Wednesday, August 29, 2018

Dogwood Sawfly Larvae - Pest or not?

I’ve just noticed dogwood sawfly (Macremphytus tarsatus) larvae on the red-osier dogwood. Squirming heaps of them are found on the underside of the leaves. At first I thought they might be caterpillars, as they look quite similar to the larvae of skipper butterflies. Heaven knows I have seen many a skipper this summer in the garden! Sliver spotted, Pecks, and many more I have a hard time identifying, have been nectaring all summer, particularly at the deep purple flowers of New York Ironweed. 

But skipper larvae are not known to feed in groups and I just learnt another way to tell them apart from sawfly larvae. They have a maximum of four pairs of stumpy prolegs, whereas sawfly larvae have a few more. So sawfly they are, but not true flies. These are the larvae of a wasp. Adult sawflies lay their eggs along the veins of leaves of their host plant. The larvae change color as they go through instars. At first they are a translucent yellow, then white with a waxy coating and finally, the stage I am viewing, they have a striking yellow underside and a black head. When the larvae stop feeding in late summer, they fall to the ground, burrow into decaying wood and pupate for nine months. 

Dogwood sawfly larvae are partial to red osier dogwood and, in some instances, are a considerable defoliating pest.  Indeed, I see that leaves on a few twigs have been chewed down to the mid-vein. But this is the end of the growing season and I planted this stand of shrubs not as an ornamental, but for wildlife habitat and to prevent erosion on a steep slope. I am confident that I will again see the plant’s striking red twigs standing in the snow again this winter. Meanwhile, the insect will be kept in check by being food for birds. 

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