Pre-dispersal seed predation in Central American Acacia farnesiana: factors affecting the abundance of co-occurring bruchid beetles

Summary

Pre-dispersal seed predation of the leguminous Acacia farnesiana by Mimosestes nubigens and M. mimosae (Coleoptera: Bruchidae) was investigated in Santa Rosa National Park, northwestern Costa Rica. The purpose of the study was to determine the patterns of resource utilization by the seed predators and the mechanisms causing such patterns. Immature, mature, and fallen fruits were monitored during the dry seasons of 1987 and 1988 from different shrubs and areas. Parameters describing plant size, fecundity, and relative plant isolation were measured on each shrub. No evidence of spatial or temporal segregation was found between the two species and the intensity of seed predation was independent of the variables measured from each plant. M. mimosae was scarcer and always occurred with M. nubigens. Both were present in areas with low and high densities of the host plant, and the frequency distributions of their emergences from the fruits overlapped through the fruiting season. urosigalphus sp., a hymenopteran parasitoid, represented ca. 40% of all insect emergences in 1987 and ca. 30% in 1988. This wasp attacked a greater proportion of bruchid eggs on pods on the shrub than on pods beneath it, and more on green than on mature fruits. Parasitism thus appears to select against bruchid females that oviposit at an early stage of pod maturation. The harsh conditions of the dry season, namely heat and desiccation, also accounted for a high level of bruchid pre-emergence mortality, especially in fallen fruits, where survival from egg to adult was only about 18%. Beneath the shrubs, bruchid females exhibited selectivity, ovipositing more on pods in the shade than on those exposed to direct sunlight. In contrast to parasitoids, abiotic factors probably impose a selective force against those bruchid females that oviposit at a late stage of maturation or on pods already dropped. Both bruchid species can have more than one generation during the fruiting period. The intensity of seed predation did not, however, change during the season. The data obtained in this study suggest that factors like natural enemies and severe weather are more likely to limit the bruchid population densities than intra- or interspecific competition.