Colony structure, provisioning and sex allocation in the sweat bee Halictus ligatus (Hymenoptera: Halictidae)

Population and colony-level sex allocation and nest productivity in the eusocial sweat bee Halictus ligatus Say were studied by excavating nests during one season. The emphasis was on measuring the provision masses, which differ in size and shape depending on the sex of the egg to be laid on them (male-producing provision masses are smaller and more or less round, whereas gyne-producing provision masses are larger and 'loaf-shaped). The primary aim of this study was to test theoretical predictions about female-bias of the sex ratio in the summer brood, both on the population level and on the colony level.
The overall sex ratio of the summer brood was moderately biased towards gynes. A significant positive correlation between the overall size of provision masses (as an estimate for the degree of female bias of the nest sex ratio) and the number of eusocial workers was found. This relationship further improved in partial analyses in which the provision mass weights were adjusted for sampling date, removing the effect of protandry. Foundress size, however, had no effect on the second brood provision masses and neither was there an effect of worker number on the size of gynes and males separately. In the first brood only the size of the foundress had a consistently positive effect on the size of the provision masses and on the size of the emerging daughter workers.
The observed increase of female bias in the nest sex ratio with increasing numbers of eusocial worker bees conforms to optimization predictions following from kin-selection theory.