Host location in stored cowpea by the egg parasitoid Uscana lariophaga Steffan (Hym., Trichogrammatidae).
Spatial distribution and an efficient host location behaviour were investigated in the egg parasitoid Uscana lariophaga foraging for clusters of Callosobruchus maculatus eggs in experimental cowpea storage containers. Females had an innate tendency to move upwards in a cowpea seed stock. Host clusters above the release point had an increased chance of being found by U. lariophaga compared to clusters below the site of release, but significant numbers of clusters below the release point were also being visited and parasitized. In host clusters with >50 eggs parasitoids were arrested, spending their entire egg load or foraging time on these clusters, leaving other clusters unexploited. Host clusters with more eggs were more frequently parasitized, irrespective of the volume of clusters or the egg density and distribution within those clusters. This increased chance of detection was thought to be the result of an increase in egg odour concentration, or in the volume of the odour sphere around clusters with more eggs. This hypothesis was tested in a tube diffusion olfactometer.