Pre-dispersal seed predation in Central AmericanAcacia farnesiana: factors affecting the abundance of co-occurring bruchid beetles |
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Authors: | Anna Traveset |
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Institution: | 1. Department of Biology, University of Pennsylvania, 19104-6018, Philadelphia, PA, USA
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Abstract: | Summary Pre-dispersal seed predation of the leguminousAcacia farnesiana byMimosestes nubigens andM. 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 withM. 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. |
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Keywords: | Acacia farnesiana Bruchidae Pre-dispersal seed predation Insect-plant interactions Insect cooccurrence |
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