Gypsy-moth caterpillars dropping like flies due to killer fungus and virus

Gypsy moth caterpillars, which earlier this year seemed on their way to mercilessly munch their way through our tree canopies in the second year of the worst outbreak since the early 1980s, have instead been dying in huge numbers in recent weeks, UMass Amherst's Center for Agriculture, Food and the Environment reports.

A fungus and a virus that seem to target the caterpillars are the cause - spurred in part by the return of normal rainfall levels after a couple of years of drought that suppressed them, but not their crawling target.

For example, a population of gypsy moth caterpillars was observed to be “otherwise healthy” and feeding on a crabapple at a location in Amherst, MA on Sunday, 6/18/2017. (They had already done a number on nearby oaks in the area.) By Friday, 6/23/17, the caterpillars were either shriveled up and hanging vertically from the trunk and branches of the tree (may be indicative of Entomophaga maimaiga) or juicy and drooping in this location in an inverted-V shape, which may be mortality caused by the NPV virus that kills gypsy moth. (The only way to truly tell which pathogen is responsible is to look at samples from the dead caterpillars under a microscope.)

The good news for trees and people sensitive to the caterpillars or just grossed out at the sound of caterpillar excrement falling from trees comes too late for many woods, particularly in the western part of the state, that were already denuded by the bugs, the center reports - adding there are still a lot of the caterpillars out there, chomping away. But there's always next year:

Just as we may be able to credit (mostly) the drought conditions experienced in 2015 and 2016 with the recent increase in the gypsy moth population (through the impediment of Entomophaga maimaiga activity), we can credit the more normal rainfall events seen in May and June of 2017 for the success of the fungus this year. Although we have been waiting for this with much anticipation and wish the caterpillar collapse occurred sooner, it is certainly better than the alternative. Hopefully, this outbreak of these pathogens in this year’s gypsy moth population will mean a reprieve for many from this insect in 2018. The Elkinton Lab reports that fungal activity was particularly pronounced on Cape Cod and the South Shore, and it seems that in many other areas across the state, the fungus has kicked in just in time.

H/t Dave Atkins.