Factors Relating to Diapause in the Alfalfa Weevil Parasite Bathyplectes curculionis (Thomson)

Biological and ecological factors regulating diapause in the ichneumonid parasite of the alfalfa weevil, Bathyplectes curculionis (Thomson), were studied. In most experiments, both parasites and weevil larvae were maintained in the environmental conditions under study from the emergence of the adult parasite until the evaluation of results. The factors either preventing or inducing diapause in B. curculionis were found to be an interaction of environmental factors. A long scotophase with cool temperatures prevented diapause. The percentage of nondiapausing parasites increased as the scotophase was increased to 15 hours. Over 15 hours of scotophase, fewer nondiapausing parasites were produced. The optimum temperature during scotophase was 7.2° C for maximum nondiapausing. The temperature during the nine-hour photophase had to be raised. The greatest percentage of nondiapausing individuals occurred when the temperature was raised to 25.0° C. No effects of relative humidity were observed between 50-80 percent. Relative humidity held at near saturation produced high mortality, while below 20 percent increased the amount of diapause to over 58 percent. A 15-hour scotophase at 7.2° C, a nine-hour photophase at 25.0° C, and relative humidity between 50-80 percent, consistently more than 95 percent of the parasites did not enter diapause. At either a long photophase or failure to alternate temperatures, essentially 100 percent diapaused. Continuous photophase or continuous scotophase, either with or without alternating the temperature cycle, produced 100 percent diapause. Part, but not all, of the regulation of diapause was determined by the effects of the photoperiod plus temperature cycle on the adult parasite just prior to oviposition. The percentage of diapause in the offspring increased markedly when the adults were transferred directly from a diapausing regime, when compared to those undergoing a five-day period with ideal conditions prior to oviposition. In both cases oviposition was under the best ...