Gypsy Moth Caterpillars Threaten Trees

In 1869, Etienne Leopold Trouvelot, an artist and amateur entomologist, brought some moths from Europe to his home in Massachusetts. Knowing something about this moth, Lymantria dispar, Trouvelot thought he could crossbreed it with the Asian moth, whose caterpillar (or worm, as we refer to it) produces silk, and he would end up with a moth that he could raise in New England that would make silk, and he would break the Asian silk monopoly and become famous and rich. To a small degree, he did become famous. He never became rich.

Trouvelot was not naive, and he knew there were risks in bringing this moth across the ocean and into America. Nonetheless, some of his moths got loose into the wild, and it seems that, in spite of knowing the risks of this moth becoming an invasive pest, the local agriculture authorities were not willing to help in capturing the escapees.

It took only a short 10 years until there were reports of the moth’s spread becoming an issue, with the first major outbreak being reported in 1889. There were reports of the moth’s caterpillars covering houses, defoliating trees and being a generally disgusting nuisance.

The gypsy moth invasion of the Northeast was under way.

The caterpillars, which are black to brown and have blue and red spots on their backs, feed primarily on oak tree foliage; when that’s not available, they’ll feed on fruit trees and a host of other vegetation, but they are partial to the oaks.

The caterpillars feed for a number of weeks and then pupate, or transition into a stage where they find shelter in the crevices of the oak tree bark or in leaf litter and form a dark brown casing around them. In about two weeks, the case splits and a moth emerges, during July and August.

The male moths seek out the females, they mate, then the females lay their eggs in hay- or straw-colored masses on the trunks of oak trees, where they remain until the following spring.

The eggs mature in early spring, and in May tiny caterpillars emerge. These caterpillars can spin tiny silks, and with just a gentle wind the silk acts as a sail and the caterpillar can be carried up to 15 miles away in the breeze. But, more often than not, the movement is fairly local, with populations generally moving about four miles a year.

As the tiny caterpillar grows, it seeks out oak trees, climbs up the nearest foliage and begins to feed. In a matter of just a few weeks, a population of these caterpillars can completely defoliate a mature tree that can be 60 feet tall and 25 feet wide.

Most oaks can tolerate this defoliation, and during the summer it will re-foliate and leaf out all over again. In most cases, the tree recovers. That is, unless it is defoliated two years in a row by a repeated infestation. In this case, the odds are against recovery.

Last year, many areas in New Jersey, New York’s Hudson Valley, some areas of the Hamptons, Connecticut and Massachusetts were stuck with hundreds of thousands of acres of trees being defoliated.

The caterpillars returned this year, but something quite fascinating is taking place in several locations.

During the last week of June, I got a phone call from an arborist telling me that he had just visited a neighborhood in Cortlandt Manor, in Westchester County, where the gypsy moth caterpillars had devastated an entire neighborhood. A few days later, I took my cameras and drone and went for a drive. I drove around reservoirs and country roads with lush forests, save for the dead ash trees and infestations of Oriental bittersweet—but there was no sign of any oak issues.

Then, I took a left-hand turn onto one long cul de sac and, sure enough, not a single oak tree with foliage. Every single tree had been eaten bare. Houses that had been built 30 to 50 years ago, with rolling lawns and 2- to 4-acre lots, were naked to the sky. Just about every property had two or more towering oak trees that were denuded.

Some of these trees had given the nearby residences shade during the day and kept them cool and certainly reduced the need for air conditioning. But not anymore. Landscapes that were once shaded were now in full sun. Lawns that once thrived in dappled or high shade were now parched, as the filtering of the oak leaves was no longer providing a scintilla of shade.

I got my equipment out and began to walk through the neighborhood—but something was missing. Usually, in a defoliation event like this, you’d expect to see caterpillars falling from the trees and meandering across driveways and sidewalks. But not a single one.

A closer examination of the tree trunks revealed hundreds upon hundreds of caterpillars, but all were dead and contorted, as if they had been tortured by something. In about 10 minutes of looking, I found only a single living caterpillar.

There were none of the yellow cards that are required when a property is sprayed by a certified and licensed pesticide applicator, and the arborist who had told me about this outbreak said his company had done no spraying.

So I did some detective work. Remember the “plant detective” column of a few weeks ago?

Turns out the same thing was being observed in Connecticut, Massachusetts, New Jersey and, yes, on Long Island. I now knew what was going on. And regular readers also should have a clue.

In 1908, researchers were dissecting dead gypsy moths and discovered that a high number of them had been killed by an entomophthiralean (insect) fungus. They propagated this fungus and released it in the Boston area in 1910 and 1911. In 1912, they determined that the fungus never became established, and that was that.

At least until the early 1980s, when a new species of the fungus was brought in from Japan, with test releases in New York in 1985. According to some documented reports, it turns out that the fungus has been unwittingly spread on the soles of the shoes of hikers and is now present in the Northeast, where the spores can persist for up to a dozen years.

But three things need to be present at the same time for the fungus to become active: the spores have to be present, gypsy moth caterpillars have to be present, and there needs to be ample moisture for the spores to reproduce.

Well, this year it seems that all three elements came together at the same time and have resulted in a fairly wide mortality of the gypsy moth caterpillars. Seems we won one this time.

So, there is reason for optimism. We should see a great reduction in the gypsy moth populations going forward, but localized outbreaks can still take place. During dry springs, these outbreaks can be quite large, like last year.

In the meantime, property owners in the Hamptons, where oaks are a very important part of the landscape, should become familiar with what the caterpillars look like, with what the egg masses look like, and with non-chemical control measures that can be taken to reduce and eliminate this pest.

Gypsy moth egg masses can easily be removed from trees, as long as you know where and when to look for them, and what they look like. The best time to do this is late fall into late winter. BT, or bacillus thuringiensis, can be applied as a spray, but the caterpillars have to ingest it while feeding.

The use of broad spectrum chemical sprays have been used, but these are very destructive to other beneficial insects, and their use to control this invasive species is totally unnecessary and bordering on unethical.

Keep growing!