THE DECREASING OCCURRENCE OF DIAPAUSE IN THE FLY PSEUDOSARCOPHAGA AFFINIS THROUGH LABORATORY-REARED GENERATIONS

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Abstract
Pupal diapause was eliminated inadvertently in the fly Pseudosarcophaga affinis auct. nec Fallén during laboratory rearing under uniform conditions through over 200 generations. In its natural environment as a parasitoid of Choristaneura fumiferana (Clemens) in British Columbia, from which the Belleville stock originated, pupal diapause shortly intervened in most individuals. At first there was some seasonal variation in the incidence of diapause in the laboratory. The development of some individuals was not interrupted by apparently 'obligatory' diapause. These always were used as breeding stock and thus this part of the population was used more often annually than those that spent 6 months in diapause. Of course this favored the increase of those whose development was not interrupted, and the somewhat biased procedures led early to the gradual elimination of that part of the population that had the potential to diapause. Moreover, cold-tolerance seems to have been decreased likewise. These observations support the hypothesis of other workers that diapause is not a fixed species characteristic, but is subject to environmental factors and that perhaps the distinction between 'obligate' and 'facultative' diapause is rarely valid.