Comments: Almost any natural or artificial situation with woody vegetation and a temperate climate can support gypsy moths, but outbreaks rarely occur except in forests where oaks (or other favorite food plants) comprise at least 15 to 25% of the stand (Nichols, 1980), usually higher. Forests comprised of over 50% oaks are especially susceptible to defoliation. The 1995 FEIS regards oak-hickory and oak-pine forest types as the most vulnerable and northern hardwoods as essentially the only immune type in most of the USA. Spruce-fir forests and others that are nearly pure conifers are also immune to the currently established European strain but the Asian gypsy moth is better adapted to conifers. Some immune types such as northern hardwoods support persistent populations but not outbreaks. Oak forests in New England, Pennsylvania and elsewhere were often nearly 100% defoliated for at least one season during the course of each gypsy moth population cycle in the mid and late 20th century and in some cases going back 100 years. In general, the degree of defoliation in peak years is directly correlated with the percentage of oaks and other highly favored species in a stand. Some good early discussions include Gerardi and Grimm (1979) and Mason, and Hicks and Fosbroke, both in Fosbroke and Hicks (1987) but usually the information in the 1995 FEIS or below should suffice. The larvae can feed on over 500 species of plants but avoid most herbs. While one sees statements to the contrary, there are many plant species including trees and shrubs that gypsy moth larvae never or rarely eat. Private operators commonly spray trees for gypsy moth control which the larvae do not normally, or never (e.g. tulip tree), eat.
The following is a summary of the suitability of common trees and shrubs based mostly on the 1995 FEIS, Nichols (1980) and observations by this author (Dale Schweitzer). In general oaks are the most important food plants, with white oaks slightly favored. Other highly favored species include mockernut hickory (Schweitzer), aspens, willows, some birches, basswood, Malus spp., most hawthorns and witch hazel. These will probably be severely defoliated if oaks are. Others that are readily eaten at least by older larvae when they are growing with oaks include beech, sweetgum, most hickories, hemlocks, and blueberries. These and some others like red and sugar maples, white pine, or rarely flowering dogwood may be defoliated in severe outbreaks when growing with oaks. First instars strongly avoid it but older larvae sometimes defoliate sweetgum. Some forest species that will not be defoliated even in severe outbreaks include true ashes, tulip tree, sycamore, magnolias, persimmons, cedars, balsam firs, bald cypress, Lonicera spp., redbud, mountain laurel, some clones of poison ivy. Most, but not all, of these are completely avoided even by starving last instars but the 1995 FEIS Appendix G pages 2-4 suggests some of these may be eaten in extreme circumstances. Black huckleberry at least usually escapes heavy damage.
Appendix D of the 1995 FEIS ranks hundreds of species as susceptible, resistant or immune. Coverage is weak for Ericaceae and some other understory groups but very thorough for trees. Carya ovata and Pinus rigida should be changed from immune to resistant like their congeners. Various sources including the 1995 FEIS Appendix G note defoliation and sometimes mortality of hickories. C. ovata growing with oaks were severely defoliated widely in New Haven County, Connecticut in June 1981. Ridgetop P. rigida can be killed (Schweitzer, pers. obs.; Quimby, 1985). In most other situations though pitch pine is at low risk, e.g., in sandy coastal pinelands. White pine is much more often defoliated than other pines.
So while virtually any kind of woodland, shaded park, backyard, or forest is potential gypsy moth habitat, not all such habitats are at risk for outbreaks. In general risk of heavy defoliation is high in wooded area composed of 50% or more oaks or other highly favored trees. It is in these habitats where gypsy moth outbreaks and efforts to control them are most likely to be potential long-term ecological concerns to biodiversity-oriented managers.