Why are Gypsy moth caterpillars dying?

I live in central Pennsylvania and gypsy moth caterpillars have been very evident this summer. Today I noticed numerous caterpillars on the trunks of our many oaks. When I went to collect them I discovered that many were desiccated, some broke in two when I touched them, and others literally exploded when touched. We are in a suburban area and I am sure that no spraying has occurred. Any idea what could cause this welcome development?


There should be parasitoids enjoying vast numbers of caterpillars!


They have likely been killed by Entomophaga maimaga fungus, one of the introduced control agents for gypsy moths: https://www.canr.msu.edu/ipm/Invasive_species/Gypsy-Moth/virus-and-fungus-disease-cause


Likely some sort of disease/parasite mix. As moth/caterpillar populations increase, so do the parasitoids and pathogens. When the population crashes from that, the cycle starts over again as the parasitoids/pathogens have no host, so then they die off. With most moths, it is about a 7 year cycle. BTW, the exploding ones were likely caused by a virus! The larvae turn into bags of black goo.


I used to work in the Tuscarora State Forest in south-central Pennsylvania. Gypsy moths there were infected with a polyhedrosis virus (https://en.wikipedia.org/wiki/Lymantria_dispar_multicapsid_nuclear_polyhedrosis_virus). It was quite dramatic as the population munched its way toward final instars but then the caterpillars would hang themselves onto tree trunks, liquefy, collapse into a V shape and then dry into powder. Biocontrol at its finest!


wow so cool! I remember seeing hundreds of those V shaped carcasses on the (outside) walls of a place I was staying a few years ago—I had assumed they’d sprayed something beforehand. not sure how I never was aware of this


Great information. Thanks much.

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