Life-history and sex allocation in Trypoxylon (syn. Trypargilum) lactitarse (Hymenoptera; Crabronidae)

Abstract

Hymenopterans have become a model for the study of factors that govern sex allocation. In 1983, Seger proposed a model to study Sphecidae wasps with a strong prediction that sex ratio for univoltine wasps should be 1 : 1 (female : male), and for partially bivoltine species it should be male-biased. The present study investigates if Trypoxylon lactitarse (Saussure, 1867) is a univoltine or a bivoltine species and if Seger's model prediction fits the pattern of sex ratio found in this species. The study was carried out at Parque Municipal das Araucárias, in the municipality of Guarapuava, state of Paraná, southern Brazil, from December 2001 to December 2004. Nests of T. lactitarse were obtained using trap-nests drilled longitudinally to a depth to 80 mm with 7.0, 10.0 and 13.0 mm opening diameter. They were placed in a very heterogeneous site filled with araucaria forests, swamps and grasslands. Trypoxylon lactitarse showed two alternative life histories: either they pupated immediately and emerged as adults later in the same season (direct development), or they entered diapause, overwintering and pupating the following spring (delayed development). Its annual sex ratios were not significantly different from 1 : 1, and both sex ratio of overwintering and sex ratio of direct-developing wasps were also not significantly different from 1 : 1, in all years of this study. By examining these results, it was possible to conclude that although T. lactitarse is a multivoltine species, with four generations per year and two alternative life histories, its sex ratio did not support Seger's model.