Abstract: To understand whether the bethylid parasitic wasp, Sclerodermus alternatusi Yang, adjusts the sex ratio of their offspring by laying the unfertilized eggs first, the oviposition traits of this parasitoid were investigated in the laboratory. Three treatments of the parasitoid eggs, including removal of the eggs laid in the first day (T₁), reserve of the eggs laid in the first day only (T₂), and reserve of all the eggs laid in all days (CK), were established for the analysis. It was found that only one out of 24 samples did not have male offspring in T₂, while all of the samples in CK had male offspring. In contrast, in T₁, there were male offspring in 15 out of 25 samples, which was significantly higher than those in T₂ and CK. There were a total of 26 males emerged from T₁ or T₂. The mean numbers of male offspring per mother wasp in T₁ (1.04) and T₂(1.08) were significantly lower than that in the CK (1.98). The results indicate that extremely female-biased sex ratio is a general phenomenon in this bethylid wasp, and the unfertilized eggs are preferentially laid first by the mother wasps. Laying unfertilized eggs first can guarantee the female-biased sex ratio of offspring in one clutch and contribute to the efficient and abundant mating for females.

Key words: oviposition sequence, sex allocation, female biased sex ratio, Sclerodermus alternatusi