Parasitoid-host relationships when host size varies: the case of Pseudacteon flies and Solenopsis fire ants

1. Phorid flies in the genus Pseudacteon are parasitoids of ants. Variation in host size preferences of four South American and two North American Pse udacteon species on monogyne and polygyne forms of their host Solenopsis sp ecies [S. invicta Buren and S. geminata (F.), respectively] was documented. 2. Monogyne Solenopsis workers were, on average, significantly larger than polygyne workers, and the average size monogyne worker attacked was signifi cantly larger than the average size polygyne worker attacked in four of the six Pdeudacteon species. 3. Three South American Pseudacteon species attacked larger than average si ze workers, whereas one attacked smaller than average size workers, in both monogyne and polygyne forms. Both North American Pseudacteon species attac ked larger than average size polygyne workers and smaller than average size monogyne workers, 4. Three Pseudacteon species were reared from eggs to adults in infected an ts in the laboratory. The size of the emergent phorid fly was related posit ively to the size of the host worker ant, with females emerging from larger hosts. Similar patterns were documented for both monogyne and polygyne for ms. 5. The mean size of worker host from which phorids emerged did not differ s ignificantly between the monogyne and polygyne forms in the subsample of ph orids reared to adults. 6. The observed patterns elucidate factors that may cause variation in Pseu dacteon sex ratios, and have implications for biological control efforts of pest Solenopsis species.