Mass rearing of Bracon cephi (Gahan) and B. lissogaster Muesebeck parasitoids of wheat stem sawfly, Cephus cinctus Norton, and temperature-induced mortality in host immatures

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Montana State University - Bozeman, College of Agriculture


Bracon cephi (Gahan) and B. lissogaster Muesebeck are host specific larval parasitoids of the wheat stem sawfly, Cephus cinctus Norton. A reliable source of these parasitoids is needed for inoculative releases into sawfly infested wheat fields in Montana. Large walk-in field screen cages were used to confine sawflies and parasitoids on wheat. Factors affecting the successful establishment of sawflies in wheat and subsequent attack by parasitoids were tested. Treatments consisted of cage modifications such as windows for enhancing the amount of light, food sources for parasitoids, increased light plus food sources, and a control. Methods for delivering adult sawflies into the cages were also investigated. The mass rearing cages with the windows had significantly greater sawfly infestation and parasitism when compared to the cages without windows. The sawfly infestation was low the first year, while the percent parasitism was quite high, indicating that the major impediment to mass rearing was obtaining greater sawfly infestation. We obtained higher amounts of infestation and parasitism for the second year when the method of introduction of C. cinctus adults into the rearing cage was changed, and this difference was more obvious than any treatment effects.
However, the sex ratio of the mass reared parasitoids was male biased indicating the absence of pre-mated females in the cages. The low number of male parasitoids in the rearing cages probably influenced this outcome. Experiments were conducted to assess possible temperature-induced mortality of sawfly immatures. Lethal temperatures and times for predicting mortality were calculated for the overwintering larval and pupal stages at 2 to 5 hr time intervals using probit analysis. The result shows that for both stages, mortality increased with increasing temperatures, and for a fixed temperature, the LT50 was lower for longer time intervals. The temperature-induced mortality experiments in the laboratory helped us to assess the lethal temperatures that should be avoided inside the rearing cages to enhance survival of the parasitoids. The temperatures inside the cages were significantly lower than those occurring in the field. These results provide the basis for wheat stem sawfly parasitoid mass rearing in walk-in cages.




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