Lymantria dispar - LDD (formerly gypsy moth) in Rhode Island


Lymantria dispar (LDD) and their larvae (caterpillars) are always present in the environment. Usually unnoticed and causing little concern, their numbers are controlled by a naturally occurring virus, Nucleopolyhedrosis (NPV), and fungus, Entomophaga maimaiga (Entomophaga), which typically kills the majority of caterpillars before they reach adulthood.

Beginning in mid-late April and continuing through late June, the caterpillars will consume leaves of many species of broadleaf trees and shrubs – such as oaks, aspen, apple, speckled alder, basswood, gray and river birch, and willow. Less desired but still attacked are maple, black, yellow, and paper birch, cherry, cottonwood, elm, black gum, hickory, hornbeam, larch and sassafras. Older LDD moths larvae devour foliage of several species that younger larvae normally avoid, such as hemlock, and pines and spruces native to the East.

LDD caterpillars avoid ash, balsam fir, butternut, black walnut, catalpa, red cedar, flowering dogwood, American holly, locust, sycamore, and yellow or tulip poplar, and shrubs such as mountain laurel, rhododendron, and arborvitae. Also consumed, but not preferred are needles of conifers – such as pines, spruces, and firs. Feeding usually occurs during night hours, but when populations are dense, caterpillars feed continuously day and night until the foliage of the host tree is stripped. They will then crawl to nearby trees in search of food.

LDD numbers can increase dramatically, resulting in an “outbreak” that can cause severe defoliation of tree canopies, such as was experienced throughout Rhode Island in 2015 through 2017. While the exact cause of that outbreak was unknown, it is believed that successive years of warm, dry, spring weather was not conducive to the growth and spread of NPV or Entomophaga. This resulted in an increase in the number caterpillars surviving to adulthood (moth stage), and therefore a large number of eggs masses for successive generations. A significant caterpillar infestation in 2017, caused extensive tree mortality throughout the State. It wasn’t until the return of near normal rainfall in the Spring of both 2017 and 2018 that NPV and Entomophaga were once again able to gain the upper hand, causing the LDD moth population to collapse to pre-outbreak levels.