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Laboratory experiments demonstrate that the adult emergence rhythms of two Trichogramma species (T. telengai Sor. and T. principium Sug. et Sor.) significantly depend on photoperiod. The synchronicity of adult emergence is maximal at day lengths of 12–16 h. Under longer days (18–20 h) adult emergence is extended, whereas under very short days (4–8 h) a substantial proportion of adults emerge before light-on. The pattern of T. telengai emergence under day length of 4 h suggests the possibility of the existence of two fractions (the first that emerges in the dark and the second that waits for light-on) which results in two peaks of emergence. As a whole, these data agree well with the hypothesis of the periods of “potential readiness” and “obligate activity” forwarded by V.B. Chernyshev. The results of the study can be used for optimization of the methods for laboratory and mass rearing of Trichogramma species.

The effects of day length on development and reproductive maturation of zoophytophagous bug Macrolophus pygmaeus (= M. nubilis) were investigated under laboratory conditions using two strains originated from the environs of Rome, Italy (41.75°N, 12.30°E and 41.95°N., 12.80°E) and Sochi, Krasnodar Territory, Russia (43.9°N, 39.3°E). The insects were kept under day length of either 10 or 16 h at a constant temperature of 20°C. Nymphs and adults were fed on the grain moth eggs. Embryonic development lasted 18–20 days, nymphal development, 25–30 days, and reproductive maturation of females, 4–6 days. The short day length (10 h) resulted in 1–2-day prolongation of nymphal development and a longer maturation time in individuals from both strains, whereas the duration of embryo development was independent of photoperiod. Under both photoperiods, males developed faster than females. Under the short day length, females of the Rome strain matured markedly faster than those of the Sochi strain. Faster development under low temperature and stronger tendency to delay the reproductive maturation observed in the Sochi strain could be explained by a relatively fast (when compared with Rome) autumnal decrease in temperature. The mechanism and adaptive value of the long-day acceleration of development are not yet clear.

The induction of diapause in the populations of Trichogramma evanescens Westw. and T. chilonis Ishii from the south of Primorskii Territory in the Russian Far East was studied at the constant temperatures of 7, 8, 10, 15, 20, and 25°C. The parental generations developed at either 25° or 20°C and a day length of 16 or 10 hours. The maximum fraction of diapausing prepupae of T. evanescens (nearly 100 %) was observed at the temperatures of 8–10°C, that of T. chilonis only at 10°C, regardless of parental photoperiodic or thermal conditions. The influence of parental temperature and photoperiod on diapause of the progeny was found only in T. evanescens. In this species, after the parental generation had been kept under short-day conditions and a temperature of 25°C, the progeny showed higher incidence of prepupal diapause in the short-day line than in the long-day line at the temperatures of 7 and 15°C.

Laboratory investigations demonstrated that the survival of the preimaginal stages and pharate adults of Trichogramma principium was not significantly dependent on temperature over the range from 17 to 30°C but the temperature of 35°C caused a two-fold decrease in the mean number of survived progeny per one female. The rate of the preimaginal development of T. principium (from oviposition to adult emergence) at the temperatures of 17–30°C could be rather closely approximated by linear regression. The sum of effective temperatures was 145–150 degree-days with the lower threshold of about 11.8°C. In addition, at a temperature of 17°C the duration of development significantly (P < 0.001 with the Student’s t-test) depended on the day length: the mean and SD constituted 26.96 ± 0.34 and 27.85 ± 0.27 days at photoperiods of 12 and 18 h, correspondingly. The adaptive role of this quantitative photoperiodic response is not clear. Potentially, the “autumnal acceleration” of development, triggered by the combination of low temperature and short day, raises the proportion of individuals that have reached the diapausing stage before the winter season.

Variations in the tendency to diapause among successive generations of laboratory strains reared under constant conditions were reported for many insects and mites. However, as far as we know, the relative importance of the endogenous and exogenous components of this variability has not been investigated experimentally. We have conducted such a study with 12 successive generations of laboratory strains of Trichogramma telengai Sor. and T. principium Sug. et Sor. In total, 12 strains originated from different females and / or reared under different photothermal conditions (temperature of 20 and 25°C and day length of 12 and 18 h) were investigated. The tendency to diapause was estimated by the proportion of the progeny diapausing at temperatures of 13, 14 and 15°C. Although the variations in the tendency to diapause in sequential generations of different strains of the same species were positively correlated, the correlation between different species was also very strong suggesting that endogenous (species-specific) factors are not the main reason for the observed changes. The data for different photothermal conditions of incubation of the studied strains and of their progeny were also correlated suggesting that occasional changes in temperature or day length are not the reason of the changes in the proportion of diapausing progeny. Evidently, the differences in the tendency to diapause among generations of the studied Trichogramma species were mainly caused by some uncontrolled environmental factors. This in no way excludes the possible role of the endogenous processes, but suggests the necessity for a critical analysis of the available data on the “spontaneous” dynamics in the tendency to diapause among successive generations of the laboratory strains of insects and mites.

