Another non-native insect has been detected in Washington State. This one – a relative of wasps, bees and ants — is eating away at alder and willow trees, harming their abilities to process through their leaves.
The non-native green alder sawfly is an insect whose larva looks like a caterpillar. The sawfly larvae have a distinctive bright green color and feed on the leaves of trees, just like caterpillars. However, they don’t turn into moths or butterflies. The adults look like wasps. They have two pair of clear wings, but unlike other wasps, they have a broad waist joining the thorax and abdomen. Many sawflies make their cocoons in the soil or in rotten wood.
Although there are many species of native sawflies that feed on all types of hardwood and conifer trees, DNR’s Forest Health Program is especially concerned about the green alder sawfly, which comes from Europe and North Africa.
Sawflies, in general, damage leaves and needles of both hardwood and conifer trees. They rarely damage buds, so trees are able to grow new leaves and recover. However, temporary loss of leaves slows tree growth and can stress trees enough to make them more susceptible to attack by other insects and pathogens (disease).
This was the first year that green alder sawfly were positively identified in Washington. But they have been here for some time… a later search through insect collections at Western Washington University found specimens collected in 1995.
The green alder sawfly has been found in Alaska, British Columbia, Washington, and Oregon. The most severe damage has been to thin-leaf alder in Alaska. In Washington, we’ve seen defoliation of red alder trees, but it’s not severe.
Scientists study characteristics to prepare for next steps
It’s difficult to predict what future damage will occur to Washington’s red alder because this is a new environment for the sawfly. We know very little about its natural controls and whether it could have a unique effect on red alder different than the tent caterpillars and leaf beetles that commonly feast on alder leaves.
DNR’s Forest Health Program is working in partnership with the U.S. Forest Service and Washington State Department of Agriculture to increase monitoring in the state, for both the insects and their damage. They are using yellow and light green sticky traps hung in alder trees to collect green alder sawfly adults. The sawfly already has been detected in 11 Washington counties, including one in eastern Washington.
The Department of Agriculture’s entomologists have been conducting the rigorous taxonomic work —classifying the organisms—to identify the sawflies that DNR catches and turns in.
This was the first year for monitoring sawflies in Washington. More monitoring is needed to determine the number of sawflies and the amount of damage done to Washington forests. The next steps for DNR and the U.S. Forest Service will be to create more monitoring sites and adding more traps in order to determine what is needed to keep the forests healthy.
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