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Estimating the impact of parasitoids on the dynamics of populations of gypsy moths
Abstract
To estimate the impact of parasitoids, one must be able to accurately measure the mortality they cause. I therefore investigated biases associated with several methods of calculating stage-specific and time-specific parasitism by Cotesia melanoscela, Parasetigena silvestris, and Brachymeria intermedia. I released laboratory-reared gypsy moths into the field to measure the timing of oviposition by parasitoids. I determined the timing of emergence of parasitoids by collecting naturally occurring gypsy moths. I also monitored the timing of host recruitment to and advancement out of the stage(s) that was susceptible to parasitism. I found that many of the methods used in previous studies of gypsy moth dynamics were affected by overlap of these processes and that several methods severely over- or underestimated parasitism. I estimated temperature-dependent development of C. melanoscela for use with the Southwood & Jepson method of calculating the number of parasitoids attacking hosts. I also studied some effects of superparasitism by P. silvestris. The dispersion of eggs of P. silvestris on gypsy moth larvae collected in the field was more aggregated when larvae were collected from under burlap bands. This resulted in lowered percentage parasitism of hosts collected in this manner. In laboratory studies, deposition of more than one egg on a single host significantly increased host mortality, but had a negative effect on survival of the immature parasitoid and the size of the puparium produced. Experimental manipulations of densities of gypsy moths revealed a strong, positive spatially density-dependent reduction in gypsy moth populations. Positive density-dependent mortality occurred during the early and mid larval stages and was primarily due to Compsilura concinnata, a polyphagous parasitoid. Oviposition by P. silvestris, an oligophagous parasitoid, was initially inversely density-dependent, but became positively density-dependent during the late larval period. I conclude that if populations of gypsy moths increase and decrease in density asynchronously on a spatial scale of a few ha, the density-dependent responses of parasitoids could suppress the populations to a point where small mammal predation would be able to prevent population increase. This phenomenon may explain the apparent stability of gypsy moth populations on a region-wide basis for the years between outbreaks.
Subject Area
Entomology
Recommended Citation
Gould, Juli Ruth, "Estimating the impact of parasitoids on the dynamics of populations of gypsy moths" (1990). Doctoral Dissertations Available from Proquest. AAI9101636.
https://scholarworks.umass.edu/dissertations/AAI9101636