EGG-LIMITED FUNCTIONAL-RESPONSE OF USCANA-LARIOPHAGA, EGG PARASITOID OF BRUCHID BEETLE PESTS IN STORED COWPEA

The functional response of the egg parasitoid Uscana lariophaga Steffa n (Hymenoptera: Trichogrammatidae) to eggs of its host Callosobruchus maculatus Fab. (Coleoptera: Bruchidae) was investigated in storage con tainers filled with a cowpea (Vigna unguiculata Walp.) seed mass. Fora ging time was limited to 4 or 24 h. These indirect experiments were su pplemented by direct observations of the parasitoid's handing time, eg g-laying capacity and initial egg load. The foraging process of U. lar iophaga can be divided into two distinct stages: the process leading t o detection of host clusters and, after arrival within a host cluster, the response of the parasitoid to the host density within the cluster . The chance that a cluster is found by U. lariophaga appeared indepen dent of the number of host eggs per cluster, but was influenced by the available foraging time. Within a host cluster, U. lariophaga demonst rated a Holling II type functional response. Parasitoids were strongly arrested in host clusters, leading to high levels of parasitism. Dire ct observations proved that handling time was not a limiting factor, b ut that U. lariophaga's initial egg load and egg maturation rate limit ed the plateau level of her functional response. As such. direct obser vations were essential for a correct interpretation of the mechanisms underlying the shape of the functional response.