Host choice and offspring sex allocation in the aphid parasitoid Aphelinus abdominalis (Hymenoptera: Aphelinidae)

The influence of host size on proportion of male and female offspring of the polyphagous aphid parasitoid Aphelinus abdominalis (Dalman) was investigated. The proportion of males decreased with increasing mummy weight. This was due to preferences of mated females which, given a choice, tended to allocate male eggs to small hosts and female eggs to large hosts. Male larvae also had a tendency to kill the host at an earlier developmental stage than female larvae of the parasitoid. Size-dependent sex allocation was similar in four host aphids: potato aphid, Macrosiphum euphorbiae (Thomas); Metopolophium dirhodum (Walker); English grain aphid, Sitobion avenae (F.); and bird cherry-oat aphid, Rhopalosiphum padi (L.). The pattern of sex allocation varied among the different-sized aphid species, but regardless of their size, parasitoid females preferred to lay male eggs in smaller hosts and female eggs in larger hosts, and host size corresponding to a 1:1 sex ratio was near the median size (except for small R. padi). Females constrained life-long to hosts of small size increased their sex ratio during the period of oviposition. Virgin females produced only male eggs and altered their host size preferences during the course of the oviposition period. Initially, they chose small hosts, but gradually allocated more eggs into large hosts.