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Abstract
Density dependent host mortality in the interaction between the solitary endoparasitoid Aphidius colemani (Aphidiidae), and its host, the green peach aphid Myzus persicae, was examined on greenhouse peppers. The experimental approach attempted to eliminate spatial interdependence in the relationship between host density and host mortality by using different plants to measure parasitism at different spatial scales. Increasing host density at the plant scale caused a significant increase in the proportion of host mortality. However, at the shoot and leaf scale, increasing host density caused a significant decrease in host mortality. This may be one of the first experimental demonstrations of a switch from inverse to direct density dependence. The pattern is assumed to be a result of searching parasitoids using different cues at different spatial scales.
Highlights
Host-parasitoid interactions have been of widespread interest in ecology for many years, and their analyses have contributed to the elucidation of mechanisms that underpin population dynamics
Host mortality in the field caused by parasitoid attack may have one of three relationships with density: 1. directly density dependent when the risk of parasitism increases with host density; 2. inversely-density dependent when the risk decreases with increasing density; 3. density-independent or “domed” when maximum mortality occurs at intermediate densities
The slopes (F = 15.71, d f 2/13, p = 0.0003) and the intercepts (F = 27.90, d f 1/12,p = 0.0002) of regression lines of host mortality on host density significantly differed between the plant scale, and the shoot and leaf scales pooled together
Summary
Host-parasitoid interactions have been of widespread interest in ecology for many years, and their analyses have contributed to the elucidation of mechanisms that underpin population dynamics. Density-independent or “domed” when maximum mortality occurs at intermediate densities How these modes of attack affect the dynamic stability of parasitoid-host systems is still not fully resolved (Mur doch & Stewart-Oaten, 1989; Hochberg & Lawton, 1990; Hassell et al, 1991; Pacala & Hassell, 1991; Godfray & Pacala, 1992; Ives, 1992; Murdoch et al, 1992; Taylor, 1993; Godfray et al, 1994; Rohani et al, 1994; Getz & Mills, 1996). As the searching behaviour of parasitoids is regarded as adaptive, parasi toids should be able to recognise those units of habitat that are more profitable to search than others
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