Factors affecting female-biased sex ratio in a trap-nesting wasp, Trypoxylon malaisei

We examined the female-biased sex ratio of a trap-nesting wasp Trypoxylon m alaisei considering the following factors: (1) local mate competition (LMC) , (2) resource quality, (3) partial bivoltinism, and (4) presence of constr ained females. The sex ratio (expressed as male ratio) at emergence was str ongly female biased, i.e., 0.30 and 0.19, in terms of the number and invest ment, respectively. To evaluate the primary sex ratio, we analyzed the data from nests where all the offspring successfully emerged, excluding nests c omposed of single-sex offspring. The primary sex ratio was also female bias ed, at 0.33 and 0.21, in terms of the number and investment, respectively. LMC was highly responsible for the female-biased sex ratio because both the nonrandom oviposition sequence [females at inner cells and male(s) at oute r cells] and earlier emergence of males allowed sib-matings to occur. In co ntrast, the other three factors little affected the female-biased sex ratio : the sex ratio was fairly constant when resource quality (nest size) varie d, partial bivoltinism was extremely rare or absent, and constrained female s were absent or did not reproduce at all.