1. Many coexisting parasitoids interact via intraguild predation (IGP); however, in IGP theory without stage structure the intraguild predator and intraguild prey coexist via this mechanism over only a relatively narrow region of parameter space. A recent formulation of IGP theory with a stage of the intraguild prey species that is invulnerable to attack by the intraguild predator shows that coexistence between guild members can occur across a much broader range of resource productivity than is predicted by IGP theory without stage structure. These general theoretical predictions are used to explore the implications for coexistence of Aphytis melinus (DeBach) and Encarsia perniciosi (Tower), a pair of parasitoids that coexist on California red scale. 2. Reanalysis of a published laboratory competition experiment between two parasitoid species suggests that Aphytis, an ectoparasitoid, survives at a lower rate when the reparasitized host contains a more developed juvenile endoparasitoid (Encarsia). This pattern is contrary to a common assumption that ectoparasitoids always consume endoparasitoids in a shared host, and may affect coexistence. The experiment covered only the first 30% of the Encarsia development period and did not describe the outcome for Encarsia, so the dynamic effects of this interaction in the context of IGP remain unclear. In addition, Encarsia is capable of parasitizing much younger hosts than is Aphytis, which the experiment did not examine. 3. The experiment was repeated, and extended to cover the full developmental duration of Encarsia. An additional experiment was performed to explore whether Encarsia could pre-empt Aphytis by attacking younger hosts. Aphytis juvenile survival was found to decrease as a function of the age of the juvenile Encarsia onto which it was laid. When Encarsia parasitized very young hosts, it was increasingly invulnerable to attack by Aphytis and increasingly more likely to survive to emergence. However, until the formation of an invulnerable pseudomoult, Encarsia was always killed in larger hosts, regardless of its age at the time of Aphytis attack. 4. According to stage-structured IGP theory, the decline in Aphytis success combined with an invulnerable Encarsia stage may promote coexistence in this system.
- Biological control