Hard choices

A garden club member recently asked me how I would deal with an infestation of gypsy moth caterpillars on a mugo pine (Pinus mugo). Last summer her tree was crawling with them. 

Nonnative gypsy moth caterpillars can cause massive defoliation

Would I use pesticides in this situation? Fortunately gypsy moth populations wax and wane on a predictable cycle. In wet years, like the one we just had, a fungus that kills the caterpillars surges. We can expect minimal gypsy moth damage this summer, so her tree should get a break. Her question raises a larger issue, though. When is it right to use pesticides?

    I was recently disillusioned to learn that the Arnold Arboretum is using a neonicotinoid pesticide to control massively destructive hemlock woolly adelgids in its vast collection of eastern hemlocks (Tsuga canadensis).

White cottony masses on this hemlock are adelgid egg cases

Following the lead of Great Smoky Mountains National Park managers, they’re taking a two-pronged approach to this nonnative pest. They’re spraying the trees with horticultural oil, which smothers the insects, and they’re drenching the soil around the trees with imidacloprid, which attacks their nervous system.

Gray trees in this view of the Smokies are hemlocks killed by adelgids-photo NPS

     In the Smokies, this treatment is reducing adelgid damage and even resulting in regrowth of some hemlock foliage (When it’s not applied to soil, the pesticide is injected into the tree’s bark). Hemlocks are foundational species in the park. Park managers decided that losing them all was unacceptable.

Drenching soil with pesticide-photo NPS

    I sympathize with their dilemma and the Arboretum’s. We resolve to limit our use of pesticides or ban them outright. Then a threat to a plant we can’t bear to lose makes us break our promise to ourselves. As an Arboretum staffer writes, “. . . we are an essential resource for a large urban population that for over 150 years has enjoyed . . . a majestic hemlock-dominated forest.”

Hemlock forest

    The problem, of course, is that all pesticides cause collateral damage. I stopped spraying hemlocks in my yard with horticultural oil for hemlock woolly adelgid two years ago when I realized that even spraying in early spring was killing some insect bystanders that I didn’t want to lose. 

Spraying for adelgids could kill bumblebees and other early foragers-photo Ivtorov

The USDA acknowledges that in the Smokies, imidacloprid affects insects in tree canopies and in the soil at their feet.

    It was human activity that brought the adelgid from Japan to North America. It has no natural predators here. Some human response seems to be needed before all our hemlocks succumb, but what should it be? Applying a pollinator-killing neonicotinoid seems like an unhappy choice. 

     One hopeful development in the Southeast: introduction of natural predators from Japan that eat only adelgids. 

Biocontrol beetle eating adelgids

Tiny beetles in the genus Laricobius have been released in the forest and succeeded in decreasing the adelgid presence without affecting native insects. The hope is that a new balance will develop between these predators and their adelgid prey. Foresters in some areas have been able to stop the chemical treatments. 

     The beetles will never kill every last adelgid, but they may allow hemlocks to survive. Was it worth using pesticides before the beetles became available? I guess so.