Seasonal change of nest survival and related aspects in an aggregation ofLasioglossum duplex (Dalla torre), a eusocial halictine bee (Hymenoptera: Halictidae)

Nest survival in an aggregation of a eusocial halictine bee, Lasioglossum duplex, was censused through an annual cycle. Out of 2,500 nests marked at the beginning of solitary phase in the spring, only 25.5% attained eusocial phase. But 60.5% of 636 nests attaining eusocial phase successfully produced sexual offspring. This shows that solitary phase is the most vulnerable period in the annual cycle. A very low productivity in eusocial phase in the census year was clarified from examination of 99 nests in the autumn. The number of prospective foundresses surviving to the next year was estimated upon the number of brood cells in examined nests and of old females surviving these nests. Deviation between this estimate and the number of nests made in the next spring was 8.8%, confirming a drop of population size to less than one third in the next year. Some nests solitarily made in the summer by dispersed females were examined. Productivity in such nests was extremely low, hence these nests should contribute virtually nothing to the next generation. Possible factors affecting the low productivity in eusocial phase were enumerated though none of these were supported by concrete evidence. Some considerations were given on the relation between obtained results and colony life cycle in eusocial insects, particularly in halictine bees.