Found to feed on hundreds of different species of plants and favoring oaks and aspens, they are a threat to Pennsylvania forests and landscapes.
Recently I was heading out the door on my way to work and notice large frass (insect larvae excrement) on the top rail of my front porch. A closer inspection revealed chewed leaves on the Fothergilla above the railing. A quick look at the shrub revealed just a couple large (bigger than I have ever seen) Gypsy Moth caterpillars.
Frass and chewed leaves evidence of Gypsy Moth Caterpillar. Photo: Kathleen Salisbury
How do I know they were Gypsy Moth caterpillars? Gypsy Moth caterpillars have distinct markings separating them from other caterpillars. In the photo below you will notice the back of the caterpillar covered with dots. Notice how the first 5 pairs of dots on the back of the caterpillar are blue and 6 pairs are red? That is the tell-tale sign of a Gypsy Moth larva.
The distinctive Blue and Red pairs of dots identify this caterpillar as Gypsy Moth Larva. Photo: Kathleen Salisbury
The Gypsy Moth (Lymantria dispar) causes damage to millions of acres of forest yearly, so these introduced insects are worth getting to know. Gypsy Moth larvae can devastate a forest. Found to feed on hundreds of different species of plants and favoring oaks and aspens, they are a threat to Pennsylvania forests and landscapes. Caterpillars are just one stage in the life cycle of the Gypsy Moth. The caterpillar stage seems to capture our attention because of the damage to our trees and it is important to become familiar with the various stages of Gypsy Moth so you can make appropriate control decisions.
Here you see pupal stage (shiny dark brown cylinders), egg masses (tan masses) and Male (dark) and Female (White) adult moths. Photo: Kathleen Salisbury
I hand-picked the larvae from my Fothergilla and dispatched them. Smaller infestations can be handled this way. Larger infestations tend to happen regionally and in cycles. A general rule of thumb is that up to 30% loss of foliage will not cause significant harm to the plants. If there is more than 30% defoliation on trees and shrubs, management options should be considered. Some years there are bumper crops of Gypsy Moths and other years they are more of a nuisance than anything else.
Gypsy Moth Caterpillar on Fothergilla gardenii--a face only a mother could love.This gypsy moth is responsible for large missing sections in the Fothergilla leaves, those smaller holes and discoloration are from 4-lined plant bugs. Photo: Kathleen Salisbury
Small mammals have been found to eat the larvae. Birds tend to stay away because of the spiny hairs that coat the caterpillar body. There are some naturally occurring viruses and fungi that will kill Gypsy Moth larvae and help control the population. Gypsy Moth "wilt" is caused by one such virus. This virus accumulates in plant material which is then ingested into caterpillars as they feed on foliage. When the infected caterpillars die, the virus invades the soil and continues to inoculate plant material. This virus doesn't do much to control small Gypsy Moth populations, but during those population explosions the virus will spread rapidly and help control forest devastation. This naturally occurring virus complex has been extensively researched and has been developed into "Gypchek". Produced by the National Forest Service, Gypchk controls only the caterpillar stage of Gypsy Moth and doesn't effect any other type of caterpillar. Gypchk is used as a biological insecticide through aerial or ground applications. Another naturally occurring fungus--Entomophaga maimaiga--will also control heavy infestations during wet springs.
Gypsy Moth "wilt" caused by a nucleopolyhedrosis virus. Photo: Kathleen Salisbury
Another control option is a naturally occurring fungus - Btk (Bacillus thuringiensis var. kurstaki) can also be used to control Gypsy Moths. Btk works on Gypsy Moth larva and a variety of other butterfly and moth larva. When using Btk, one must be aware of other non-target butterflies and moths in the area. Btk should be applied while the caterpillars are actively feeding as it is a stomach poison and needs to be ingested in order to be effective.
There are also many chemical insecticides labeled for control in a variety of landscape and forest conditions. These should be applied in May after most of the egg masses have hatched and the larvae are still small.
As with all pesticides, be sure to follow all label instructions when applying these products.