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

Abstract. 1. Phorid flies in the genus Pseudacteon are parasitoids of ants. Variation in host size preferences of four South American and two North American Pseudacteon species on monogyne and polygyne forms of their host Solenopsis species 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 significantly larger than the average size polygyne worker attacked in four of the six Pseudacteon species.
3. Three South American Pseudacteon species attacked larger than average size workers, whereas one attacked smaller than average size workers, in both monogyne and polygyne forms. Both North American Pseudacteon species attacked 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 ants in the laboratory. The size of the emergent phorid fly was related positively to the size of the host worker ant, with females emerging from larger hosts. Similar patterns were documented for both monogyne and polygyne forms.
5. The mean size of worker host from which phorids emerged did not differ significantly between the monogyne and polygyne forms in the subsample of phorids reared to adults.
6. The observed patterns elucidate factors that may cause variation in Pseudacteon sex ratios, and have implications for biological control efforts of pest Solenopsis species.