Late-night snacking of the black-dotted brown moth

by | Jul 17, 2013

An oak tree with its bark stripped by animals eating the black-dotted brown moth caterpillars.

In the spring and early summer of 2012, an event was taking place throughout the Piedmont. The bark of oak trees was being stripped off, and no one knew why. There are some tree diseases that can cause the bark of trees to slough off, but this appeared different. After much speculation, observers began to notice squirrels and birds, such as woodpeckers and nuthatches, popping the bark off of trees to expose and devour hiding caterpillars.
The birds and squirrels were going nuts because there was an unprecedented outbreak of black-dotted brown moth caterpillars. To the animals, this was an unlimited buffet, and not even the bark of trees could deter the feasting. This was also going on in parts of South Carolina, Georgia, Alabama and Virginia.
One of the reasons the caterpillars were not immediately identified as the culprit was because of their stealthy behavior. Caterpillars hide under the bark during the day, but when darkness falls, they ascend the trunk of the tree to feed on the leaves of oaks. One-time defoliation of hardwood trees usually does not cause long-term health consequences.

Black-dotted brown moth larvae, hiding under the bark (left) and in the palm of Mark Bost, NCFS (right).

While this late-night munching on oak leaves was certainly mysterious, the caterpillars are not always so secretive. They flock to lights at night, leave innumerable droppings (called frass) beneath treesĀ  and even venture into homes. Because of this, many people consider them pests, not just because they were damaging trees, but because they were annoying to live with.
But why would a native species, never known to cause an outbreak before, suddenly cause widespread defoliation of oak trees? In a recent publication in American Entomologist, Dave Coyle of the University of Georgia and his colleagues shed light on the sudden population explosion and rapid oak defoliation, hypothesizing that recent drought and an unusually warm winter last year added to the epidemic.
Thankfully, this year, an outbreak of the moth caterpillars did not occur, likely due to a cooler spring.