Look For Gypsy Moth Egg Masses And Prepare For Hatch; DATCP Slow-The-Spread Treatments Announced

By Bill McNee, DNR Forest Health Specialist, Oshkosh, Bill.Mcnee@wisconsin.gov or 920-360-0942

Typically, gypsy moth egg masses hatch in April as temperatures warm. Now is a great time to do an egg mass inspection to look for unknown infestations and treat or remove any masses within reach. Each mass can result in 500 to 1,000 leaf-eating caterpillars.

Egg masses are tan-colored lumps and vary from about the size of a nickel to a quarter. They can be found on many outdoor surfaces such as tree trunks, the undersides of branches, buildings, rocks, fences, retaining walls, firewood piles and picnic tables.

Gypsy moth egg masses on the underside of a maple branch

Gypsy moth egg masses on the underside of a maple branch. 

If egg masses are found, there are two options to help reduce pest numbers. Horticultural oils can be directly sprayed on the egg masses to suffocate them. These oils are typically applied when temperatures are above 40 degrees and not expected to dip below freezing for a few days. Adding a little food coloring to the spray mix helps identify the treated masses and shows that oiling has been done. Egg masses within reach can also be scraped into a can of soapy water and left to soak for a few days before being thrown in the trash. Target egg masses that were produced during the previous summer. Older masses that are faded and feel spongy do not contain viable eggs.

In the summer of 2020, there was a significant rebound of the gypsy moth population after several years of weather conditions that didn’t favor their reproduction. It is also believed that the cold temperatures in February 2021 did not cause heavy egg mortality in most of the state. The highest populations are usually found where oaks and other preferred species are growing on mowed lawns, street terraces or sandy soil. A survey method for predicting the level of this summer’s defoliation can be found here.

Insecticide treatments (injections, soil drenches or sprays) may be appropriate for larger trees with many egg masses. Some treatments are applied before the eggs hatch, and some are done while the caterpillars are small. A directory of for-hire certified arborists is available on the Wisconsin Arborist Association website, and other businesses offering insecticide treatments may be found online or in a phone book.

Additional information about physical controls, including sticky barrier bands and burlap collection bands, is available on the Wisconsin gypsy moth website. It also contains a guide to help those interested in an aerial spray to protect larger areas of high-value, susceptible hosts.

Slow-The-Spread Aerial Treatments Announced

Map of proposed 2021 gypsy moth slow the spread treatment sites in Wisconsin.

Map of proposed 2021 DATCP gypsy moth slow-the-spread treatments. Mating disruption (pheromone) treatments are shown in blue and bacterial insecticide (Btk) treatments are shown in purple.

The Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) has announced its plan for the 2021 Slow The Spread of Gypsy Moth Program and treatments in western counties. About 95,000 acres are scheduled for treatment at 45 sites in 14 counties, using low-flying airplanes.

Counties with sites scheduled to receive aerial treatments include Barron, Chippewa, Crawford, Douglas, Dunn, Grant, Green, Iowa, La Crosse, Lafayette, Rusk, Trempealeau, Vernon and Washburn. A total of 8,133 acres will be treated two times with a bacterial insecticide containing Bacillus thuringiensis kurstaki (Btk). A mating disruption (pheromone) treatment will be applied once over 86,466 acres.

Treatments will likely begin in May and continue through late July or early August. The treatment areas are primarily rural forests, but there is some treatment acreage within a village or city’s boundaries. Additional information, detailed maps and treatment updates can be found online at the DATCP Gypsy Moth website.

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