Partial bivoltinism in a ground-nesting bee: the biology of Diadasia rinconis in Texas (Hymenoptera, Anthophoridae)

A central Texas population of Diadasia rinconis, a common, oligolectic, gregariously nesting solitary bee, is shown to be partially bivoltine. Most individuals are univoltine, over wintering as prepupae in cocoons in their natal cells. However, a portion of the mature first generation larvae does not spin a cocoon but rather immediately pupates and emerges. Since this second emergence usually occurs well after the peak bloom of its floral hosts, we suggest most second emergence individuals are unsuccessful because they neither nest nor over winter and simply die without leaving any progeny. In addition, in some years a high proportion of a local population may skip that year, spending an additional year in diapause despite the local abundance of the appropriate floral resources. Males patrol the emergence sites and mate with emerging females during both the first and second emergences although some mating also occurs at flowers. First emergence females commonly provision two to three cells per day, taking eight to fifteen pollen-gathering trips per cell. Only Cactaceae pollen is used in provisioning nest cells, primarily that of several species of Opuntia. Data are provided on nectar and pollen production of three primary floral hosts (O. leptocaulis, O. macrorhiza and O. engelmannii var. lindheimeri).