The Development of Host Discrimination and the Prevention of Superparasitism in the Parasite Pseudeucoila Bochei Weld (Hym.: Cynipidae)

1. The solitary endoparasite Pseudeucoila bochei oviposits in larvae of Drosophila species. The wasp is able to discriminate between parasitized and unparasitized hosts and also between hosts with different numbers of parasite eggs. Although discrimination appeared to be very strong, in a number of cases considerable degrees of superparasitization were still found during earlier research (BAKKER et al., 1967, 1972). Therefore, six possible causes of superparasitism in P. bochei were analysed: a. a female lays more than one egg at an oviposition; b. a female does not recognize hosts parasitized by other females; c. a female lays a second egg after the first oviposition within the period needed for building up the factor which causes avoidance of superparasitization; d. two or more females lay eggs simultaneously in one host; e. a female's tendency to oviposit increases when she encounters only parasitized hosts for a long period; she will lay eggs in these hosts; f. a female has not yet learnt to discriminate. 2. ad a: Per oviposition never more than one egg is laid (chapter III). 3. ad b: Hosts parasitized by a conspecific female were rejected by the wasps as properly as hosts parasitized by themselves (Table II, chapter IV). 4. ad c: The period needed for building up the factor which causes avoidance of superparasitization after the first oviposition appeared to be max. 70 seconds (Fig. 4). Acceptance or rejection of a once-parasitized host during this period depends on the interval between the oviposition and the second contact with that host, and on the distance between the places where the host was pierced at the first and second time (Fig. 5). The host is marked during or after the actual egg deposition, probably by substance(s) from the wasp's abdomen. Due to the fast marking after an oviposition, only a very small proportion of superparasitization will take place by this cause (chapter V). 5. ad d. Simultaneous oviposition by several wasps in one host was seldom observed, and thus may bring about only very few superparasitizations (chapter VI). 6. ad e: A high tendency to oviposit will cause high degrees of superparasitization, only when the wasps do not have the opportunity to leave a site with parasitized hosts or when, in the field and under laboratory conditions, the number of parasites is large relative to the number of hosts. In general a wasp will migrate from a place where she parasitized a large percentage of the hosts, or found parasitized hosts only. This tendency to migrate is very high and only after the wasp is forced to stay with parasitized hosts for hours, superparasitization may occur, perhaps because the tendency to oviposit increases so high that the oviposition threshold for parasitized hosts is crossed temporarily (Fig. 8, 9 & 10, chapter VII).