Photoperiodic time measurement and seasonal adaptation of the southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera : Pyralidae)

"Photoperiodism is an adaptation which enables organisms to maintain maximum reproductive and survival efficiency under periodically changing environmental conditions. Since daylength is astronomically stable, it gives organisms the most reliable, noise-free signal to construct a developmental program before a favorable or hazardous season approaches. Photoperiodism is a widespread phenomenon including developmental polymorphism, and behavioral and physiological periodicities. Since Bunning's "physiologische Uhr" was published in 1958, a variety of publications have appeared on this subject. Among them, the Cold Spring Harbor Symposium, held in 1960, was a turning point which stimulated much research in the 1960's and 1970's. This research has been summarized in symposia held in 1965, 1971, and 1975. Excellent reviews about insect photoperiodism have been published (Andrewartha, 1952; Lees, 1955; 1968; Danilevskii, 1961; Danilevskii et al. 1970; Masaki, 1961; deWilde, 1962; Beck, 1968b; Saunders, 1976; Brady, 1974; Rensing, 1973; and Tauber and Tauber, 1973c, 1976; Tyshchenko, 1977). The experimental insect used for the present study, the southwestern corn borer, Diatraea grandiosella Dyar, is a neotropical species which first invaded the United States around 1913. Thereafter, the species expanded its range into the midwestern corn belt (Chippendale and Reddy, 1974a). Frontier populations must survive under severe environmental pressure. Most usually, the timely completion of the life cycle and the institution of a dormant state are critical to survival under winter conditions. In this respect, diapause may be a relatively recent adaptation in this insect as the result of its territorial expansion. Chippendale and Reddy (1973), and Chippendale et al. (1976) have studied the photoperiodic and temperature control of the seasonal life cycle and diapause determination of D. grandiosella. They found that (1) diapause induction and termination are controlled by photoperiod and temperature; (2) chilling is not required for diapause termination; and (3) temperature cycles affect diapause determination in some way. In addition, the juvenile hormone theory of larval diapause was first proved experimentally using D. grandiosella. Considerable data are available about the endocrinology of this insect (Yin and Chippendale, 1973; Chippendale, 1977). Bridging between these two aspects, this study was directed at finding a more rigid basis for the working principle of the photoperiodic clock and a more accurate accessment of the phenology of D. grandiosella. These are interrelated because phenological data provide important clues about the working principle of the clock. Once the mechanism of the photoperiodic clock is determined, one can predict when the insect goes into diapause and when it resumes active growth, relying on the clock as much as the insect itself does. For this purpose a double hourglass model was devised, and the model was compared with other models which have already been proposed. A photoperiod temperature interaction was shown to determine the seasonal appearance of immature stages. How the insect responds to natural fluctuating temperatures and changing photoperiods was shown to be required information for prediction. The seasonal change in the intensity of diapause and in the sensitivity to both diapause terminating and maintaining photoperiods were also investigated. To test the data obtained in the laboratory, the seasonal life cycle of three geographic strains was compared using a photothermogram."--Introduction.