Assessment of the population dynamics, foraging activity, and pollination efficiency of Agapostemon angelicus Cockerell in male-sterile cotton

Abstract

Field studies were conducted on the solitary ground nesting bee species, Agapostemon angelicus Cockerell (Hymenoptera: Halictidae), a pollinator of potential value in hybrid cotton production programs on the Texas High Plains. A methodology using sticky traps was developed to estimate relative densities of this species. The seasonal population dynamics of A. angelicus were studied at four locations from 1983 to 1985, and the within-field spatial distribution pattern was determined. Additionally, components of pollination efficiency of A. angelicus were examined. A yellow sticky trap using two plastic cups was the most time and cost efficient device for collecting A. angelicus females. Trap position with regard to direction, height, and adjacent floral hosts did not significantly influence bee collection. Traps replaced at intervals of seven days or less optimized trap effectiveness. Sticky traps were shown to be useful in comparing A. angelicus densities and for constructing seasonal population profiles. In addition, numbers of A. angelicus collected on traps prior to cotton bloom were significantly related to visitation levels and yield components (p<$.05) at study sites. A. angelicus densities showed significant variation between locations and season (p<$.05). Females increased in abundance concurrent with cotton bloom. In addition, importation of honey bees (Apis mellifera L.) did not competitively displace A. angelicus at study sites. Adjacent crops and pastureland did not signficantly influence the distribution of A. angelicus in cotton and foragers of this species were present across entire cotton fields. These findings indicated this species could provide nearly uniform coverage of cotton fields needing pollination; however, more extensive examinations are needed to further characterize this species' foraging activity. Laboratory studies of A. angelicus in cotton provided qualitative and quantitative evidence that foragers of this species collect cotton pollen and are capable of transferring pollen grains to the stigmas of male-sterile cotton. Individual females carried an average of 1315 cotton pollen grains per single pollen load and low levels of non-Gossypium grains were present, indicating that competition from other plant species is minimal. In controlled laboratory studies, A. angelicus deposited an average of 32.9 pollen grains to a male-sterile cotton stigma in a single one minute visit. Assuming that these studies simulate natural conditions, it is expected that more than one visit by A. angelicus to a male-sterile flower is necessary to completely set a full complement of 28 - 32 seeds in a cotton boll.