Density-related mortality in Cameraria hamadryadella (Lepidoptera: Gracillariidae) at epidemic and endemic densities

To determine the role of natural enemies and host-plant quality in the population dynamics of Cameraria hamadryadella. we examined survival patterns and causes of mortality of a single population of C. hamadryadella during the epidemic vcars of 1982-1984 and the endemic years of 198,S-1990. C hamadryadella is a leat-mining microlepidopteran whose primary host is white oak, Quercus alba. Populations of C. hamadryadella range in density over four orders of magnitude from endemic densities of 0.004 individuals per leaf to outbreak densities of 22.79 per leaf. In an epidemic year (1984). 53.5% of the individuals studied died because of "other causes" which we contend is a combination of host resistance and intraspecific competition. Parasitism and predatit)n accounted for 21.2"o and 24.7"o ot' total mortality, respectively, and only 0.6"o of the cohort survived. Survival was signifieantly greater in the endemic year (1990). and death by "other causes" was also 29.2% lower in the endemic year. The reduction in mortality due to other causes in the endemic year is due to lower rates of starvation, and to reduced intraspecific competion. However, mortality from starvation, parasitism, and predation did not differ between epidemic and endemic years indicating a lack of temporal densitydependence. During the epidemic, only mortality caused by host and intraspecific competition was spatially positively density-dependent on the scale of individual leaves. The overall impact of parasitoids was density-independent, and the effect of predators was signifieantly inversely density-dependent on the leaf scale. Our results suggest that natural enemies are not responsible for the decline in abundance of C. hamadryadella observed between 1984 and 1990. However, starvation possibly arising from host resistance and. secondarily, intraspecific competition could account for the observed population crash.