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New Mexico State University

New Mexico State University

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Tent Caterpillars Defoliate Aspen

LAS CRUCES -- A leaf-eating caterpillar has defoliated thousands of acres of aspens in the Santa Fe national forest and may kill a number of the trees, said a New Mexico State University forest entomologist.

"We're in the sixth year of an outbreak of the western forest tent caterpillar, and it's probably going to cause significant aspen loss this year," said Bob Cain of NMSU's Cooperative Extension Service.

No control measures will be taken to eliminate the insects. "While an insect population explosion like this one can be very dramatic, it is a natural occurrence in the forest ecosystem," Cain said. "Letting nature take its course is the best way to repress this pest."

"Caterpillar numbers will rise, until disease and parasites build up in the population, causing the population to crash," Cain said. "Also, overcrowding will weaken the caterpillars and lead to starvation."

Although the caterpillar problem has existed since 1990, this year's defoliation will be especially stressful because of the dry conditions in the forest. Leaves often develop later in the summer, after an attack by the caterpillars. But this year, with so little moisture, the trees will have a harder time regenerating foliage, Cain said.

Weather conditions also can affect the situation. Last year's cool spring temperatures kept the trees from leafing out until mid-June, and a foliar disease in late summer caused an early leaf drop, Cain said.

A fungus called Cytospora also is attacking and killing aspens this year. The insects and drought have weakened some older trees to the point that they can no longer resist the fungal disease, he said.

The tent caterpillars have completely devoured the aspen leaves, leaving mountainsides above the capitol city colored gray, rather than their usual vibrant green. The leafless trees are filled with dirty-white tents and their branches and trunks are covered with silken cocoons.

"The Santa Fe ski basin road is covered with caterpillars," Cain said. "The forest floor is layered with them, and the little stream running through the area is chock full of their bodies. There's so many caterpillars up there eating the foliage, that the pitter-patter of their droppings hitting the ground sounds like rain."

Forest tent caterpillar outbreaks occur fairly regularly in New Mexico. "While outbreaks throughout the rest of the United States don't seem to cause much damage, these eruptions in the Southwest seem to last a little longer and destroy more aspen," Cain said.