Phenology and social biology of two sibling species of Halictus in an areaof sympatry

Fortnightly samples of foragers of Halictus ligatus Say and its recently di stinguished sibling species Halictus poeyi Lepeletier were collected in an area of sympatry. Both species were active from late April until autumn, al though H. ligatus ceased foraging about 1 month earlier than H. poeyi. Phen ological differences were minor and insufficient to preclude hybridization opportunities between the species. The two had a similar proportion of ovar ially developed workers, but the proportion that mated in H. poeyi was doub le that found in H. ligatus, and size dimorphism between castes was greater in H. poeyi than in H. ligatus. When these data are compared with data fro m other populations, in sympatry the two species appear to be more similar to each other in several sociobiological parameters than each does to allop atric conspecific populations. This suggests a prime role for ecological fa ctors in determining details of social organization in halictine bees.