Sex ratio variation in sawflies.
The ecological and evolutionary implications of sex ratio bias for Symphyta are discussed in this chapter. Three hypotheses where unequal returns on paternal investment in male and female offspring could result in a departure from 1:1 sex ratio are discussed: sex ratios when host size or plant quality vary or when mating is nonrandom. The literature is reviewed and the sex ratios of 22 tenthredinids, 18 diprionids and an argid are reported, with sex ratio, sample size and factors determining sex ratio being recorded. The direct causes of sawfly sex ratio variation are outlined, including variation in primary sex ratios due to obligate parthenogenesis, unmated females, parental sex ratio control, environmental sex determination and differential cost of male and female egg production, and variation in secondary sex ratios due to differential mortality caused by male haploidy and life history traits. The adaptive significance of sex ratio variation for Symphyta is examined in relation to plant quality and nonrandom mating. Finally, a model predicting sawfly extinction and population growth under different sex ratio regmies is presented.