Females leave home for sex: natal dispersal in a parasitoid with complementary sex determination

The detrimental fitness effects of inbreeding are exacerbated in hymenopterans with single-locus complementary sex determination (sl-CSD) such as the parasitoid wasp Cotesia glomerata L. (Hymenoptera: Braconidae). Under sl-CSD, haploids are male, whereas diploids are female when heterozygous at the sex determination locus, but male when homozygous. Inbreeding leads to increased diploid male production. As diploid males can be of inferior fitness and are produced at the expense of females, species with sl-CSD are expected to avoid inbreeding. Natal dispersal of one or both sexes reduces the incidence of sibling matings. However, inbreeding avoidance is not necessarily the unique trigger of natal dispersal. Other factors, such as habitat variability and kin competition can also promote natal dispersal. Dispersal can be split into three stages, emigration, movement between patches and immigration, and revealing the ultimate causes of natal dispersal requires studying the proximate causes affecting these different stages. We monitored the postnatal behaviour of C. glomerata males and females in an experimental habitat that allowed us to track all three stages of dispersal and to manipulate the proximate factors triggering dispersal. None of the tested males, but more than one-third of the females dispersed when intersexual kin interactions were the only driving force of natal dispersal. Furthermore, we provide evidence that females dispersed to a new patch in search of mating opportunities. Our results demonstrate that inbreeding avoidance is the major cause of female natal dispersal in C. glomerata.