Intraguild interaction between egg parasitoids: the role of the host egg mass size and parasitoids’ reproductive capacities

In Sicily, field collections of egg masses of the southern green stink bug, Nezara viridula (L.) (Heteroptera:Pentatomidae), revealed that two solitary parasitoids, Trissolcus basalis (Wollaston) (Hymenoptera: Platygastridae) and Ooencyrtus telenomicida (Vassiliev) (Hymenoptera: Encyrtidae), can exploit the same egg mass. Previous finding showed that parasitoid species differ in host location and larval competitive abilities and a host finding-larval competition trade-off may be important for coexistence. The aim of this work was to evaluate the role played by the host egg mass size and the importance of species reproductive capacities in the outcome of interspecific competition. Under laboratory conditions, parasitoids were introduced in an arena and allowed to exploit an egg mass made of 10, 20, 30 or 40 unparasitized hosts. Under field conditions, unparasitized egg masses consisting of 10 or 40 hosts were exposed in a tomato crop and recollected after one week. To assess the reproductive capacity of T. basalis and O. telenomicida, investigations were conducted by: 1) counting the number of ovarian eggs of female parasitoids; 2) measuring the size of the fresh laid egg; 3) recording the host handling time. Results indicated that a higher proportion of T. basalis emerged from large egg masses (N=40) while O. telenomicida dominated in small egg mass (N=10); intermediate results were obtained from egg masses of medium size. Analysis of reproductive capacities of parasitoid species indicated that T. basalis has superior abilities in exploiting unparasitized egg masses compared with O. telenomicida. The ecological role of these results is discussed in the context of intraguild interactions.