A halictid bee with sympatric solitary and eusocial nests offers evidence for Hamilton's rule.

The validity of Hamilton's rule has been confirmed among cooperative breeders where helping behaviour is transient; however, Hamilton's rule has not been validated among eusocial insects where helpers commit for life. Here we conduct a direct test of Hamilton's rule using field populations of Lasioglossum baleicum bees, which inhabit sympatric solitary and eusocial nests. Our results show that the indirect fitness of sterile first-brood workers is higher than the direct fitness of solitary first-brood females, and spring foundresses achieve a large direct fitness by having helpers. These fitness benefits are attributed to markedly higher larval survival rates in multiple-female nests, and intruding into an unrelated nest yields a moderate degree of direct fitness, but coexistence with unrelated females also increase overall brood survival. We discuss reasons why various types of cooperation are maintained in Lasioglossum baleicum with relation to that how a multiple-female nesting improves larval survival.