Parasites floral resources and reproduction in natural population of bumblebees
Schmid Hempel, P.; Durrer, S.
Oikos 62(3): 342-350
Weekly censuses of bee abundance, flower availability, resource use and infestation of bees by parasitic flies (Conopidae, Diptera) were made throughout a seasonal cycle in eight populations each of Bombus pascuorum and B. terrestris-B. lucorum (pooled as B. terr/luc). Several life history parameters, together with factors describing environmental conditions were used to characterise the populations. Length of the life cycle varied from 47 to 99 d in B. pascuorum and from 43 to 61 d in B. terr/luc. Also, time of appearance of males in relation to life cycle, the temporal delay between worker and male production, duration of male emergence, number of males produced, and number of males emerged per worker varied substantially among populations. To account for this variation, a multiple regression analysis was used that included as independent variables the seasonal average of resource availability (range of flower densities: 7.7-24.2 flowers m-2), resource demand (the ratio of conspecific worker abundance and resource availability), and frequency of conopid infestation in worker bees (range: 19.7-62.5%) as important sources of environmental variation. Low flower availability throughout the year was accompanied by earlier reproduction in both species. Average frequency of parasites infestation in worker bees was correlated with later reproduction in B. ter/luc but earlier in B. pascuorum. When separating the environmental factors for the first and second half of the year, early resource availability and demand explained a significant amount of life history variation, whereas for the second half, frequency of parasitisation was significant. Our evidence is correlative and further experimental studies must establish the causal relationships suggested here. Nevertheless, our findings suggest that in addition to food shortage, the adverse effects (e.g., increased mortality rate of workers) caused by heavy infestation by parasitic conopid flies could also be important for reproduction in natural populations of bumblebees.