Abstract
In the Hymenoptera, female-biased sex ratios are often attributed to Local Mate Competition (LMC), which occurs when related males compete for access to the same mates, reducing fitness returns for male offspring relative to females. In social species, advantageous interactions between cooperating females can also promote female bias via Local Resource Enhancement (LRE). Despite recent theory suggesting potential interactions between these phenomena, studies of LMC in social species remain scarce. This study investigates female-biased sex ratio adjustment in the silk wasp Microstigmus rosae , a species exhibiting both incipient sociality and likely localised mating. Our analyses reveal several genetic indicators of local mating, including heterozygote deficiency and high inbreeding coefficients. Moreover, pairwise relatedness values between mothers and their inferred mates indicate that 35% of mating pairs were siblings or parent-offspring. In further support of the role of LMC, we find that our observed sex investment ratio of 0.35 is statistically equivalent to the theoretically expected evolutionary stable sex ratio, as predicted by partial LMC. We also find a clearly defined male-first sequential sex allocation strategy (M-F-F), a pattern typically attributed to extreme LMC. In contrast, we find no perceivable effect of cooperative behaviour (LRE) on the brood sex ratio. This rare demonstration of inbreeding and LMC in a social aculeate hymenopteran highlights the potential role of these factors in driving the evolution of social group formation in this species.