Good news for trees

Spotting a little green caterpillar on a weed I pulled from the perennial bed reminded me that this is the time when winter moth larvae (Operophtera brumata) make their way up trunks of trees such as the old oak that stands over this area of the garden.

Winter moth caterpillar

    That caterpillar was in for a surprise, though. Thanks to a team led by University of Massachusetts entomologist Joseph Elkinton, a parasitoid tachinid fly, Cyzenis albicans, is laying its eggs on leaves consumed by winter moth caterpillars. The fly’s offspring will kill most of the caterpillars that eat those leaves. What used to be an epidemic of nonnative pests defoliating trees in eastern Massachusetts has turned into no big deal.

    We used to notice clouds of male winter moths when they appeared in late fall, often lighting on windows and doors and sometimes making their way into the house. 

Winter moth caterpillars in November 2016

The wingless females can’t fly, but they can climb tree trunks to lay their eggs in bark crevices. The larvae emerge in March and devour leaf and flower buds, or they produce strands of silk that allow them to “balloon,” floating to other trees to spread the destruction. 

     Winter moth surged in Massachusetts in 2003 and found no native predators. At the height of the moths’ population boom, they affected most of our street trees, favoring the plentiful maples and oaks. It was alarming to walk down the street and see every tree leaf riddled with holes.

Birch leaf chewed by winter moth caterpillars, 2015

    To protect ornamental trees, the usual approach was to apply pesticides, and I did that for several years. I was telling myself that the treatment was relatively benign, because it used spinosad, an agent derived from naturally occurring soil bacteria.

Although it's a natural product, spinosad kills bees as well as winter moths

     Elkinton’s team took a different tack. In Europe, where it originated, winter moth has numerous insect predators. The scientists tested these to see which would be safe for release in New England, seeking an insect that targeted only winter moth and wouldn’t harm native insects. For the past 14 years, they’ve been releasing the parasitoid fly Cyzenis albicans to kill winter moth caterpillars. 

Cyzenis albicans-photo James K. Lindsey

    This year Elkinton has declared victory. After years of painstaking monitoring, his team has determined that winter moth populations are down to manageable levels. The parasitoid fly has become established, maintaining sufficient populations to keep winter moth under control.

    Predators need prey. If the flies eradicated all the winter moths, they’d have no food for their larvae. They would die out too. With both maintaining a stable presence, they can continue their interdependent dance.

    This is one of those scientific and ethical dilemmas where human beings have created a problem—in this case by unintentionally importing nonnative winter moths—and there’s no perfect response. If we did nothing, we’d likely lose many trees. 
  

     If we kept spraying pesticides, even targeted ones, we’d inevitably cause collateral damage to other insect species. That's why I stopped applying spinosad. So far, introducing nonnative Cyzenis albicans seems like the happiest solution for a thorny environmental problem.

Winter moths won't be troubling our oak anymore

Fighting caterpillars from within

Winter moths are appearing on the storm doors again,

so I was eager to attend a recent talk by Professor Joe Elkinton. Elkinton is an entomologist at UMass who not only studies infestations of nonnative insects, he actually does something about them. His speech about gypsy moths and winter moths alerted me to a gardening quandary I’ll need to resolve.

Birch leaves chewed by winter moth caterpillars

    Gypsy moths (Lymantria dispar) came to Boston in 1868, imported by Étienne Léopold Trouvelot, an astronomer in Medford, Massachusetts who was trying to mate them with silk worms. In an all-too-common scenario, they escaped from his custody and traveled across Massachusetts defoliating trees over vast areas. The last major outbreak was in 1981.

Life stages of the gypsy moth

     In an interesting ecological twist, gypsy moth populations were decimated soon afterward by a newly introduced fungus. This year’s drought changed the balance by limiting the fungus, so gypsy moths surged. 

    Elkinton said that biological control—introduction of organisms to kill unwanted insects such as gypsy moth—was tried as far back as 1905. In this approach, insects that control the pests in their homeland are brought over to do the same here. 

     Early on, one introduced parasitoid killed native insects, such as the beautiful luna moth (Actias luna).

The luna moth is a North American native

A parasitoid, unlike a parasite, ultimately sterilizes or kills its host. 

    That brought me to what I wanted to know from Elkinton’s talk. How can we be sure we won’t cause more problems by releasing new nonnative insects to control the ones we don’t like? His answer was, “Host range testing.” This means carefully evaluating the insect you’re planning to release, checking whether it will use native insects as hosts. Entomologists in this field only release parasitoids or predators that specialize in one nonnative insect, unlike goofballs like Trouvelot. 

    Winter moth (Operophtera brumata), also from Europe, got to the US in 1950 and has been a major problem in eastern Massachusetts since around 2001. The larvae, little caterpillars, tunnel into buds and eat young leaves of most of our deciduous trees. Elkinton has been releasing a tiny tachinid fly, Cyzenis albicans, to parasitize winter moth larvae. 

Winter moth caterpillar

     This is a long process. It takes three to five years after introduction to establish a population of the fly in a new location. To determine whether their efforts are succeeding, his team and volunteers harvest 120,000 caterpillars in May and check whether each contains flies.

    Elkinton revealed that my neighborhood has an established population of C. albicans. He predicted there will be enough of them next summer to make a significant dent in the winter moth population.

    Thereby hangs a sustainability dilemma. If I continue having my trees sprayed to kill winter moth caterpillars, I’ll kill the little flies inside the larvae as well. I’d like to let the flies do their job, but if they don’t, the caterpillars will defoliate the trees. I think I'll take a chance and skip the spraying.

Last foliage of the year. This ornamental plum could be defoliated by winter moth next summer if I guess wrong.