HOST LOCATION IN STORED COWPEA BY THE EGG PARASITOID USCANA-LARIOPHAGA STEFFAN (HYM, TRICHOGRAMMATIDAE)

A good spatial distribution and an efficient host location behaviour a re prerequisites for successful biological control of stored product p ests. These attributes were investigated in the egg parasitoid Uscana lariophaga Steffan (Hym., Trichogrammatidae), foraging for clusters of Callosobrachus maculatus Fab. (Col., Bruchidae) eggs in experimental cowpea (Vigna unguiculata Walp.) storage containers. Females have an i nnate tendency to move upwards in a cowpea seed stock. Host clusters a bove the release point have an increased chance to be found by U. lari ophaga compared to clusters below the site of release, but still, sign ificant numbers of clusters below the release point are also being vis ited and parasitized. In host clusters with more than 50 eggs, parasit oids are arrested and may spend their entire egg load or foraging time , leaving other clusters unexploited. Host clusters with more eggs are more frequently parasitized, irrespective of the volume of clusters o r the egg density and distribution within those clusters. This increas ed chance of detection is thought to be the result of an increase in e gg odour concentration or in the volume of the odour sphere around clu sters with more eggs. This hypothesis was tested in a tube diffusion o lfactometer.