Parasitism of the grasshopper Melanoplus sanguinipes by a sarcophagid fly, Blaesoxipha atlanis: influence of solitary and gregarious development on host and parasitoid

Blaesoxipha atlanis (Aldrich) (Diptera: Sarcophagidae) is a common parasitoid of the grasshopper Melanoplus sanguinipes (F.) (Orthoptera: Acrididae) in western Canada. We tested the hypothesis that B. atlanis can develop as either a solitary or a gregarious parasitoid, and assessed the influence of parasitism on the growth and survival of infected grasshoppers. Males and females of M. sanguinipes were parasitized manually with one, two, or three first-instar larvae of B. atlanis in the laboratory. Parasitism was more deleterious to males than females of M. sanguinipes; females are larger than males. Host survival and longevity declined with the number of larvae per host in a sex-specific manner. In females, 39%, 24%, and 8% of hosts containing, respectively, one, two, and three sarcophagid larvae survived parasitism. Although 41% of single-parasitized males survived, all males containing more than one larva died. Variations in host quality as measured by dry mass explained much of the response to parasitism in male, but not female, hosts. Parasitoid larvae, apparently, did not cause significant physical damage to host organs and tissues but instead functioned as a metabolic sink. The greater metabolic activity associated with egg production could account for the relatively higher tolerance to parasitism of female, as opposed to male, grasshoppers. Developmental time, adult size, and percentage survival of B. atlanis declined with the intensity of parasitism, especially in parasitoids developing in male hosts. Females developing gregariously contained fewer ovarioles at eclosion than counterparts developing as solitary larvae. The mean body size of field-collected B. atlanis did not differ from that of laboratory-reared parasitoids developing singly in a host. Gregarious development is an alternative strategy to solitary development that may enable B. atlanis to maintain population numbers during periods of grasshopper scarcity.