Studying insect photoperiodism and rhythmicity: Components, approaches and lessons

Abstract

Abstract.Components of daily and seasonal timing systems in insects are reviewed. Photoperiod indicates seasonal position reliably, but signals can be much modified by habitat, latitude and season. Several receptor features and pigment systems are known, with different daily, seasonal and general functions, including differences between circadian and seasonal reception. Clocks can serve several different purposes, functioning as daily oscillators, interval timers or through successive requirements. The molecular functioning of circadian clocks is best known, but even so there is considerable complexity and diversity and much remains to be discovered. We know relatively little about the internal states that provide information for timed responses (such as the photoperi odic "counter"), about the central controlling mechanism, or about the effectors that transmit output signals. Nevertheless, temporal responses serve a very great range of purposes in insects, and the reported complexity in all of the components of timing systems reflects complex ecological needs across daily and seasonal intervals. The variety of components and the complexity of interactions reported (even within species), as well as the diversity of such elements as photosensitive pigments, molecular clock function and potential neurotransmitters, suggests that -unlike some earlier expectations -there is no single master clock for all timing functions in insects.Insect photoperiodism and rhythmicity have been studied by both observational or direct approaches (examination of system ele ments or devices, and qualities such as survival), and by inferential or indirect approaches (such as interpretation of various responses to photoperiod, modelling, and estimating fitness). Many students work with only one approach, but the power of different approaches is not equal, and knowledge at one level may not give answers at another. These difficulties tend to limit our under standing of the linkages among components.This overview suggests several lessons for the study of photoperiodism and rhythmicity. There are multiple elements, complex integration and a diversity of clocks, showing that different processes serve different purposes. The diversity of findings also results from the fact that different investigative approaches, which depend on the question being asked and on the perspective of the investi gator, can influence the outcome of the investigation. Given these complexities, I believe that the key to interpreting photoperiodic and circadian responses is their ecological value. Notwithstanding the interest of timing mechanisms or their parts and of specific responses, daily rhythms and seasonal timing are best understood through the essential context provided by the ecological demands on the actual organisms under study.

Effect of photoperiod on parasitization by Trichogramma principium (Hymenoptera: Trichogrammatidae)

“…Adaptive value of this reaction is not clear. In diapause regulation, this pattern of photoperiodic response is usually connected with aestivation or summer diapause (Danilevski, 1965;Tauber et al, 1986;Zaslavski, 1988;Denlinger, 2002;Saunders, 2002;Danks, 2003). But T. principium and many other Trichogrammatidae are typical "long-day species" with a facultative winter prepupal diapause (Zaslavski & Umarova, 1990;Boivin, 1994;Reznik et al, 2002;Reznik & Kats, 2004).…”

“…Some of them are the components of the "diapause syndrome", while others are more or less independent of diapause (Danilevski, 1965;Zaslavski, 1988;Denlinger, 2002;Saunders, 2002;Danks, 2003). In particular, reproduction may be controlled by photoperiod both qualitatively (as a switch between reproduction and adult diapause) and quantitatively (as an influence on the duration of the pre-oviposition period, oviposition intensity, total lifetime fecundity, etc.).…”

Effect of temperature on the reproduction and development of Trichogramma buesi (Hymenoptera: Trichogrammatidae)

“…Côn-soli & Parra, 1995a). However, the effect of thermoperiod on the percentage of T. buesi females ovipositing revealed a very sharp, almost irreversible response, which was strongly dependent on the position of the thermophase in the photoperiod, which was earlier revealed only in the regulation of daily and seasonal cycles (Tauber et al, 1986;Zaslavski, 1988;Danks, 2003). Earlier (Reznik et al, 2001(Reznik et al, , 2003 we suggested that induction of parasitization, as of diapause or active development, is based on neuro-endocrine mechanisms.…”

“…Third, temperature may serve as a key factor regulating seasonal or daily cycles and thus indirectly influence various aspects of insect biology (Tauber et al, 1986;Zaslavski, 1988;Danks, 2003). In this case, temperature often interacts with other environmental cues, particularly photoperiod.…”

Insect adaptations to cold and changing environments

“…271-272;Lankinen and Riihimaa 1997). Unfortunately, most of the recent work on circadian responses has focussed on timing mechanisms rather than on ecological relevance (Danks 2003(Danks , 2005b.…”