The effects of day length on preimaginal development and reproductive maturation of females of laboratory strains which originated from two native: Irkutsk (Siberia, Russia) and Daegu (South Korea) and two invasive: Sochi (Caucasus, Russia) and Pardubice (Czechia) populations of Harmonia axyridis were investigated under the laboratory conditions. In all the 4 strains, the acceleration of development caused by the short day (all other conditions being equal) was relatively stronger in those larvae that later, at the adult stage, delayed reproductive maturation. This correlation can be considered as an indirect proof that both reactions are based on the common physiological mechanism. However, comparison of the parameters of photoperiodic responses showed that although the photoperiodic effect on reproductive maturation in individuals from Sochi and Pardubice invasive populations was relatively weak, the effect of day length on the rate of their preimaginal development was even somewhat stronger than that of individuals from Irkutsk and Daegu native populations. This uncorrelated interpopulation variation of the quantitative and qualitative photoperiodic responses suggests that in spite of the common physiological mechanism, the responses influencing preimaginal development and reproductive maturation of H. axyridis are to a certain extent independent and thus the evolution of their parameters (in particular, adaptation to new climate conditions) occurs independently.

The distribution limits of the beet webworm, Loxostege sticticalis L., with its sustainable development in one, two, three, or four successive generations per season in the former USSR territory were mapped for the first time using routine GIS procedures. The expected number of generations was determined based on the sum of daydegrees (above a threshold of 12°C), critical day length value (14 h), and Selyaninov’s hydrothermal index (> 0.3). The compiled map was in good agreement with that prepared for China (Luo Li-zhi and Li Guang-bo, 1993).

Variations in the tendency to diapause were investigated in 5 successive generations of 6 parthenogenetic clones of Trichogramma telengai Sor. reared under constant laboratory conditions (the temperature of 20°C and the day length of 18 h). The tendency to diapause was estimated by the proportion of the progeny which diapaused at temperatures of 13, 14 and 15°C. Although the variations in the tendency to diapause in successive generations of different clones were positively correlated, the dynamics of the proportion of diapausing progeny in one of the studied clones was significantly different from that in the other clones reared under the same conditions. Moreover, the differences among subclones originated from the same female were also revealed. Analysis of the results suggests that the interclonal variations in the proportion of diapausing progeny were at least partly caused by endogenous factors. The differences between clones can arise during the development of one generation but can also damp very fast or, at least, they are not accumulated over generations. These variations are probably based on the same epigenetic mechanisms that form the basis of the maternal effect on the progeny diapause in Trichogramma species.

Field experiments conducted in the environs of St. Petersburg (Russia) with a local population of Calliphora vicina showed that induction of larval diapause under natural conditions was significantly dependent on day lengths and temperature. The maternal photoperiodic response had a distinct threshold: the first diapausing larvae hatched from the eggs laid in the middle of August when the day length was 16 h; at shorter photoperiods, the fraction of diapausing larvae depended only on temperature. At the mean temperature of 16°C, larval diapause was rarely recorded; at 12–13°C, about 50% of the larvae entered diapause; at 7–9°C, nearly all the larvae entered diapause. These results of the field experiments agree well with the parameters of photoperiodic and thermal responses studied in the laboratory at constant temperatures and photoperiods.

The adult photoperiodic responses of two Trichogramma species were investigated by exposing adults during 3 days to one of the seven light: dark regimes: L: D = 4: 20, 8: 16, 12: 12, 14: 10, 16: 8, 18: 6, and 20: 4. The preimaginal stages of these individuals developed under short (L: D = 12: 12), long (L: D = 20: 4) or intermediate photoperiods (L: D = 14: 10 and L: D = 16: 8 for Trichogramma principium and T. embryophagum, respectively). The progeny of these females developed under short day L: D = 12: 12 and at two moderately diapause-inducing temperatures (13 and 14°C for T. principium, 14 and 15°C for T. embryophagum). In both Trichogramma species which developed at both temperatures, the percentage of diapausing prepupae was significantly dependent both on the photoperiodic conditions of the preimaginal development of the maternal generation and on the photoperiod which influenced the adult females. The adults showed a typical long-day photoperiodic response with a threshold day length of ca 13 h in T. principium and ca 16 h in T. embryophagum, which practically coincided with the thresholds of the pupal photoperiodic responses of these species revealed in our previous studies. However, the ultra-short photoperiods (L: D = 4: 20 and 8:16) caused a relatively stronger diapause-inducing effect on the progeny when applied to the adult females than when it was applied to the pupae. Thus, in both the Trichogramma species studied, the patterns of photoperiodic responses of pupa and adult were somewhat different although they almost coincided in the “ecologically significant” part of the photoperiodic scale.

Interaction of the photoperiodic conditions of development of maternal females (day lengths of 2 to 22 h at 20°C) with the thermal regime of development of their progeny (temperature of 12 to 15°C at day length of 12 h) in determination of prepupal diapause in Trichogramma piceum was studied under laboratory conditions. At 15°C the diapause was practically absent. At lower temperatures, the proportion of diapausing prepupae was maximal (25% of larvae at 14°C, 70% of larvae at 13°, and 80% of larvae at 12°C) if the maternal females developed under short day conditions (10–12 h). When maternal females developed at day lengths of 18–20 h, diapause was rarely recorded at all temperatures, while ultra-short (less than 8–10 h) days also caused a decrease in the proportion of diapausing progeny. The right (ecologically important) threshold of this maternal long-day photoperiodic response was about 14–15 h independently of the temperature during the progeny development. These results make it possible to clarify the mechanism of the “maternal photoperiodic correction of the progeny thermal response.” Although the impact of the maternal photoperiodic response can be revealed only within a very narrow thermal range, the relative strength of the diapause-inducing effect of different day lengths is independent of the temperature regimen of the progeny development.