Field and laboratory studies were conducted to determine how Apanteles scutellaris and Agathis gibbosa (Hymenoptera: Braconidae) coexist as solitary endophagous parasitoids of potato tuberworm (PTW) larvae, Phthorimaea opercullela (Lepidoptera: Gelechiidae), in southern California. The competitive characteristics of Orgilus jennieae (Hymenoptera: Braconidae), an exotic parasitoid of PTW larvae, were compared with those of the native species to determine its potential for establishment. Emergence of the egg-larval parasitoid Chelonus phthorimaeae (Hymenoptera: Braconidae) from field-collected PTW larvae was compared with those of the other parasitoids, but its competitive interactions were not considered. Seasonal density changes, mine characteristics, and within-plant distributions of PTW larvae were studied to determine their possible impacts on parasitoid interactions.

Apanteles scutellaris and A. gibbosa oviposited in similar host instars, similarly responded to vertical distributions of PTW larvae in plants, and were capable of ovipositing in larvae mining in nearly all potato plant tissues. However, A. scutellaris with its short ovipositor more efficiently oviposited in hosts in leaflets and distal portions of petioles, while A. gibbosa with its long ovipositor more efficiently oviposited in hosts in buds, stems, and basal portions of petioles. Females of A. scutellaris also could detect hosts parasitized by A. gibbosa, but A. gibbosa females readily multiply parasitized hosts. Eggs of A. scutellaris appeared to be adversely affected during host paralysis and oviposition by A. gibbosa, but larvae were not affected. Consequently, when nondiscriminating A. gibbosa females oviposited before the eggs of A. scutellaris hatched, A. gibbosa emerged from multiply parasitized hosts; otherwise, A. scutellaris emerged. Results suggest that differences in oviposition efficiencies relative to the location of PTW mines and the discrimination abilities of A. scutellaris are the primary mechanisms that enable these species to coexist. Influences of alternate hosts and PTW densities, along with the evolution of these competitive strategies, are discussed.

Like A. gibbosa, O. jennieae most efficiently oviposited in PTW larvae mining potato buds, stems, and basal portions of petioles of potato plants. In addition, O. jennieae readily multiply parasitized hosts previously parasitized by A. gibbosa, but avoided hosts previously parasitized by A. scutellaris. Neither native species discriminated against hosts previously parasitized by O. jennieae. In multiply parasitized hosts, O. jennieae immatures were nearly always killed by A. gibbosa immatures or during host paralysis, despite oviposition sequences and time intervals between ovipositions. When A. scutellaris multiply parasitized hosts previously parasitized by O. jennieae, the species whose egg hatched first subsequently emerged. Orgilus jennieae exhibited a narrower oviposition preference for younger PTW instars than did the native species, possibly increasing the probability of multiple parasitization by the native species. These results, especially those relating to interactions with A. gibbosa, along with considerations on searching capacities and potential population increases, suggest that O. jennieae is not likely to become permanently established in southern California. Previous failures to establish exotic species of Orgilus in southern California and the importation of exotic parasitoids against other native pests that possess coevolved natural enemy complexes are discussed.