Effects of host age and size on clutch size and sex ratio of Oomyzus sokolowskii(Hymenoptera: Eulophidae), a larval-pupal parasitoid of Plutella xylostella(Lepidoptera: Yponomeutidae)

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We investigated the effects of host age and size on clutch size and sex ratio of Qomyzus sokolowskii (Kurdjumov) (Hymenoptera: Eulophidae). The number of eggs laid (primary clutch size) tended to increase with host age and was significantly larger for the late fourth-stadium larvae than for the second- and third-stadium larvae. The number of wasps that emerged from a host (secondary clutch size) tended to increase with host age and was significantly larger for the late fourth-stadium hosts than for the second-stadium hosts. In the late fourth-stadium hosts, secondary clutch size tended to increase significantly with host size. There were no significant differences in sex ratios among host ages or sizes. Where fewer than 10 wasps emerged from a host, most clutches produced only 1 male; larger clusters tended to produce 2 or more males. This implies that O. soko/owskii may deposit male eggs in the early eggs of a clutch and place the second male eggs in the tenth or later eggs of a clutch. By this mechanism, 0. soko/owskii is thought to produce an invariable sex ratio regardless of clutch size.
2002-05-25