EFFECTS OF HOST EGG MASS SIZE ON SEX-RATIO AND OVIPOSITION SEQUENCE OF TRISSOLCUS-BASALIS (HYMENOPTERA, SCELIONIDAE)

This study examines the influence of the size of the host patch (egg m ass size) on the sex ratio produced by the scelionid egg parasitoid Tr issolcus basalis (Wollaston) under laboratory conditions. The influenc e of this factor on the sequence in which male and female eggs are lai d also is analyzed. Our detailed study shows that the size of the host patch offered to females strongly influenced both the overall sex rat io they produced, and the organization of the sequence in which male a nd female eggs are laid. Females adjusted their sex ratio with host nu mber per host egg mass in qualitative agreement with the predictions o f the local mate competition model and independently of oviposition se quence. Variations in the sex allocation sequences produced by T. basa lis do not seem to depend on the ability of female wasps to assess the number of hosts per patch before oviposition. Females start egg-layin g sequences by laying male eggs early in an oviposition bout in accord ance with the males Ist strategy. As long as the sequences are complet ed, the pattern of sex allocation changes as an indirect modification in response to variation in the size of the egg mass. Such a modificat ion in sex-sequence patterns can be considered to be an adaptive mecha nism leading the females to adjust their sex ratio to different enviro nmental conditions.