Effect of photoperiod and thermoperiod on daily rhythms of adult eclosion in insects

		*Laboratory of* Experimental Entomology and BioControl Theory Russia, 199034, St.-Petersburg, Zoological Institute. Phone: +7 (812) 714 0442; Fax: +7 (812) 714 0444; E-mail: sreznik@zin.ru

Effect of photoperiod and thermoperiod
on the daily rhythms of adult emergence in insects

Project is currently conducted by S.G. Karpova, S.Ya. Reznik and T.Ya. Umarova.

The project was aimed to investigate a relative importance of endogenous and exogenous factors in the control of daily rhythms of adult eclosion in insects, with special reference to intra- and interspecies variation in responsiveness to light and temperature.
Experimental studies are conducted mainly with egg parasitoids species of the genus Trichogramma. Another part of the research deals with blood-sucking mosquitoes (genera Aedes and Culex) and includes both laboratory experiments and field observations.
Considerable attention is given to exogenously driven effects in the rhythm, in contrast to majority of previous studies, which focused mainly on endogenous oscillation controlling adult eclosion rhythms in insects.

In many insect species with a pupa covered by various shells (puparium, host remains, etc.) the pupal-adult ecdysis and the emergence to the open air represent two discrete steps. However, in Trichogramma, as well as in other insect parasitoids, these two processes have never been studied separately. We investigated temporal patterns of pupal-adult ecdysis and of adult emergence from the host chorion in Trichogramma embryophagum Htg. (Hymenoptera, Trichogrammatidae) in laboratory conditions. Adult ecdysis was arrhythmic, while adult emergence showed a strong rhythmicity. The time lag between ecdysis and emergence varied from one to almost two days, depending on the circadian time of the ecdysis. The proportion of ecdysed adults stayed in the host chorion ranged up to 60% (just before the highest peak of emergence). The cumulative percentage of ecdysed adults gradually increased with time, independently of whether the light was turned on in accordance with the entrained circadian rhythm or 4 h earlier. This arrhythmic ecdysis could be explained by the fact that the ecdysed adults get into a well protected space inside the host chorion and the timing of this event is adaptively neutral.

Exogenous and endogenous components of the adult emergence rhythm in Trichogramma

The dynamics of adult emergence in various Trichogramma species was studied under aperiodic conditions, photoperiod at constant temperature, thermoperiod at constant light, and joint action of photoperiod and thermoperiod. The rhythm can be synchronised by photo- or thermoperiod so that the maximum of emergence occurs near the onset of photo- or thermophase. Since the rhythm persisted during several days in aperiodic conditions, it was considered to be endogenous. Exogenous stimuli (light-on or temperature rise) can induce emergence in a considerable fraction of individuals practically at any time of day, thus significantly modifying the pattern of the overt rhythm ("masking effects"). The sensitivity to these factors, measured as a fraction of adults eclosing within 2 h after the stimulation, was found to be modulated by a circadian oscillation. The magnitude of the response was determined by the phase of this oscillation and by a stimulus intensity.
The evidences were obtained that the studied laboratory strains are polymorphic in relative importance of endogenous and exogenous factors controlling the rhythm of adult eclosion.

Interrelation of photoperiod and thermoperiod in determining the adult eclosion rhythm in Trichogramma

Comparative investigation of synchronization of the rhythm to photoperiod under constant temperature and to thermoperiod under constant light was conducted in laboratory conditions with 9 strains of 7 Trichogramma species. The strong positive correlation was revealed between the percentage of adults emerging during scotophase at photoperiod and constant temperature and the percentage of adults emerging during cryophase at thermoperiod and constant light. This correlation provides support for the previous findings that light and temperature signals are apparently integrated on some levels of circadian system.

Study of entrainment of a rhythm to joint light and temperature cycles repeated daily at various combinations demonstrated that the overt rhythm depended on the position of thermophase in a light-dark cycle. In some cases, the emergence distribution curve became bimodal. This suggests an apparent individual variation in relative reactivity to light and temperature as entraining factors.

Investigation of the influence of non-repetitive light and temperature pulses applied at different combination was conducted with Trichogramma embryophagum and T. principium in order to investigate a capacity of the rhythm to be modified immediately by unpredictable influence of light and/or temperature stimuli. The fraction of adults emerging during 2 h after the beginning of the action of an exogenous factor depended not only on the preceding scotophase duration, but also on the stimulus itself (light, temperature or both). When two stimuli were combined, the response was significantly intensified or inhibited, depending on the order of their application. The evidences were obtained that stimulus given early can modify the responsiveness to the later one, acting by shift of the circadian oscillation. These experiments revealed also interspecies differences in the relative sensitivity to light and temperature stimuli.

