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Southern New England Trees Get Help from a Fungus and a Virus -- and Rain

This devastation shows how thoroughly gypsy moth caterpillars stripped these Michigan oak trees in June 2007. Photo Credit: Trees eaten by Gypsy Moth Caterpillars by ellenm1. Licensed under CC BY-NC 2.0, via Flickr.

Southern New England, now in its second year of a severe gypsy moth outbreak, is finally seeing some relief, thanks to a fungus that went dormant during an extended drought in the region and a virus that spreads like the common cold.

The caterpillar phase of the gypsy moth (Lymantria dispar) is one of the most destructive pests to hardword forests in eastern North America. Their preferred food is the leaf of the white oak, but they will also devour leaves from other oak, maple, hickory, birch, poplar, willow, apple, and hawthorn trees and several species of flowering shrubs, and then will move on to conifers such as pine and spruce once they've eaten everything else in sight.

Gypsy moths are an invasive insect species introduced to the United States from France in the late 1860s by E.L. Trouvelot, an artist and scientist, who brought the species to his Massachusetts home to study whether they might be useful for creating local silk production. He was breeding them in trees in his backyard when some of the blue and red spotted caterpillars escaped. The first North American outbreak happened in his town, Medford, in the early 1880s. Outbreaks occur periodically, with particularly bad cases in 1971, 1981, and the past two years. Last year deforestation in Massachusetts alone exceeded 350,000 acres, and the lack of leaves was noticeable in satellite photos taken from space. This spring, researchers estimate that 1.3 trillion caterpillars hatched in the region.


Photo Credit: Gypsy moth caterpillar by GollyGForce - Living My Worst Nightmare. Licensed under CC BY 2.0, via Flickr.

Scientists theorize that cycles of drought are tied to increased numbers of gypsy moths, which are controlled naturally by predators, such as the white-footed mouse and the nuclear polyhedrosis virus (NPV), which lives in soil. In years when seeds and nuts are not abundant to support the white-footed mouse population, such as when there has been little rainfall, their population shrinks and, in subsequent years, gypsy moths' grow. NPV, which can be killed off by exposure to high temperatures and UV light, would also seem to be less prevalent in drought years. These conditions allow the gypsy moths to flourish.

Southern New England, which had suffered a drought over the past three years, has had significant rainfall this spring, which helped to trigger the spores of a fungus (Entomophaga maimaiga) that can lie dormant in the top layer of soil for a dozen years.

In the early 1900s, after failing to control the gypsy moth population through traditional means, New England researchers learned that gypsy moths in Japan died after exposure to a fungus. In 1910 and 1911, they released this Maimaiga fungus in the region, but a lack of results made them think it had been a failure. However, in 1989, scientists found the fungus present in dead caterpillars. (They have not yet determined why there was such a lag between introduction and quantifiable results.)

Caterpillars pick up the Maimaiga fungus from the soil and climb back up trees, but ultimately die. The fungus spreads through their dead bodies and can infect live caterpillars nearby. As the population dwindles thanks to the fungus, the NPV also has a chance to reassert itself against the caterpillar. The virus is triggered by stomach acids, so if caterpillars consume something with NPV on it, the virus will infect the stomach cells, spread to the blood, and then kill the caterpillar. The virus is then present in any bodily fluid released by the decomposing corpses, which might then be spread onto the food source of other living caterpillars.

People in New England are reporting that thousands of caterpillars have suddenly died in the last two weeks or so, providing anecdotal evidence that the fungus and virus have finally started affecting the gypsy moth population in significant numbers. Scientists are hopeful this will reduce the numbers of moths that emerge later this summer to mate and lay eggs and that the gypsy moth population will shrink again next spring.

Many of the trees attacked by the gypsy moths can withstand a season or two of deforestation, but having all their leaves or needles eaten does make them more vulnerable to secondary attacks by other pests. Trees that do not make a comeback also increase the risk of forest fires in the region. Trees are not the only organisms affected by the gypsy moth: airborne caterpillar feces, or frass, and hairs can irritate people when inhaled or when they touch the skin.

Want to learn about another moth outbreak? Check out this interactive, based on the classic story of evolution by natural selection—the story of the peppered moths in England during the Industrial Revolution. Check out the Giant Leopard Moth, which can grow to the size of your fist, and the Hummingbird Hawkmoth. You can also learn about caterpillars that trick ants, attack tomatoes (which fight back), and eat tobacco. Then find out more about the life cycle of a butterfly (and how a moth's differs).

More concerned about the trees than the moths? Gypsy moths aren't the only threat to trees. Find out about the forest habitat, and see why a threat to the northeast's forests is a big problem for the country. Hear about the effects of groundwater pumping on the health of riverbank ecosystems and how deforestation can lead to drought. Learn why squirrels moving out of the forest has been bad, as is the disappearance of large predators. Find out about the unintended consequences of military invasions on a region's natural environment and what happens when trees get stressed.

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