In many insect species with a pupa covered by various "shells" (puparium, host remains, etc.) pupal-adult ecdysis and 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 the temporal pattern of pupal-adult ecdysis and of adult emergence from the host chorion in Trichogramma embryophagum Hartig (Hymenoptera: Trichogrammatidae) in laboratory conditions (12L : 12D, 20°C). 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 that 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.
1_Maternal and grand-maternal photoperiodic responses of Trichogramma buesi, T. embryophagum, T. evanescens, T. piceum, T. principium, and T. telengai were investigated in laboratory conditions. During the experiment, grand-maternal and maternal generations developed at 20°C and one of the 4 photoperiodic regimes: L : D = 12 : 12, 14 : 10, 16 : 8, and 18 : 6 (in total, 16 combinations) while the progeny developed at L : D = 12 : 12 and one of the 3 thermal regimes: 13, 14, and 15°C. The proportion of diapausing individuals in the progeny of all the studied species was significantly dependent on the direct influence of temperature and on the maternal photoperiodic response. The influence of the photoperiodic conditions during development of the grand-maternal generation was statistically significant in 5 of the 6 studied species, being relatively weak in T. embryophagum and T. telengai, whose geographical ranges extend up to north-western regions of Europe (possibly, these wasps enter diapause so early that the grand-mothers of the diapausing generation develop under long day conditions). Comparative analysis showed that the thresholds of the maternal and grand-maternal photoperiodic responses coincided or almost coincided. The grand-maternal effect was stronger in the progeny of maternal females which developed under short day conditions than in those that developed under long day conditions. This pattern of interaction probably synchronizes the life cycle with seasonal changes because diapause is induced under decreasing day length and thus mothers of diapausing individuals develop at shorter daylength than do grand-mothers., 2_We conclude that the grand-maternal and the maternal effects on Trichogramma progeny diapause are based on one and the same photoperiodic response. In nature, the grand-maternal effect increases the proportion of diapausing individuals in the progeny of females which have developed under short day conditions during two generations, thus achieving a "cumulative" photoperiodic effect., Natalia D. Voinovich, Nina P. Vaghina, Sergey Ya. Reznik., and Obsahuje seznam literatury
The effect of photoperiod on parasitization of the eggs of the Angoumois grain moth, Sitotroga cerealella (Olivier, 1789) by Trichogramma principium Sugonyaev & Sorokina, 1976 was investigated under several photoperiodic regimes of L : D = 3 : 21, 6 : 18, 9 : 15, 12 : 12, 15 : 9, 18 : 6 and 21 : 3. In all regimes, certain wasps delayed ovipositing in this non-preferred host. Potential fecundity of T. principium females (the number of mature ovarial eggs at emergence) and subsequent oogenesis (estimated by the number of mature ovarial eggs in non-ovipositing females) was independent of photoperiod. However, the percentage of females that oviposited was higher for females that developed and were kept under 6-12 h long photophase than for those that developed and were kept under ultra short (3L : 21D) and under long (18L : 6D and 21L : 3D) photophases. The average duration of the pre-oviposition (egg retention) period showed the opposite pattern to the photoperiodic response. A possible explanation of this reaction is that the delay in oviposition is adaptive if the probability of finding a better host is high. In autumn, when the last Trichogramma females are still active but their lepidopteran hosts are already much less abundant, then parasitization of any suitable host is the best strategy.
At emergence females of Trichogramma had a lot of mature eggs in their ovaries, but some delayed parasitization or refused to parasitize a laboratory host. The effect of constant and alternating temperatures on the percentage of Trichogramma buesi females parasitizing Sitotroga cerealella eggs and the duration of the pre-parasitization period were investigated. The temperature dependencies of the rate of preimaginal development, pre-emergence survival, number of eggs laid daily, and total lifetime fecundity were also determined. As the temperature was increased from 12 to 35°C, the median pre-oviposition period decreased from 5 days to 3 h, with maximum values of 24 and 1.5 days, respectively. The rate of induction of parasitization (reciprocal of duration of the pre-parasitization period of the females that parasitized) increased with temperature like the rate of preimaginal development and average number of eggs laid daily by a parasitizing female. Total cumulative percentage of parasitizing females reached a maximum (ca 60%) at temperatures of 25-30°C, while at 12 and 35°C, respectively, 25 and 50% of females parasitized the S. cerealella eggs. Average lifetime fecundity and pre-emergence survival showed a similar dependence on temperature. The influence of the thermorhythm (25°C for 4 h and 15°C for 20 h) was strongly dependent on its position within the photoperiod. When thermophase coincided with photophase, the percentage of females that parasitized was close to that recorded at a constant temperature of 25°C. But when the high temperature pulse coincided with the dark period, the percentage of parasitizing females was the same as at 15°C. Thus, the temperature dependence of ethogenesis (supposedly, an increase in motivation to parasitize or search for a host) in Trichogramma females was similar to that of morphogenesis, although the reaction to alternating temperatures may have been complicated by interaction with the light : dark regime.
