Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2001
Publication Date: 5/2/2002
Citation: Ode, Paul; Heinz, Kevin. 2002. Host-Size Dependent Sex Ratio Theory and Improving Mass-Reared Parasitoid Sex Ratios. Biological Control 24:31-41. Interpretive Summary: A common problem associated with augmentative biological control programs is the high cost of producing sufficient numbers of natural enemies to suppress pest populations within the time constraints imposed by ephemeral agroecosystems. One such cost in the mass rearing of many parasitic wasps is the overproduction of males. Because only female wasps kill hosts directly via oviposition and/or host feeding, a male-biased wasp populatio will be less effective in controlling a pest population than a similar- sized female-biased population. Overproduction of males (defined as the production of more males than is necessary to mate all females) may limit the economic feasibility of biological control programs relying on mass- reared wasps. Such costs exist in mass rearing of Diglyphus isaea, a commercially available parasitic wasp that attacks economically damaging leafminer flies. Although very effective in suppressing leafminer damage, mass-reared D. isaea populations are frequently very male biased, making the cost of repeated releases of this natural enemy too expensive for many agricultural clientele. We manipulated the sex ratios of D. isaea by steadily increasing the size of hosts that were presented. In this way, we were able to produce more female-biased sex ratios with little increase in mass-rearing costs. The modified mass-rearing protocol we present here will allow growers to pay for fewer releases of D. isaea at significant cost savings, yet still realize a high degree of leafminer control.
Technical Abstract: 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 dreduce the need for frequent releases in biocontrol programs. By presenting D. isaea females groups of sequentially larger leafminer hosts to attack, we are able to generate progressively more female-biased sex ratios. After three d 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 behavior allow us to manipulate sex allocation behavior. First, D. isaea is a solitary idiobiont; resources available to each offspring are present at the time o attack allowing the ovipositing female 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.