Primary sex-ratio and differential progeny survivorship in solitary haplo-diploid parasitoids

ABSTRACT.

• 1 A method of partitioning emergence sex-ratio to give estimates of primary sex-ratio and male and female offspring survival from oviposition to emergence in solitary haplo-diploid parasitoids is presented.
• 2 This method is applied to sex-ratio data from a larval parasitoid, Aphidius ervi Haliday, and a pupal parasitoid, Coccygomimus turionella L., parasitizing a range of host types.
• 3 There was no evidence of facultative control of primary sex-ratio in either species. Parasitoid emergence sex-ratios were similar for all host sizes attacked by C. turionella. In contrast, emergence sex-ratios of A.ervi showed a significant male bias in smaller hosts. This shift in emergence sex-ratio was attributable to differential progeny survival.
• 4 Pre-emergence mortality in both species was a function of host size, with few offspring surviving from small hosts. This suggests that host size may be an important component in the dynamics of host-parasitoid interactions.
• 5 The evolution of sex-ratio regulatory mechanisms in solitary haplo-diploid parasitoids is discussed in the context of parasitoid life-history. We suggest that there is a constraint to the evolution of the facultative control of primary sex-ratio in parasitoids attacking larval stages as a result of the uncertainty of future host resource acquisition rates.