The status of the locust fly, Wohlfahrtia pachytyli (Diptera : Sarcophagidae), in the Karoo and the impact of locust control operations on its abundance

The status of the sarcophagid fly, Wohlfahrtia pachytyli (Townsend) (Diptera : Sarcophagidae), a facultative parasite of the brown locust, Locustana pardalina (Walker), in the Nama-Karoo biome, is reviewed. Information on the distribution, life cycle and larviposition of this ubiquitous flesh fly is presented. As facultative parasites, flies deposit microscopic larvae on the immobile bodies of locusts while they undergo ecdysis. Larvae enter the body cavities, consume the entire inner body contents and kill the locusts. Alternatively, the fly behaves as a saprozoic scavenger, attracted to decaying flesh and faecal matter. With parasitism rates of 0.1 % for fifth instar brown locust hoppers and 6 % for brown locust adult fledglings in the field, the impact of these facultative parasites on gregarious brown locust populations was low. However, a sharp increase of 37 % was registered after 48 hours when brown locust hopper bands, sprayed with a deltamethrin ultra low volume formulation (Decis® UL), were utilized as hosts. In the brown locust, this insecticide elicited a curious shade seeking response followed by a protracted paralysis, which W. pachytyli flies exploited. Despite the presence of relatively high deltamethrin residues, following spraying with Decis® UL, a heavy influx of flies regularly flew into spray areas and readily larviposited on sprayed hoppers. Resultant maggots successfully completed their life cycle within such toxic hosts. Since Decis® is in widespread use or brown locust control operations in the Karoo, such an abundance of additional paralysed hosts would result in a burgeoning W. pachytyli adult fly population, thereby enhancing parasitic potential. Paradoxically, the positive side-effects reported here on the impact of locust control operations with Decis® on a non-target, beneficial agent such as W. pachytyli, provides an anomalous example of a broad-spectrum insecticide actually benefiting a natural enemy.