A model for the photoperiodic counter in the aphid Megoura viciae

Abstract

The photoperiodic counter in the aphid Megoura viciae was analysed by means of a previously developed general model of the photoperiodic counter mechanism in insects and mites. It was shown that the counter in M. viciae accumulated long scotophases as well as short scotophases. It could further be shown that the inductive strength, or “value”, of long and short scotophases depended to a certain extent on photophase duration: long, 12-h scotophases accompanied by long photophases (> 18 h) decreased in value, whereas the inductive strength of short, 8-h scotophases decreased when accompanied by short photophases (< 16 h). Sensitivity to photoperiod changed slightly during the 10-day postnatal sensitive period at 15°C. Although sensitivity to short-night cycles (16 h light-8 h dark) showed a maximum at around day 7 and thereafter decreased sharply, sensitivity to long-night cycles (12 h light-12 h dark) did not appear to change. In M. viciae the “required day number”, which is the number of long- or short-night cycles required to elicit a 50% response, could only be determined at 12 and 15°C, as at higher temperatures (17 and 20°C) the long-night response (ovipara-production) was attenuated. For the low temperatures it was found that the accumulation of short-night cycles was far more temperature sensitive than that of long-night cycles. The observed attenuation of the long-night response at the higher temperatures could be explained in terms of the model, by assuming that the “minimally required induction sum” ( S min) is extremely sensitive to temperature, with a temperature coefficient much higher than the temperature coefficients of the photoperiodic values of both long-night cycles and short-night cycles.