Since 1991 the pteromalid parasitoid Spalangia nigroaenea has been mass-released annually in more than 20 feedlots in southwest Kansas to control filth flies. This wasp not only parasitizes the host to produce progeny but also kills host pupae by feeding on them directly. This mortality is important in reducing fly populations in confined cattle feedlot environments.
In experiments conducted in 1994 and 1995 in Garden City, Kansas, researchers evaluated the efficacy of the wasp at different parasitoid-to-host ratios and the effect of temperature on parasitization.
S. nigroaenea performed better when its density was higher, but a conflict seems to exist between efficiency and effectiveness. When parasitoid densities were varied, while host densities were held constant, significantly more offspring were produced as the parasitoid-to-host ratio changed from 1:5 to 1:50, but the percentage parasitism actually decreased. The amount of host feeding did not differ much in 1994, but in 1995 there was a direct correlation between host-feeding and parasitoid-host-ratios.
For major fly reduction, enough parasitoids to maintain the parasitoid-to-host ratio at 1:5 to 1:10 would be necessary. From a practical standpoint, that would not be economically feasible, even with high mortality from parasitization and host-feeding. A more efficient parasitoid-release program might employ a 1:20 ratio to achieve mid-levels of parasitism and host-feeding.
During the fly breeding season (May to October) manure temperature (where the hosts are) greatly influenced parasitism. Temperatures over 95°F reduce parasitism, so above this temperature the hotter it gets, the fewer hosts the wasps parasitize and fewer offspring are produced per female. Therefore, weather conditions should also be considered when determining the best release rate for each situation-but of course, more research is needed to figure out what those rates should be.
Greene, G. L., Y.-J. Guo, and H.-Y. Chen. 1998. Parasitization of house fly pupae (Diptera: Muscidae) by Spalangia nigroaenea (Hymenoptera: Pteromalidae) in cattle feedlot environments. Biol. Control 12:7-13.
Return to Commodity Menu Vol. V No. 10
Return to Contents Menu Vol. V No. 10
Go To Index