Gypsy Moth Invasions and the Odyssey of Biological Control
I live in Massachusetts and remember a huge gypsy moth outbreak in the Northeast in 1981 that defoliated millions of acres of forests and shade trees. In the quiet of the evening, you could stand in your yard and hear what sounded like rain falling on the trees. But there was no rain. What sounded like rain was actually the droppings of the caterpillars falling through the trees. Pavement beneath trees was carpeted with black dots that looked like poppy seeds but were actually caterpillar droppings.
The outbreak was so great that reports spoke of infestation rates of one to two gypsy moth caterpillars per leaf. And they’re voracious eaters; their sheer numbers easily defoliating mighty oaks and most other species’ of trees. Healthy trees will recover, but with each defoliation the tree weakens, its growth is stunted and it heads toward a likely premature demise.
For the better part of the 20th century, outbreaks were occurring about every ten years, however most of New England enjoyed what the professionals call low-density, non-outbreak status in the years following that 1981 outbreak. Gypsy moth caterpillars were off the front page, but there was more than the decennial cycles at play here, which sets the stage for a tale of biological intrigue.
The First Prong of the Two-Pronged Biological Counter-Attack
In 1910 a winter-hardy Japanese fungal pathogen with an appetite for gypsy moth larvae, called Entomophaga maimaiga (E. maimaiga) was released in the Boston area as a way to biologically combat the gypsy moth larvae. However there was no indication that the fungus took hold and scientists gave up on it. Fast forward 79 years and all of a sudden, in 1989, E. maimaiga appears again in the Northeast, devastating gypsy moth caterpillar populations in several states. Curiously enough, the fungus doesn’t appear to be lethal against most other moth and butterfly caterpillars.
How E. Maimaiga Is Effective Against Gypsy Moth Larvae
The fungus overwinters in the soil and on the bark of trees as what scientists call resting spores. The resting phase ends in May or June and the fungus produces sticky spores at the end of a stalk, protruding from its body, which the gypsy moth larvae come in contact with. The sticky spores burrow their way inside the gypsy moth caterpillar, killing their host usually within a week. Airborne spores are released from the caterpillar cadaver, killing healthy caterpillars they settle onto.
E. Maimaiga’s Kryptonite
In order to successfully release the airborne spores from the caterpillar cadavers, E. maimaiga must be bathed in rainfall during May and June. My area experienced drought conditions beginning in May, 2015 and continuing through 2016, and the gypsy moth caterpillars made a comeback. The drought ended and there was plenty of rain in 2017, but die-off from the fungus usually occurs in late June, after defoliation has already occurred, so 2017 also saw widespread defoliation. At the same time, there was evidence of a widespread epidemic among the caterpillars caused by the fungus.
The Second Prong of the Two-Pronged Attack
The gypsy moth caterpillar die-off of 2017 also revealed evidence of a second biological agent, this time a virus, the name of which is abbreviated as NPV. Around since the early 20th century, it was the main pathogen responsible for widespread gypsy moth caterpillar collapse until E. maimaiga appeared. NPV is usually seen only when caterpillar populations attain high outbreak levels, as seen in 2017, while the fungus is present even in sub-pest concentrations.
Solving the Murder Mystery
The cadavers leave telltale signs of what killed the caterpillar. Those killed by the fungus will usually stay attached to stems of bushes or branches and trunks of trees, their bodies stiff and at a right angle to the tree. Those killed by the virus also stay attached, but their bodies usually hang in a limp, inverted V position. Scientists can also dissect the cadavers and examine the body cavity under a microscope. Finding spores indicates the caterpillar was killed by the fungus, while those killed by the virus reveal molecules called occlusion bodies, which protect the virus particles when they’re released into the environment.
If They Survive as Larva, There’s No Guarantee They’ll Survive as Pupa
Mortality rates in the larval stage, in most populations, often exceed 90%. The majority of those deaths are from the fungus, but the virus is also a factor. When you add predation mortality during the egg and pupal stage, the rate jumps up to 99%. Predators include birds, mammals, insects and amphibians. Finally, there’s a biological agent homeowners can use. Called B.t. for short, bacillus thuringiensis is a naturally occurring, soil-dwelling bacterium that’s fatal to a variety of insects. The bottle-dwelling bacterium is available where you get garden supplies. There are numerous subspecies of B.t, with B.t.k. (Bacillus thuringiensis var. kurstaki) used most often against the gypsy moth larva.
Massachusetts Division of Fisheries and Wildlife (MassWildlife)
Massachusetts Wildlife, Vol. 68, No. 1, 2018
Gypsy Moths in 2017: A Pathogen Epidemic
By Joe Elkinton, Jeff Boettner, and Valerie Pasquarella
Cornell Mushroom Blog
Entomophaga maimaiga—The caterpillar killer
March 19, 2009
Michigan State University
Michigan State University Extension Bulletin E-2604 adaptation by Dave Smitley, Deborah McCullough, and Lyle Buss, Michigan State University
A Natural Enemy of Gypsy Moth
Questions & Answers
© 2018 Bob Bamberg