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672 Temperature and moisture have also been suggested as key factors in the termination of diapause, especially for some overwintering forms. However, it is often unclear whether these factors are serving as token stimuli for terminating diapause or are enabling postdiapause development to begin (Denlinger, 1986; Tauber et al., 1986). According to Hodek (2002) and others, diapause typically terminates naturally simply by the passage of time; that is, no specific temperature or moisture stimulus is required and any effects of these factors is exerted on the quiescent phase that follows diapause.

In contrast to circadian clocks for which, as noted earlier, there is now a reasonable understanding of their mechanism, the photoperiodic clock that regulates insect seasonality remains largely a "black box." Despite extensive study no single hypothesis predominates; rather, a series of complex models have been proposed, details of which are beyond the scope of this text. Most of the models incorporate two interdependent components: a circadian-type clock system that measures the number of hours of light (or darkness) in each day, and a counter that sums the number of cycles. Adjustments are made for variations in temperature, latitude, food, etc. The information gathered is stored in a "memory link" until a "critical value" is reached, at which point an effector (e.g., the neuroendocrine system) is activated. For further information, refer to Takeda and Skopik (1997), Vaz Nunes and Saunders (1999), Tauber and Kyriacou (2001), and Saunders (2002).

For the great majority of insects that exhibit a photoperiodically induced diapause it is the absolute day length that is critical rather than daily changes in day length. Most insects studied to date show a long-day response to photoperiod (Figure 22.5A). That is, when reared under long-day conditions, they show continuous development, whereas at short day lengths diapause is induced. Between these extremes is a critical day length at which the incidence of diapause changes abruptly. Examples of insects that show a long-day response are the Colorado potato beetle, L. decemlineata, and the pink bollworm, P. gossypiella. In

FIGURE 22.5. Different types of diapause incidence-day length relationships in insects. (A) Long-day; (B) short-day; (C) short-day-long-day; and (D) long-day-short-day. The hatched line in Figure 22.5A indicates that in some long-day species diapause incidence is less than 100% at very short day lengths. [From S. D. Beck, 1968, Insect Photoperiodism. By permission of Academic Press, Inc., and the author.]

FIGURE 22.5. Different types of diapause incidence-day length relationships in insects. (A) Long-day; (B) short-day; (C) short-day-long-day; and (D) long-day-short-day. The hatched line in Figure 22.5A indicates that in some long-day species diapause incidence is less than 100% at very short day lengths. [From S. D. Beck, 1968, Insect Photoperiodism. By permission of Academic Press, Inc., and the author.]

FIGURE 22.6. Effect of photoperiod on diapause incidence in Acronycta rumicis (Lep-idoptera) populations from different northern latitudes. [From S. D. Beck, 1968, Insect Pho-toperiodism. By permission of Academic Press, Inc., and the author.]

a number of species, including the silkworm, Bombyx mori, diapause is induced when the day length is long, while at short day lengths development is continuous. Such insects are said to show a short-day response (Figure 22.5B). The European corn borer, O. nubilalis, and the imported cabbage worm, Pieris brassicae, have a short-day-long-day response to photoperiod; that is, the incidence of diapause is low at short and long day lengths, but high at intermediate day lengths (14-16 hours of light per day) (Figure 22.5C). The ecological significance of such a response is unclear, since under natural conditions, insects would already be hibernating when the day length was short. A few northern species of Lepidoptera behave in the opposite manner, namely, show a long-day-short-day response to photoperiod (Figure 22.5D). All photoperiods except those with 16-20 hours of light per day induce diapause. Again, however, the ecological value of such a response is uncertain.

The precise value of the critical day length for a species varies with latitude (Figure 22.6). For example, the sorrel dagger moth, Acronycta rumicis, studied in Russia by Danilevskii (1961), is a long-day insect which, near Leningrad (latitude about 60°N), has a critical day length of about 19 hours. In more southerly populations the critical day length is gradually reduced and is, for example, only 15 hours on the Black Sea coast (43°N).

In the dragonfly Anax imperator and perhaps a few other insects, diapause is induced by daily changes in day length rather than by absolute number of hours of light per day. Anax larvae that have entered the final instar by the beginning of June are able to metamorphose the same year. Those that reach the final instar after this date enter diapause and do not emerge until the following spring. It seems that larvae are able to determine the extent by which the day length increases. When the daily increment is 2 minutes or more per day larvae can develop directly, whereas at smaller increases or decreases in day length diapause is induced (Corbet, 1963).

Temperature may profoundly modify or overrule the normal effect of photoperiod on diapause induction. For example, the critical photoperiod depends on the particular (constant) temperature at which insects are maintained: in A. rumicis a 5°C difference in temperature results in a 1-hour difference in the critical day length. At extreme values the effects of temperature may overcome those of photoperiod with reference to induction of diapause. In long-day insects exposure to constant high temperature may completely prevent diapause induction regardless of photoperiod. Conversely, in short-day insects high temperature induces diapause, even under long-day conditions.

In nature temperatures normally fluctuate daily about a mean value. This daily fluctuation (thermoperiod) also may modify the influence of photoperiod according to whether

Beekeeping for Beginners

Beekeeping for Beginners

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