Sex ratio and maternal investment in the multivoltine large carpenter bee Xylocopa sulcatipes (Apoideh: Anthophoridae)

Abstract

Abstract. 1. Females of the multivoltine carpenter bee Xylocopa sulcutipes (Maa) (Hymenoptera: Anthophoridae) usually excavate a straight tunnel in dead twigs and mass provision a linear array of up to ten brood cells with pollen and nectar. An egg is deposited upon each food mass within one cell.2. Female offspring generally receive a higher provisioning mass (0.180 ± 0.048 g) than males, a significant difference (P > 0.001). There are, however, male larvae that receive as much food or more as their sisters or female larvae reared in another nest.3. There is a close positive association between the size of a mother and the weight of provisions for individual daughters, but not for sons.4. Female offspring are positioned in the innermost brood cells (Gositions 1, 2 and 3). The sex ratio of the outer cells is either significantly male biased (positions 4–6) or skewed towards males (positions 8 and 9). Positions 7 and 10 are in equilibrium.5. Solitary females produce a significantly female biased sex ratio (P < 0.01). Sex ratio in social nests is skewed toward females, but not significantly so (P < 0.2). There is no significant difference between the sex ratio of solitary and social nests (P= 0.361). The population sex ratio (pooled sex ratio of all broods produced) is significantly female biased (P= 0.003).6. Females kept in the laboratory produced female biased sex ratios whilst unmated females produced all‐male broods indicating that insemination and ovarian development are not causally related.7. The expected sex ratio (ESR) under equal investment, calculated as 1/CR (CR = mean male provision weight/mean female provision weight), is 137.5:117.5 (males:females), and differs significantly from that observed, 104:151 (males:females) (P < 0.001). The ‘Local Resource Enhlancement’ hypothesis best explains the female biased sex ratio found in X.sulcatipes and its maintenance in the population.