Egg-limited functional response of Uscana lariophaga, egg parasitoid of bruchid beetle pests in stored cowpea.
The functional response of the trichogrammatid egg parasitoid Uscana lariophaga to eggs of its host Callosobruchus maculatus was investigated in storage containers filled with a cowpea (Vigna unguiculata) seed mass. Foraging time was limited to 4 or 24 h. These indirect experiments were supplemented by direct observations of the parasitoid's handling time, egg-laying capacity and initial egg load. The foraging process of U. lariophaga can be divided into 2 distinct stages: the process leading to 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 independent of the number of host eggs per cluster, but was influenced by the available foraging time. Within a host cluster, U. lariophaga demonstrated a Holling type II functional response. Parasitoids were strongly arrested in host clusters, leading to high levels of parasitism. Direct observations showed that handling time was not a limiting factor, but that U. lariophaga's initial egg load and egg maturation rate limited the plateau level of its functional response. As such, direct observations were essential for a correct interpretation of the mechanisms underlying the shape of the functional response.