Comparative responses of four Pseudacteon phorid fly species to host fire ant alarm pheromone and analogs

Abstract

Several species of Pseudacteon phorid flies, including Pseudacteon cultellatus, P. curvatus, P. obtusus, and P. tricuspis, have been introduced in the southern United States for biological control of imported fire ants, Solenopsis spp. Previous studies showed that Pseudacteon flies use the alarm pheromone and venom alkaloids of fire ants to locate host ant workers. A recent study by our group demonstrated attraction of one of the species, P. tricuspis, to certain alkylpyrazine analogs of fire ant alarm pheromone. If effective, alkylpyrazine analogs of fire ant alarm pheromone may provide cost-efficient alternatives as phorid fly attractants. To test the hypothesis that other phorid fly species are attracted to alkylpyrazine analogs of fire ant alarm pheromone, we compared electroantennogram (EAG) and behavioral responses of four Pseudacteon species, P. cultellatus, P. curvatus, P. obtusus, and P. tricuspis, to the commercially available alarm pheromone isomer (2-ethyl-3,6(or 5)-dimethyl pyrazine) and six structurally related alkylpyrazine analogs at physiologically relevant doses (ranging from picogram to 10 µg). The alarm pheromone isomer and two analogs, 2,3-diethyl-5-methyl pyrazine and 2 ethyl-3,5 + 6 methyl pyrazine, elicited the strongest EAG activity in all four species at all tested doses. Results from four-choice olfactometer bioassays testing the above three EAG-active compounds showed that all four species were strongly attracted to the alarm pheromone isomer at doses ranging from 0.001 to 1 µg. The flies also showed significant attraction to the two analogs (2,3-diethyl-5-methyl pyrazine and 2 ethyl-3,5 + 6 methyl pyrazine) compared to the solvent control but not as strong as the attraction elicited by the alarm pheromone isomer. These results confirm that multiple species of phorid flies use fire ant alarm pheromone as a host location cue. However, the flies are not strongly attracted to the tested alkylpyrazine analogs of the alarm pheromone.