Diapause detection and monitoring in the Mediterranean corn stalk borer

Abstract. Sesamia nonagrioides Lefebvre diapauses as mature larvae. Once diapause is induced, larvae continue to grow and moult with no apparent changes, so distinction between non-diapausing and diapausing larvae is difficult. In the present work two physiological markers of diapause induction are obtained and their efficacy in detecting diapause onset and in monitoring diapause development in field populations is evaluated. The first marker is based on the differential capacity of two photoregimes, LD 0:24 h and LD 16:8 h, to reverse diapause induction. When larvae of the first and second stadia were subjected to the diapause-inducing LD 12:12 h light cycle and then transferred to LD 0:24 h, larvae pupated after a development duration significantly higher than that of the larvae transferred to LD 168 h. This criterion was used to monitor diapause development in overwintering larvae until field-collected larvae submitted to LD 0:24 h pupated synchronously with those submitted to LD 16:8 h. This occurred between late January and early March. The second marker refers to development of imaginal wing discs. A relative index of the tracheal mass area divided by the total disc area diminishes steadily during development in non-diapausing sixth-instar larvae until it becomes zero just before pupation, whereas it remains constant in diapausing larvae. Though wing disc development was blocked in diapausing sixth-instar larvae, the disc continued to grow and, consequently, the total area of the disc was not a suitable marker of diapause induction. When the constancy of the relative area index was used to monitor diapause development and termination in overwintering larvae, the results agreed with those obtained using the first criterion and with previous field observations.     

Normal vision can compensate for the loss of the circadian clock

Circadian clocks are thought to be essential for timing the daily activity of animals, and consequently increase fitness. This view was recently challenged for clock-less fruit flies and mice that exhibited astonishingly normal activity rhythms under outdoor conditions. Compensatory mechanisms appear to enable even clock mutants to live a normal life in nature. Here, we show that gradual daily increases/decreases of light in the laboratory suffice to provoke normally timed sharp morning (M) and evening (E) activity peaks in clock-less flies. We also show that the compound eyes, but not Cryptochrome (CRY), mediate the precise timing of M and E peaks under natural-like conditions, as CRY-less flies do and eyeless flies do not show these sharp peaks independently of a functional clock. Nevertheless, the circadian clock appears critical for anticipating dusk, as well as for inhibiting sharp activity peaks during midnight. Clock-less flies only increase E activity after dusk and not before the beginning of dusk, and respond strongly to twilight exposure in the middle of the night. Furthermore, the circadian clock responds to natural-like light cycles, by slightly broadening Timeless (TIM) abundance in the clock neurons, and this effect is mediated by CRY.     

Unity and diversity in the insect photoperiodic mechanism*

This review examines some of the models to account for time measurement in insect photoperiodism. It considers the supporting evidence for these models and the attempts to discriminate among them. Although hourglass timers may exist, it is suggested that most photoperiodic mechanisms, including many hourglass‐like timers, are circadian‐based, making Bünning's original hypothesis, that the circadian system somehow provides the essential “clockwork” for photoperiodic timing, the most persuasive unifying principle. The apparent diversity among modern species in their modes of time measurement is probably the result of differences between the underlying circadian systems that were adopted for seasonal night length measurement as the insects, or groups of insects, moved northwards into areas with a pronounced winter season. Photoperiodic time measurement, therefore, exhibits both unity (in their common circadian basis) and diversity in detail. Attention to this diversity may provide invaluable insights into the problem of photoperiodic time measurement at comparative, and molecular, levels.     

Non-canonical Phototransduction Mediates Synchronization of the Drosophila melanogaster Circadian Clock and Retinal Light Responses

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Signification differentielle des photoperiodes pour l'horloge biologique de Pieris brassicae (Lepidoptera)

Abstract

Photoperiodic insects are able to distinguish between long days and short days. In various models the long day response is classifically considered the “actively” induced state. The short day response is thought to be “passive”, caused by failure of light to coincide with a photosensitive part of the night or failure of coincidence of constituent oscillators. The photoperiodic response curve of Pieris brassicae showed that diapause is induced by short days (4–14 h), and non‐diapause state by several conditions (natural and non‐natural): long days (16 h or more), LL, DD and ultrashort days (0.1 h). By reciprocal transfers of larvae between non‐diapausing determining and diapause determining conditions, it was proved possible to estimate the differential capacity of four non‐diapausing conditions vs. the diapausing action of LD 8: 16 in decreasing sequence: LD 16:8 > LL > DD = LD 0.1: 23.9. DD may be considered a “neutral” condition. In darkness the development seems to be determined by an endogenous program without external influence. LL, although beingan aperiodic signal as DD, has a weak antidiapausingeffect.Thebiological clock of Pieris differentiates between two constant conditions. The four non‐diapausing conditions have the same effect on the development when applied during the entire larval life, but have different effects when only applied during a few days. Both ecological conditions LD 16:8 and LD 8:16 have an action on the development but in an opposite way. There was not a “passive” state caused by failure of another inductive photoperiod. Ultra‐short days, DD and LL are without ecological meaning. Nevertheless, in these experiments, they provided informations in attempts to determine the mechanism of the time measurement. The external coincidence model of Pittendrigh and Minis (1964) was the more adequate to explain theearlier results on the biological clock of Pieris. However, this model has to be modified to account for the differential significance of several non‐diapausing conditions.     

