gypsy moths
Lymantria dispar
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An illustration of a few life stages
DESCRIPTION Gypsy moth caterpillars and tent caterpillars are sometimes confused. The gypsy moth caterpillar is identified by five pairs of blue spots and six pairs of red spots (think of a gypsy wearing bright red). It is not native to the North America. It was introduced from Eurasia in the 1860s and has been spreading out ever since. Most females do not fly, so they lay eggs where ever they pupated or crawled. This is usually on tree trunks and branches, but it can be other places. The egg is the stage that overwinters and the tan fuzzy covering can contain a hundred to over a thousand eggs. The larvae hatch while trees are budding during the two months of spring. They can disperse by hanging from their windblown thread or be transported with the help of humans moving items with egg clusters on them. As the eat, they moult, increasing in size with each instar. If the population is very high, they eat night and day, strip trees and crawl to another tree.
The caterpillars pupate in mid-June through early July. This can take 1 to 2 weeks. The brown males hatch first and start flying in search of pheromone emitting mates. After the females lay their eggs, they and the males die.
HOST PLANTS This list is long. It includes alder, aspen, gray and white birch, hawthorn, larch, linden, mountain ash, oaks, Lombardy poplar, willows, and witch-hazel. The older instars will feed on beech, red cedar, chestnut, hemlock, plum, pine, and Colorado blue spruce.
Over 50% removal of a trees foliage is considered heavy defoliation. This may kill a hemlock, pine or spruce. Deciduous trees will probably sprout new leaves in August, but this is very taxing on the tree and weakens its health. Two or more years of this can result in mortality. Poor health attracts other pests and diseases.
CONTROL Planting trees not eaten by the caterpillars is an organic way to avoid tree mortality. This list includes tuliptree (yellow poplar), arborvitae, ash, balsam fir, catalpa, true cedars, dogwood, holly, honeylocust, horsechestnut, black locust, mountain laurel, mulberry, and sycamore.
Entomophaga maimaiga is a fungal pathogen that can have an effect on keeping the caterpillar populations at bay during a wet spring. Ground beetles, white-footed mice and birds are all natural predators. There are also some introduced species of fly and wasp parasitoids of the gypsy moth that help to keep populations low over time. Overpopulation and reduced food availability tend to cause stress which helps a virus called 'wilt' bring about high mortality rates.
Insecticides can be used to manage this pest if timed after most have hatched but before serious defoliation. The young instars chew small holes in the leaves as opposed to the older ones that start chewing the edges of leaves. A professional application is advised because of the danger of the chemicals and the difficulty of spraying all the way to the tops of trees where the young ones are.