Predación de semillas de Acacia aroma por el brúchido Pseudopachymerina grata en función de la posición de las semillas y el número de semillas por vaina

Abstract

The torulose, indehiscent pods of Acacia aroma contain seeds that are preyed upon by larvae of the bnichid beetle Pseudopachymerina grata. The presence and position (relative to base of pod) in the pod wall of an adult exit hole reveals the presence and position of a preyed seed. In August 1989, a .sample of 7800 pods were collected from 42 shrubs near Vipos, Tucumán, Argentina. The number and distribution of exit holes in pods containing different numbers of seeds were recorded. At the tune of collection, pods had an average of 0.4 exit holes/pod. Among those pods having 1 exit hole, holes occurred more frequently (60%) on the basal half than on the stilar half. After being stored for about 1 year, with free access provided to several generations of adult beetles, pods exhibited an average of 4 exit holes/ pod with rat significant difference in the proportion of exit holes between basal and stilar halves. These residís suggest that under natural conditions P. grata preys differentially on seeds occupying basal positions, which are slightly heavier than stilar seeds (78 vs. 71 rag). Pie presence of a larger proportion of egg shell remnants at basal positions than at stilar positions also supports the differential predation hypothesis. Pie number of seeds per pod varied between 1 and 16 with an average of 7.4 seeds. Pie percentage of seeds preyed upon increased with number of seeds/pod. This trend could explain, at least in part, why most A. aroma contain a much .smaller number of seeds than the maximum they could mature.