Population of spongy moth at record high

Spongy moth populations reached record levels in much of eastern and central Wisconsin, in 2023. To combat that, the Wisconsin Department of Agriculture, Trade and Consumer Protection’s (DATCP) 2024 treatment plan, is designed to prevent these large populations from spreading into uninfested regions of western Wisconsin and causing damage to timber, nursery and tourism industries.

Beginning in May and continuing through July, the DATCP plans to treat selected areas in western Wisconsin, including Chippewa County, using low-flying planes. Approximately 28,590 acres, at 23 sites in nine counties, are scheduled for treatment.

These areas have been identified as having increasing populations of spongy moth, a destructive non-native insect, with an appetite for hundreds of different species of trees and shrubs.

Property owners should examine their trees for spongy moth egg masses and determine whether control measures are needed this spring. Spongy moths thrive in warm and dry weather, but even with “average” weather conditions this spring and summer, the current spongy moth outbreak, which led to the defoliation of a state record 373,000 acres of woodland last year, is likely to continue to spread.

Those with oak, birch, crabapple, aspen, willow and linden (basswood) trees should be exceptionally watchful, because the caterpillars of this invasive insect prefer these species. However, they will feed on many other kinds of trees and shrubs.

Their egg masses look like tan-colored lumps about the size of a nickel or quarter. These egg masses are found on trees, buildings and other outdoor objects. They may also be found inside sheltered places, such as firewood piles and birdhouses. Egg masses produced in 2023, will feel firm and appear darker in color than older egg masses, which will appear faded, feel spongy to the touch and do not contain viable eggs.

These egg masses will typically begin to hatch in midto- late April, in southern Wisconsin. Horticultural oil, available from many lawn and garden centers, can stop these egg masses from hatching, if applied when temperatures are above freezing.

A knife, chisel or similar utensil can be used to scrape unhatched egg masses into a can of soapy water. Once egg masses have been removed, leave them to soak in the water for a few days, before disposing in the trash.

Once the caterpillars hatch, insecticide treatments, including foliage sprays, and soil and trunk injections, may be a suitable control option. A professional treatment is likely needed when there are high numbers of egg masses on large trees.

When low or moderate numbers of egg masses are present, physical controls can often be used to reduce pest impacts and make insecticide unnecessary. Insecticide treatments are most effective when the caterpillars are small (mid-to-late May, in southern counties, and late May or early June, in northern counties).

Host trees that suffered heavy defoliation or drought stress in 2023, are at a higher risk of dying, especially if they are not protected with insecticides and they suffer noticeable leaf loss again this year. Low-vigor oak trees are likely infested by the two-lined chestnut borer, a native beetle, contributing to tree decline and death. Consult an arborist or DNR forester for additional management recommendations.

For more information, visit smaerialspray.wi.gov or spongymoth.wi.gov.