The relationship of provision weight to adult weight and sex ratio in the solitary bee, Ceratina calcarata

The female of the solitary bee Ceratina calcarata (Robertson) (Hymenoptera: Anthophoridae) excavates a tunnel in a pithy twig and then constructs and provisions a linear series of brood cells that make up her nest. Adult females are, on the average, 1.3 times heavier than the males, a significant difference ( P <0.001). There is no difference between the sexes in the amount of weight gained per unit of larval food. Larger females occur because their provision masses are, on the average, 1.3 times heavier than male‐producing provision masses, a significant difference ( P <0.001). Because mothers invest more time and energy in their daughters, Fisher's theory predicts that they should produce more sons. When available resources are fewer in a given year as reflected in lighter provision masses, more males are produced during the year. The observed sex ratio did not differ significantly from the expected, calculated as mean female weight/mean male weight and was male‐biased. Unlike species which nest in pre‐formed tunnels, the sex of any brood cell except the innermost is random with respect to that cell's position in the nest and the tunnel's depth and diameter. The innermost position contained offspring with a female biased sex ratio ( P <0.005).