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Sunday, December 6, 2015

The best among bad choices?

This is the time of year when small gray-brown winter moths (Operophtera brumata) congregate on windows and storm doors at night, sometimes fluttering into the house. These nonnative insects have staged a major infestation in Massachusetts since 2001, and they have me in a dilemma.

Male winter moths hanging out on a window

    Starting around Thanksgiving, the moths mate, and the females lay their eggs on trunks and major branches of trees. They particularly like maples, oaks, crabapples, and blueberries. Those are some of the key species in my yard. The larvae, tiny caterpillars, hatch in spring, wiggle into buds, and eat young leaves. 

    Weakened by losing a large proportion of their leaves to the caterpillars’ depredations, the trees may die because they lack the energy to put out a new crop of foliage. I can see this happening to street trees in the neighborhood. Some have died; many are very stressed.

This birch lost many leaves to winter moth caterpillars

    I’ve been having my trees sprayed with a (somewhat) natural product called spinosad that kills winter moth caterpillars when they’re eating leaves in spring. I do it to save the trees, but I don’t feel good about it. Spinosad kills by its action on the caterpillar’s nervous system. It’s extracted from the bacterium Saccharopolyspora spinosa, which is fermented to derive the active ingredient. 

    Spinosad has low toxicity for mammals and birds and spares many insect predators and parasitoids that we count on to control leaf-eaters. The problem is that it kills not only winter moth larvae but other insects too. I was alarmed to learn that it can kill honeybees if it’s applied while they’re active, as when flowering trees are blooming in spring and bees are foraging. Once the spinosad dries, it becomes less dangerous to bees. 

I don't want to kill bees by spraying spinosad

    So which choice is correct, to spray or not to spray? I’m torn between wanting to save trees I love--losing the hundred-year-old oak near the house would be particularly sad—and recognizing that by spraying spinosad, I’m altering the balance of insects in my yard and killing off some natives participants in the local food web. I could let the winter moth caterpillars do their worst and see which plants survived, but so far I haven’t had the heart for such an austere approach. 

I'd hate to lose this red oak

    In the next few years, biological control may let me off the hook. Winter moth originated in Europe. Because it didn’t evolve here, it has no native predators. A team led by UMass scientist Joseph Elkinton has spent a decade releasing a predator of winter moth in New England.    

     The silver bullet is a European parasitic fly, Cyzenis albicans, which controlled winter moth outbreaks in Nova Scotia and the Pacific Northwest. The flies eat winter moth caterpillars when they pupate in the soil. They’re specialists, so they don’t affect other species. Their population is expanding. 

Cyzenis albicans

    I like the idea of getting rid of winter moths without chemicals. I wonder, though, about unforeseen risks of introducing nonnative insects to kill the nonnative insects we’ve already introduced accidentally.

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