CHRYSOMYA MEGACEPHALA (FABR) IS MORE RESISTANT TO ATTACK BY CH-RUFIFACIES (MACQUART) IN A LABORATORY ARENA THAN IS COCHLIOMYIA-MACELLARIA (FABR) (DIPTERA, CALLIPHORIDAE)

Chrysomya Robineau-Desvoidy blow flies recently introduced to the Amer icas include two species, Ch. megacephala (Fabr.) and Ch. chloropyga W iedemann (= Ch. putoria) with purely saprophagous larvae, and two, Ch. albiceps (Wiedemann) and Ch. rufifacies (Macquart), that are facultat ive predators on other maggots. Patterns of adult abundance suggest th at the invading species suppress the saprophagous native Cochliomyia m acellaria (Fabr.), and do so more effectively in combination than indi vidually. We hypothesized that Ch, megacephala, historically sympatric with Ch. rufifacies, is relatively resistant to predation by Ch. rufi facies, which could provide it with a competitive advantage over a mor e vulnerable C. macellaria when larvae of all three occur together. To test this hypothesis, larvae of both prey species were individually p aired with larvae of Ch. rufifacies in the laboratory. C. macellaria w ere consistently killed at a higher rate than were Ch. megacephala.