Host-size-dependent sex ratio theory and improving mass-reared parasitoid sex ratios
2002
Ode, P.J. | Heinz, K.M.
Although an effective parasitoid of agromyzid leafminers, Diglyphus isaea (Walker) (Hymenoptera: Eulophidae) is an expensive biological control agent in terms of production costs. In part, these costs arise from the production of male-biased offspring sex ratios. Here, we present a mass-rearing technique that will increase the proportion of females produced and reduce the need for frequent releases in biocontrol programs. By presenting female D. isaea groups of sequentially larger leafminer hosts to attack, we are able to generate progressively more female-biased sex ratios. After three days of providing increasingly larger hosts, we were able to reduce the sex ratios produced by individual females from 57% male to 36% male; sex ratios produced by groups of females dropped from 64% male to 45% male. Several attributes of D. isaea sex allocation allow us to manipulate sex allocation behavior. First, D. isaea is a solitary idiobiont; resources available to each offspring are present at the time of attack allowing the ovipositing fema1e to accurately assess host quality. Host size positively affects both male and female wasps. Females laid more daughters in larger hosts and more sons in smaller hosts. We show that the observed relationship between host size and offspring sex ratio is due to maternal sex allocation decisions rather than differential mortality. Furthermore, assessment of the size threshold was relative to prior host encounters rather than an absolute assessment. Our simple memory model suggests that while females are influenced most strongly by recent encounters, females also base their assessment of the host-size threshold on prior host encounters.
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