Gypsy moth: more complicated than you'd think

While waiting last week for my computer to return from urgent care, I contemplated a talk about biological controls by Joseph Elkinton, a professor of environmental conservation at the University of Massachusetts, that I heard last November. 

Gypsy moth caterpillar

    What Elkinton said about the history of gypsy moth in North America turns out to be unexpectedly relevant. We had a gypsy moth surge in 2016, and this year there has been significant defoliation by gypsy moth caterpillars, especially in southeastern Massachusetts.

Voracious gypsy moth caterpillars shred leaves

    A complicated web of factors influences the gypsy moth population. It all goes back to acorns.

    Gypsy moth (Lymantria dispar), native to Europe, was introduced to Medford, Massachusetts by Étienne Léopold Trouvelot, an astronomer dabbling in silk worm breeding. He thought he could cross the gypsy moths with native silk moths to produce silk in the US. The insects escaped from his home in the 1860s, and over 50 years they slowly established themselves in the wild. They’ve spread as far as Minnesota and North Carolina.

Life stages of the gypsy moth

    Elkinton explained that until 1989, gypsy moths experienced population outbreaks about every 10 years. Throughout the twentieth century, predator insects were introduced in hopes of controlling the gypsy moth population, but none did enough.

Predatory wasps kill caterpillars, but not enough

     Meanwhile, research into the ecology of the gypsy moth uncovered a system of natural controls. Robert Campbell of the US Forest Service found that the white-footed mouse was the most significant predator of the caterpillars, with birds less of a factor.

White-footed mouse, Peromyscus leucopus, in captivity

     Mouse populations rose and fell with the supply of their main food, acorns, which fluctuated with weather conditions. The mice couldn’t expand their population enough to control gypsy moth outbreaks, when caterpillar numbers spiked exponentially.

    Outbreaks were brought to an end by Nuclear Polyhedrosis Virus (NPV), which eventually killed up to 99 percent of the horde of larvae.

    This cycle continued until another accidental foreign introduction in 1989: Entomophaga maimaiga, a Japanese fungus. Infected by the fungus, gypsy moth caterpillars suddenly died all over southern New England.

    So with mice, viruses and fungi controlling its population, why is gypsy moth surging again? The levels of fungus are highest in wet years. Last year’s severe New England drought allowed the gypsy moth population to escape from its fungal control. This summer, caterpillars chomped foliage until June, when they started to die off. Caterpillars examined by Elkinton’s team showed a combination of viral and fungal infections. Wet weather had turned the tide.

    This means, thank heavens, that we don’t have to make the difficult decision whether to spray for gypsy moth.

    The future outlook for New England trees is mixed. Gypsy moths won’t die out completely, and our trees face a number of stresses from drought and other nonnative insect attacks. A tree can survive one defoliation, but repeated losses deplete its energy stores. Elkinton worries about major losses of native oaks, which the caterpillars particularly enjoy. Then what will happen to the white-footed mice?

 

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.