A comparative study of the direct influence of single, non-repetitive changes of light and/or temperature on the daily pattern of adult eclosion was conducted in laboratory conditions with the egg parasitoid, Trichogramma embryophagum (Hartig). In all experiments, the entire development occurred under the same regimen (12L : 12D, 20°C). However, just in the day when records of eclosion were made, the time of light-on was changed (from the total absence of a dark period to the 12-h-long scotophase). In addition, in different treatments of each experiment a thermophase (30°C during 2 or 4 hours) was applied in various positions relative to light-on. Results showed that light-on or a temperature step-up could induce eclosion in a certain percentage of individuals practically at any time of the 24-h-long cycle (masking effects). 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). The temperature stimulus was found to modify the responsiveness to light considerably, depending on the order of application of the stimuli. The simultaneous action of light-on and a temperature step-up exerted a more considerable influence than could be expected based on their separate impacts. On the contrary, a high temperature pulse preceding the onset of the photophase inhibited the response of individuals to light-on, evidently acting by phase delay of a circadian rhythm of light responsiveness. These effects were more pronounced in the first half of the 12-h-long scotophase. The data suggest a complex interaction between the exogenous factors in their influence on the daily pattern of adult eclosion.
A laboratory study was carried out on photoperiodic control of prepupal diapause in the egg parasitoid Trichogramma embryophagum (Hartig). All experiments were conducted with an isofemale parthenogenetic strain. The maternal generation was reared at 20°C and photoperiods of L:D = 3:21, 6:18, 9:15, 12:12, 15:9, 18:6, 21:3 or 24:0. The tendency to diapause in the progeny was estimated by rearing the daughter generation at 15°C in the dark. Experiments revealed a long-day type response based on maternal influence on the progeny prepupal diapause. However, significant endogenous fluctuations in the pattern of the photoperiodic curve were revealed in successive laboratory generations reared under constant conditions. The left threshold day-length was very variable, while the right threshold kept relative constancy. Experiments with individual females sequentially offered new host eggs demonstrated that the probability of the progeny entering diapause depends significantly on maternal age. At 20°C and 18L : 6D, the percentage of diapause was maximal (ca 15%) in the progeny eclosed from the eggs laid during 1st - 2nd days of maternal life. Then the proportion of diapausing progeny decreased to 0-5% at days 9-11 of female life and later slightly increased in 15-17 days old females. Thus, endogenous factors play an important role in maternal influence on progeny diapause, particularly in environments close to threshold temperature and photoperiod.
Trichogramma dendrolimi, T. ostriniae, T. confusum and T. evanescens are the four most commonly occurring Trichogramma species with overlapping distribution in China. They are the most frequently used egg parasitoids for biological control of lepidopterous crop pests in China. It is difficult to differentiate Trichogramma species because of their small size and lack of differences in morphological characters. Different molecular markers were employed to molecularly characterize and differentiate these species, including direct amplification of the internally transcribed spacer 2 (ITS2) of ribosomal DNA by polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD) and species-specific primers. The results showed that direct amplification of ITS2 could not clearly discriminate these species, but they could be differentiated using RFLP pattern obtained with endonucleases EcoRI and HindIII. The banding pattern produced by RAPD is irreproducible so it is not a suitable way to identify Trichogramma species. Finally, the species-specific primers designed based on ITS2 sequences could unequivocally distinguish the four species. The species-specific primer-based protocol proved to be the most convenient and time saving method for the identification of Trichogramma species by creating a unique PCR product, which can be used in surveying natural populations of Trichogramma species. This is the first report of the prompt identification of the four most commonly occurring Trichogramma species in China.