Intraguild predation in larval parasitoids: implications for coexistence

Summary 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 ofAphytis melinus(DeBach) andEncarsia perniciosi(Tower), a pair of parasitoids that coexist on California red scale. Reanalysis of a published laboratory competition experiment between two parasitoid species suggests thatAphytis, 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 theEncarsiadevelopment period and did not describe the outcome forEncarsia, so the dynamic effects of this interaction in the context of IGP remain unclear. In addition,Encarsiais capable of parasitizing much younger hosts than isAphytis, which the experiment did not examine. The experiment was repeated, and extended to cover the full developmental duration ofEncarsia. An additional experiment was performed to explore whetherEncarsiacould pre‐emptAphytisby attacking younger hosts.Aphytisjuvenile survival was found to decrease as a function of the age of the juvenileEncarsiaonto which it was laid. WhenEncarsiaparasitized very young hosts, it was increasingly invulnerable to attack byAphytisand increasingly more likely to survive to emergence. However, until the formation of an invulnerable pseudomoult,Encarsiawas always killed in larger hosts, regardless of its age at the time ofAphytisattack. According to stage‐structured IGP theory, the decline inAphytissuccess combined with an invulnerableEncarsiastage may promote coexistence in this system.