Use of a Secondary Host by Non‐Outbreak Populations of the Gypsy Moth

Oak species are the favored host of gypsy moth populations in the northeastern United state, although the herbivore expands its host range dramatically during an outbreak. Pitch pine, a secondary host because of its unacceptability for early development, was found to be frequently used for oviposition in oak—pitch pine forests with non—outbreak populations. This observation led to the study of ecological and behavioral factors that can contribute to the use of a secondary host under low—density conditions by an irruptive herbivore species. A series of manipulative field and laboratory experiments plus a study of natural history provided data on the pattern of pitch pine use during the life cycle of the gypsy moth, the effect of pitch pine on larval growth, and the differential impact of natural enemies depending on host use. I found that (1) egg masses occurred far more frequently on pitch pine than was expected based on the frequency of pitch pine in forests with low—density gypsy moth populations; (2) in the laboratory, early—instar larvae could not survive on pitch pine while late—instar larvae grew well; (3) in the field, larvae began to use pitch pine to feed and rest after the onset of the fourth instar. Compared to oak, (4) egg masses on pitch pine experienced less parasitism; (5) the microhabitat of pitch pine held less nuclear polyhedrosis virus (NPV), a major mortality agent of the gypsy moth; (6) individuals hatching from eggs laid on pitch pine were less infected with NPV; and (7) larvae dosed with a known amount of NPV survived longer when feeding on pitch pine foliage. The use of pitch pine by individuals in low—density gypsy moth populations appeared to be beneficial and may have an important effect on population dynamics. The mobility associated with host switching by late—instar larvae and with dispersal by first—instar larvae oviposited on unacceptable food may represent an important mechanism for host—range extension.