Abstract
The influence of photoperiod on the thermal requirements for development was discovered for the first time in insects during experiments on the linden-bug, Pyrrhocoris apterus. The effect of photoperiod on the duration of linden-bug development at five constant temperatures (20, 22, 24, 26 and 28°C) was measured and the thermal requirements for development at three photoperiods (14, 17 and 20 h light per day) were calculated. Bugs from four geographic populations were used in these experiments: Pyatigorsk (44°02´N, 43°04´E), Borisovka (50°36´N, 36°01´E), Mikhailov (54°15´N, 39°0´E) and Ryazan (54°36´N, 39°42´E). From the values of individual development times at different temperatures the coefficient of linear regression of development rate (the inverse of the duration) on temperature and the thermal threshold for development were calculated. Both these parameters were found to decrease significantly with decrease in day-length for all four populations studied. It means that at shorter day-lengths nymphal development is less dependent on temperature compared to the development at longer day-lengths. These effects seem to be adaptive. The development times of nymphs at relatively high temperatures (above 24-25°C) are shorter under long-days than under short days which should be advantageous at the height of summer when the days are long and the weather is warm. In the contrast, at relatively low temperatures (below 24-25°C) the nymphs develop significantly faster under short-days than under long days, which is advantageous at the end of summer as it allows the nymphs to reach the adult stage, the only stage capable of overwintering. The influence of photoperiod on the thermal reaction norm appeared to be more or less gradual, i.e. the shorter the day-length the shallower the slope of the regression line of development rate on temperature and the lower the thermal threshold for development. An analysis of the literature shows that this effect of photoperiod on the thermal requirements for development is widespread among insects but has been overlooked by previous authors. The authors conclude that the variation in the development time observed in insects at different seasons, photoperiods or food regimes, or from different populations, etc., are generally due to some modification of the thermal reaction norms and more specifically to differences in the thermal requirements for development.
Highlights
The rate of growth and development of insects is usually regulated by direct influence of temperature but can depend on other factors that act indirectly and serve as seasonal cues inducing or terminating diapause
Using the generalizations given by Danks (1987) it is possible to classify all these species into 3 groups in which: (1) Short days retard development compared to long days; (2) Short days accelerate development compared to long days; (3) Intermediate photoperiods retard or accelerate development compared to short and long day conditions
The results of this study confirm the findings of previous authors (Saunders, 1983, Numata et al, 1993) that the development of the linden-bug is accelerated when reared under typically short-day conditions at a temperature of 24°C and below (Figs 1, 2)
Summary
The rate of growth and development of insects is usually regulated by direct influence of temperature but can depend on other factors that act indirectly and serve as seasonal cues inducing or terminating diapause. (1) Short days retard development compared to long days;. (2) Short days accelerate development compared to long days;. (3) Intermediate photoperiods (which are as a rule close to the critical day-length) retard or accelerate development compared to short and long day conditions. According to Musolin & Saulich (1997) all three types of photoperiodic control of development mentioned above are found in bugs. This was confirmed recently (Nakamura, 2002; Musolin & Numata, 2003). The second type (the acceleration of development under short days compared to long days) is much more widespread. In some bug species photoperiod does not influence the duration of development
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