Exploring the foraging environment of a natural enemy of Callosobruchus maculatus: Spatial egg distribution in stored cowpea

Knowledge of the spatial distribution of stored product insects may reduce the dependency on chemicals for control of these insects. Biological control, for instance, could be improved based on such knowledge. In this paper we describe the three-dimensional spatial oviposition pattern of Callosobruchus maculatus in stored cowpea. Individual C. maculatus females oviposited in clusters of 70 ± 15 (SD) eggs. These clusters were variable in shape. In any cluster 90 to 95% of the eggs fitted into a volume of 19.1 ± 3.5 cm³. The egg density was highest (0.6 eggs bean^-1) at the center of a cluster and decreased towards the periphery. A statistically significant relationship existed between the number of eggs n in a cluster and the cluster volume, V(cm³): V = 11.5 + 0.11n. We also investigated the spatial egg distribution of beetles which emerged from egg clusters such as those produced by individual females. Their oviposition was not confined to one specific area but was scattered throughout the bean mass. A point pattern analysis showed that the density of the 'parent' cluster had no effect on the spatial egg pattern. These results give insight into the foraging environment which the egg parasitoid Uscana lariophaga, a promising candidate for biological control of C. maculatus, is facing. We argue that the probability p of encountering at least one other bean with eggs after a parasitization is a function of the number n of beans that are visited: p = 1 - 0.42 (0.37)^(n-1).