The study deals with species of Aedes (Ochlerotatus) genus and Culex pipiens complex. Both laboratory experiments and field observations were curried out in order to reveal the factors controlling the daily dynamics of emergence from pupae in nature (Northern-West Russia, in latitude 62 North). In natural conditions, all species investigated so far showed morning and evening peaks of adult emergence. Laboratory experiments have revealed that thermoperiod is the main factor determining the daily dynamics, while photoperiod is less important. Endogenous control of the rhythm is rather week. This feature may be of adaptive value, since in a northern climate it is important to react directly to favorable but unpredictable changes of environmental factors at any time of day.

Recently comparative investigation of the adult emergence rhythm in autogenous and anautogenous forms of Culex pipiens pipiens was started. The preliminary results suggest the differences in some features of the circadian rhythm between C. p. p. f. molestus, inhabiting mostly urban underground biotops, and anautogenous C. p. pipiens from overground reservoirs.

Reznik S.Ya., Voinovich N.D., Vaghina N.P., Karpova S.G. 2008. Arrhythmic adult ecdysis but rhythmic emergence from the host chorion in Trichogramma embryophagum (Hymenoptera: Trichogrammatidae). Eur. J. Entomol. Vol. 105. pp. 81-85. PDF

Reznik S.Ya. and Karpova S.G. 2006. Method for synchronization of Trichogramma eclosion. Plant Protection (Zashita rasteniy). No 2. pp.54-55.

Karpova. S. G. 2006. The role of endogenous and exogenous factors in regulation of synchronous emergence of Trichogramma embryophagum Hartig and T. principium Sug. et Sor.(Hymenoptera, Trichogrammatidae). Entomological Review. Vol. 86 (3). pp. 252-263 PDF

Klyueva N. Z., Ryzhov D. B., Karpova S. G., and Reznik S. Ya. 2005. Influence of phenybut on regulation of daily rhythm of adult eclosion in Trichogramma embryophagum Htg. (Hymenoptera, Trichogrammatidae). Entomological Review. Vol. 85 (4). pp 347-351 Abstract

Karpova S.G., Reznik S.Ya. 2003. Temperature dependency of the daily pattern of adult eclosion in two Trichogramma species (Hymenoptera, Trichogrammatidae). Ann. Reports Zool. Inst. RAS. Vol. 299. P. 61-68.

Karpova S.G., Reznik S.Ya. 2002. Masking effect of light and temperature on the daily rhythm of adult eclosion in Trichogramma embryophagum (Hymenoptera: Trichogrammatidae). Insect Photoperiodism and Rhythmicity (Abstracts of 2nd Czech-Japanese Seminar on Entomology, Ceske Budejovice, Czech Republic, August 5-8, 202). P. 11.

Klyueva N.Z., Reznik S.Ya., Karpova S.G.. 2002. Influence of β-agonist of GABA aded to carbohydrate food on behavior of Trichogramma (Hymenoptera, Trichogrammatidae). Abstracts of XII Congress of Russian Entomological Society (St.Petersburg, 19-24 August 2002). P. 159.

Karpova S.G., Reznik S.Ya. 2002. Interaction of exogenous factors (light and temperature) in their influence on the daily pattern of adult eclosion in Trichogramma embryophagum (Hymenoptera: Trichogrammatidae). Europ. J. Entomol. 99: 427-436. PDF

Zaslavski V.A., Zinovjeva K.B., Umarova T.Ya. & Reznik S.Ya. 1999. Interaction of circadian rhythm synchronized by photoperiod and thermoperiod with direct influence of light and temperature in adults eclosion rhythm determination in two species of the genus Trichogramma (Hymenoptera, Trichogrammatidae). Entomol. Obozr. 78: 3-14. (English translation: 1999. Entomological Review. 79: 1-10).

Reznik, S.Ya., Zinovjeva, K.B., Umarova, T.Ya. & Zaslavski, V.A. 1998. Dependence of eclosion rhythm on photoperiod and thermoperiod in species of the genus Trichogramma Westw. (Hymenoptera, Trichogrammatidae). Entomol. Obozr. 77: 17-25.

Zinovjeva, K.B., Reznik, S.Ya., Zaslavski, V.A. & Umarova, T.Ya. 1996. Effect of photoperiod and thermoperiod on the eclosion rhythm of adults of Alysia manducator (Hymenoptera, Braconidae) and Trichogramma evanescens (Hymenoptera, Trichogrammatidae). Zool. Zhurn. 75: 1496-1504. (English translation: 1999. Entomological Review. 76: 1244-1250).

Zaslavski, V.A., Zinovjeva, K.B., Reznik, S.Ya. & Umarova T.Ya. 1995. Effect of photoperiod and thermoperiod on the eclosion rhythm of Trichogramma evanescens. Entomol. Exp. Appl. 74: 99-104.