Day 323: The identity of this critter may come as a surprise. Despite appearances, it is NOT a caterpillar. True caterpillars are the immature stage of butterflies and moths. These larvae are in fact those of the Mountain-ash Sawfly (Pristiphora geniculara), a species of wasp and currently the bane of my garden. I should have investigated more closely when I first noticed leaves disappearing from the smaller twigs, but I assumed that the tree was heat-stressed or suffering in response to a pesky Sapsucker's drilling. When a second Mountain-ash (different species) began exhibiting the same symptoms, I took a closer look. Pulling a branch down to eye level, I was surprised when a little yellow "caterpillar" assumed an uncharacteristically defensive stance. Then I noticed its cousins hard at work on one of the remaining leaves.
Mountain-ash Sawfly was first noticed in Washington in the summer of 2009, apparently a European immigrant. At that time, it was confined to Snohomish County, but it has since spread to much of the western portion of the state. There are two remedies for this pest. I will only resort to chemicals as a last resort, preferring to go with my father's tried-and-true method: removal and burning of each infected twig or branch showing evidence of the insects or their egg cases. The trees will have to be monitored again next spring because I'm bound to miss a few.
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