Sex ratios in natural populations ofAphelinus mali (Hym.: Aphelinidae) in relation to host size and host density

Abstract

In an apple orchard at Armidale, the Northern Tablelands of NSW, population sex ratios of Aphelinus marl (Haldeman), an endoparasitoid of the woolly apple aphid, Eriosoma lanQerum (Hausmann) varied from 0.51 (proportion of males) at low host densities to female-biased at high host densities (proportion of males ranged from 0.35-0.39). This shift in sex ratio seems to be caused by the differences in allocation of sons and daughters to hosts of different sizes. In the field A. mali parasitizes all life stages (four nymphal instars and adult) of the woolly aphid upon encountering. According to Hughes '(1979) optimal diet model, such general host acceptance seems to be the best strategy. However, it allows the host nymphs or adults to continue to develop or reproduce until about to mummify (pupate). No mortality was observed when first or second-instar hosts were parasitized in the laboratory. Field collected small mummified hosts yielded male-biased sex ratios whereas large mummified hosts produced mainly females. In the laboratory, progeny from smaller hosts (first to third-instars) produced sex ratios which were not significantly different from 0.5 whereas progeny from larger hosts (third and fourth-instars) produced female-biased sex ratio. During winter (June-August) and early spring (September-October) when the host populations in the orchard were predominantly nymphs, the parasitoid tended to allocate equal resources to male and female offspring. In contrast, at peak population densities in summer and autumn (December-May) when larger hosts were available, the sex ratios were female-biased. The host size of E. lanigerum and A. mali is, therefore, an important component in the dynamics of host-parasitoid interactions.

Functional Responses of the European Earwig and Two Species of Coccinellids to Densities of Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae)

“…Since E. lanigerum populations are generally low and comprised of a higher proportion of nymphs of the early instars during the winter and early spring (Asante and Danthanarayana 1993), the present observation suggests that when large numbers of F. auricularia and H. conformis are available in the spring, they would act as important natural control agents. On the other hand, adult P. australasiae appears to be an inefficient predator for E. lanigerum.…”

Sex Ratios of Commercially Reared Biological Control Agents

“…are solitary endoparasitoids of aphids. In the species that have been studied, females are disproportionately produced from large hosts and female-biased sex ratios are produced when sufficient large hosts are available (Raney et al, 1971;Asante and Danthanarayana, 1993;Yokomi and Tang, 1995;Honek et al, 1998a). Honek et al (1998b) also showed that larger parasitized aphids dispersed farther from drought-stressed plants than did smaller parasitized aphids.…”

Comparison of a Canadian and a Dutch strain of the parasitoid Aphelinus mali (Hald) (Hym., Aphelinidae) for control of woolly apple aphid Eriosoma lanigerum (Haussmann) (Hom., Aphididae) in the Netherlands: a simulation approach

“…Aphelinus mali is a solitary, arrhenotokous parasitoid. It lays its eggs inside the aphid where the unfertilized eggs develop into males and fertilized eggs develop into females (M UELLER et al., 1992; A SANTE and D ANTHANARAYANA , 1993). The sex ratio in the field depends on host density and host age structure.…”