2014 Caterpillar Outbreak

Combo, including gypsy moths, defoliates trees

Posted on June 23, 2014 in Science

Hawk Mountain this spring observed a unique outbreak of several defoliating caterpillars, including gypsy moths within the Sanctuary forest. Fall cankerworm was spotted in large numbers in and around Hawk Mountain in mid May and defoliated many oaks. Gypsy moths numbers increasing in late May which expanded the defoliation further. Naturalists also observed high levels of oak leaf roller activity affecting both oak and witch hazel trees, although less numerous than either the cankerworm or gypsy moth. Tent caterpillar has been observed on site, as well.

Together, these different species have caused heavy defoliation in the Hawk Mountain forest with a primary focus on oaks, the favored food of several species.

By mid June, the gypsy moths began exhibiting signs of natural population controls: an infection from the NPV virus that causes the v-shaped droop in their bodies, and a fungal infection that kills them, but the bodies remain linear on trunk (see photo). Large numbers of dead or dying caterpillars now (late June) can be seen clinging to the bottom of tree trunks along the trail.

Hawk Mountain has seen this same crash in late June during past gypsy moth outbreaks and usually most of the trees put out a second round of leaves and survive. Rain in next few weeks will assist trees in surviving this defoliation and the abundance of rain this spring will also help the trees to ward off the stress from defoliation. The biologists at Hawk Mountain have seen swarms of cedar waxwings and common grackles eating the caterpillars. In past years, grackles have swarmed in July, post-breeding, to feast on the abundant protein, so keep an eye out for this behavior.The bigger story is that Hawk Mountain has not used chemical sprays to control gypsy moths since the 1960s and as a result, outbreaks here are typically less severe than elsewhere. State forest lands are rarely sprayed anymore as well, and also rely on natural controls.

The NPV virus, which spreads through gypsy moths when they reach high population numbers, appears to be killing large numbers of gypsy moths this summer. A natural fungus also has been a major component of limiting them here since late 1980s as well. Natural insect predators—wasps, beetles and flies—then take over to attack the remaining numbers and keep the population from growing in subsequent years. The lack of spraying for decades ensures that Hawk Mountain has this healthy population of insect predators that have adapted to prey upon them. In short, by not spraying, the community of natural controls at Hawk Mountain is healthy and the trees here should re-leaf in July which will help most of them survive this outbreak. 
If you observe other caterpillars active in the forest this year or see insect predators at work, please share photos with us by emailing [email protected].

Source: Weiser State Forest, PA Bureau of Forestry foresters, Penn State University extension office; US Forest Service.