Circadian clock regulates photoperiodic responses governed by distinct output pathways in the bean bug,Riptortus pedestris

Effects of RNA interference (RNAi) targeted against circadian clock genes on two distinct types of photoperiodic responses – ovarian development and lipid accumulation – were investigated in a bean bug Riptortus pedestris, to explore which physiological process in the photoperiodic response involved the circadian clock. Ovarian development and lipid accumulation are known to be regulated by distinct output pathways. Control insects showed clear photoperiodic responses; i.e. induction of ovarian development and suppression of lipid accumulation under long-day conditions, whereas opposite characteristics under short-day conditions. We found that RNAi directed against period, a negative element of the circadian clock, produced a long-day effect for both the ovarian development and lipid accumulation, while RNAi directed against Clock, a positive element of the circadian clock, produced a short-day effect for both, irrespective of photoperiod. These results indicate that the circadian clock comprised of these genes regulates a process governing both distinct photoperiodic responses.     

Photoperiodic induction of pupal diapause in Sarcophaga argyrostoma: Temperature effects on circadian resonance

Larval cultures of the flesh-fly Sarcophaga argyrostoma maintained in circadian ‘resonance’ experiments produced a high incidence of pupal diapause when the period of the light cycle was close to (T) 24, 48 or 72 hr, but a low incidence of diapause at T 36, 60 or 84 hr. Cultures pre-programmed for diapause by exposing pregnant females to long nights indicated the induction of non-diapause development at T 36, 60 and 84, whereas cultures pre-programmed for diapause-free development by exposing females to continuous light indicated the induction of diapause at T 24, 48 and 72.Raising the temperature reduced the heights of the diapause peaks whereas lowering the temperature raised them. With progeny from long-night-reared flies the lowest temperature tested (18°C) produced a result indistinguishable from an ‘hour-glass’ response, warning that ‘negative’ resonance experiments may merely indicate non-permissive conditions for demonstrating the involvement of circadian rhythmicity in insect photoperiodism.The results of the ‘resonance’ experiments and the effects of temperature are interpreted in terms of a multioscillator ‘external coincidence-photoperiodic counter’ model for the clock.     

Characterization and distribution of digestive proteases of the stalk corn borer,Sesamia nonagrioides Lef. (Lepidoptera: Noctuidae)

Larval midgut extracts from the noctuid Sesamia nonagrioides Lef. were assayed for protease activity. Total proteolytic activity, as measured by azocasein hydrolysis, showed a pH optimum in the range 10.0 to 11.5, suggesting a digestive system based largely on serine-like proteases. The ability of midgut extracts to hydrolyze specific synthetic substrates, the elucidation of the pH at which maximal hydrolysis occurs, and their sensitivity to protease inhibitors confirmed the presence of the serine endoproteases: trypsin, chymotrypsin, and elastase; and the exopeptidases: carboxypeptidase A, carboxypeptidase B, and leucine aminopeptidase. The distribution of these digestive proteases along the gut sections and among the different midgut regions was examined. All types of endoproteases and exopeptidases were mainly located in the midgut, with less than 5% of the activity in the foregut and hindgut. When the two halves of the midgut were compared, all proteolytic activities were higher in the anterior portion of the midgut. Trypsin, chymotrypsin, elastase, and carboxypeptidase B activities were mainly located in the endoperitrophic space of the midgut, with some activity in the ectoperitrophic space, whereas aminopeptidase and carboxypeptidase A activities were preferentially located in the midgut epithelium. © 1996 Wiley-Liss, Inc.     

Age-stage two-sex life table analysis of Sesamia nonagrioides (Lep.: Noctuidae) reared on different host plants

The effects of maize and sugarcane were studied on the demographic parameters of Sesamia nonagrioides under controlled environmental conditions. Life table parameters were estimated using age-stage two-sex life table method. Total developmental times of immature stages were 53.51 ± 0.42 and 39.58 ± 0.14 days on maize and sugarcane, respectively. The fecundity values were found to be 93.07 ± 2.94 and 122.95 ± 6.72 eggs/female on maize and sugarcane, respectively. Based on the analysis, the intrinsic rates of increase (r) and the mean generation time (T) of S. nonagrioides were determined as 0.0909 ± 0.0027 and 0.0579 ± 0.0021 day^?1; 42.09 ± 0.21 and 56.83 ± 0.63 days on sugarcane and maize, respectively. Consequently, the sugarcane was a better host for development and population increase of S. nonagrioides than maize. These results can be used to predict population dynamics of the pest on the natural hosts and to improve mass rearing technique on natural diets in insectariums.     

Adult emergence rhythm of the egg-parasitoid Telenomus busseolae

The adult emergence rhythm of Telenomus busseolae, an egg parasitoid of Sesamia nonagrioides, was examined when parasitoids were exposed to different light-dark regimes. Most of the adult parasitoids emerged throughout the whole period of the photoperiodic cycle. Peak male emergence occurred 2–5 hours earlier than that of females. Adult emergence was asynchronous in continuous darkness or light. However, regimes of alternative light and dark phases such as L4:D20, L8:D16, L12:D12, L16:D8 and L20:D4 h generated a population rhythm with a period length of 24 hours. The peak of the emergence activity moves from the scotophase to the middle of the photophase with an increase of the photophase from 4 to 20 h. Rhythmical activity of adults was synchronised within 2 cycles when immature stages of parasitoid grow under continuous light conditions (LL) and then transferred to L12:D12. Moreover, emergence rhythm persisted and continued in a free-run with a period length of less than 24 hours by transferring a rhythmic culture from L12:D12 h to LL or RR (continuous red light) conditions, indicating the existence of a circadian rhythm. The ecological implications of the expression rhythm relate to better survival of the parasitoids.