On a June day in 1986, you could have driven for 50 miles through oak-dominated forests in Pennsylvania and not seen a single leaf, except on a black walnut or tulip tree, which are just about the only species that gypsy moths turn their noses up at. Having seen devastating gypsy moth defoliation on a smaller scale in Connecticut, I can understand why a person might have thought the world was coming to an end. An outbreak of this species is frightening and sickening. After hatching, each successful larva consumes about 9 square feet of leaf area. Some of this food gets converted into caterpillar, but a vast quantity falls from the trees as waste matter in a shower of disgusting coffee grounds. Then millions of the hairy 2-inch caterpillars drop to the ground and look for a place to pupate. The forest is as gray as in February.
Once a non-native insect such as the gypsy moth gets a good foothold, the advance is relentless. Most insects don’t spread very fast on their own, but they are aided by rides on all kinds of wheeled conveyances. The gypsy moth, which has now been an unwelcome guest on this continent for about 136 years, has survived and continued to expand its range despite being attacked with everything in the entomological arsenal. Hundreds of millions of dollars have been spent studying and fighting it. Now most everyone realizes that these moths are here to stay, and moth management is the best we can expect.
After their famous escape from captivity near Boston around 1868, gypsy moths were brought from Europe a second time in the 1920s, this time on spruce trees shipped to New Jersey from the Netherlands.
Because the females are unable to fly, the moths move across the land at only 1.25 miles per year and, without human help, would by now have traveled about 170 miles and just be approaching New York City (the New Jersey arrivals would be 100 miles from their ship). Instead, gypsy moths have occupied all of New England and New York state for decades and now have reached Wisconsin, with about 1,000 miles under their fat belts. In recent years they have spread at an average rate of 13 miles per year. As the distance from Boston increases, the area inhabited by gypsy moths grows exponentially.
Currently, gypsy moths occupy only a third of the area in which they are likely to cause severe damage. At their present rate of expansion, they will reach all of the suitable habitat – forests that are predominantly hardwood – in North America in 100 years. If their advance into new territory could be reduced just by half, that journey would take 200 years. Merely slowing the rate at which they move across the country might at first seem like a pessimistic proposition, but speed makes a huge difference, and, for several reasons, slowing them down is easily worth the cost.
Keep in mind that losses from this insect are tremendous: many trees are killed, and growth rates of the survivors are greatly reduced. In addition, by favoring and killing more oak than other species, they have already begun to change the composition of forests. Though the first defoliation by gypsy moths is usually the most destructive, there are likely to be 2- to 4-year-long outbreaks every 5 to 10 years once they’re established. The losses are not just one-time affairs.
A slower rate of advance gives enemies of the moth – viruses, fungi, and various predators and parasites – time to build in strength. One fungus, Entomophaga maimaiga, is particularly promising. It was imported in 1910 and 1911 to control gypsy moths and for 80 years appeared to have been a failure. In the late 1980s, it was unexpectedly identified as the organism that was killing caterpillars on a gratifying scale. In addition, improved formulations of nuclear polyhedrosis virus, another gypsy moth killer, are being developed. With more time, these and perhaps other agents can be made more effective and can be deployed at the leading edge of gypsy moth expansion instead of following in its wake.
“Slow the Spread” is a 5-year-old national program run by the U.S. Forest Service in cooperation with state governments. In a pilot project, the rate of gypsy moth advance was slowed to 5.25 miles per year. Pheromone traps are set out to lure any stray male gypsy moths in the uninfested areas along the edge of the insect’s range. If males are found in the traps, more pheromones are dropped from airplanes to confuse them so that they are unable to find females. Some gypsy moths will slip through the net, to be sure, but the magnitude and speed of the moths’ advance will surely be diminished.
In our region, where gypsy moths have a relatively long history, outbreaks have been minimal in recent years, probably because of the fungus. At the front of an invasion, it’s a very different story.
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