Forest Service Anticipates Douglas-fir Tussock Moth Outbreak; Aerial Treatment to Start in Early Summer

Release Date: Jun 1, 2019

Central Washington — High along the slopes of Blewett Pass, USDA Forest Service Entomologist Connie Mehmel records new data on tussock moth egg masses. For several months Mehmel’s team, in conjunction with the Washington Department of Natural Resources, has been carefully monitoring these masses to determine if a moderate outbreak is likely for the summer of 2019. Largely based on these surveys, she anticipates outbreaks on Okanogan-Wenatchee National Forest lands in the Methow Valley, Lake Wenatchee, and Blewett Pass areas. Although periodic outbreaks are a natural part of the ecosystem of the Cascades, Forest staff are also planning to utilize a natural bio-control in high-use recreation areas to minimize public safety issues.

“Public safety is always our top priority. The natural bio control we’ll use has no chemical insecticide whatsoever,” said Mehmel. “Our goal is to spray caterpillars before they grow big enough to cause damage in high-use areas.”

This year the project will involve aerial application of a naturally occurring virus toxic only to the tussock moth, called TM Biocontrol-1, on up to 8,000 acres of public lands north of Mazama in Okanogan County, both sides of Blewitt Pass in Kittitas and Chelan Counties as well as the Lake Wenatchee area. The virus would be applied using helicopters as early as the second week in June and as late as the last week in June, depending upon when insects hatch out and caterpillars are most vulnerable to the virus. 

“The biocontrol is sticky- it’s a mixture of water, molasses, and the virus,” added Mehmel. “Visitors will want to avoid areas on days when spraying occurs, as you probably wouldn’t want to get this sticky substance on your clothing or your vehicle.”

Forest visitors should also be aware that hairs from tussock moth caterpillars can cause allergic reactions for some people and their pets in an outbreak area. Long-term, Forest managers will continue to utilize restoration tool such as thinning and prescribed fire to increase forest health and resilience to disease and insect outbreaks such as tussock moth. The last severe outbreak in Washington occurred from 1999 to 2002, with more than 45,000 acres defoliated. 

USDA Forest Service staff will utilize a natural bio-control in high-use recreation areas to minimize public safety concerns due to the anticipated Tussock Moth outbreak on the Okanogan-Wenatchee National Forest in 2019. Credit: USDA Forest Service map.

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