage of stressed trees from drought. The confluence of these insects put some oak trees in risk of mortality.
“If you see more than 30% dieback, the prognosis is not great,” Cigan said. “Trees can recover from 30% loss of leaves but it’s going to be more difficult and make the situation more tenuous for the tree’s survival.”
Trees with most of their crowns or have been refoliating are likely to recover, while those that are older or have had other injuries are more than likely not going to make it through the cycle, Cigan said.
No additional defoliation will occur this summer as pupation of the caterpillars has started and moth flight will begin soon. During recovery red leaves will appear on trees that were defoliated. The second set of leaves could be red because of a high concentration of the pigment anthocyanin, which acts like a sun screen, Cigan said. It’s a normal process and should not be cause for alarm.
Limiting further stress on trees is the easiest way to avoid long-term issues. Cigan said those with ornamental trees in their yards should try and keep trees healthy. He notes mulching trees and watering every 7-10 days during dry weather. For those with forests, it’s important to monitor recovery or note if there’s any additional decline in tree health.
“We really kind of dodged a bullet with getting good moisture this year, and that’s really going to help with tree recovery,” Cigan said.
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