Biological characteristics of a non‐photoperiodic‐diapause strain of the cabbage beetle Colaphellus bowringi (Coleoptera: Chrysomelidae)

Comparison of biological characteristics between diapausing and non‐diapausing strains of insects provides some insights into the mechanisms regulating diapause. In this study, biological characteristics, especially diapause characteristics and life‐history traits, of a non‐photoperiodic‐diapause (NPD) strain of the cabbage beetle Colaphellus bowringi were compared with those of a normal, high‐diapause (HD) strain that enters diapause in response to either long day length or low temperature. The NPD strain did not enter diapause at any photoperiod at 22°C or higher, but still had a capacity to enter diapause at low temperatures. Although diapause could be induced in both strains by exposure to 20°C, the proportion of individuals having shorter diapause duration was greater in the NPD strain compared to the HD strain. The NPD strain had significantly lower hatching and larval survival rates, longer developmental duration of immature stages, smaller body size and lower longevity and female fecundity compared to the HD strain. The NPD strain of this species is a promising model for investigating diapause regulation in insects in general.     

Pre‐wintering conditions and post‐winter performance in a solitary bee: does diapause impose an energetic cost on reproductive success?

1. Diapause is a dynamic process of low metabolic activity that allows insects to survive periods of harsh conditions. Notwithstanding the lowered metabolism, and because diapausing insects have no access to food, diapause has an energetic cost that may affect post‐diapause performance. 2. Previous studies on the solitary bee Osmia lignaria have shown that prolonged pre‐wintering periods (the time during which individuals already in diapause remain at warm temperatures) are associated with elevated lipid consumption, fat body depletion, and body weight loss. The present study investigated whether prolonged pre‐wintering also affects reproduction, i.e. whether the costs associated with diapause could have an effect on post‐diapause performance in this species. 3. Females were exposed to a range of pre‐wintering conditions, and ovary development and individual post‐wintering performance were monitored throughout their adult life span. 4. No evidence of an effect of pre‐wintering duration on post‐diapause reproductive success was found. Expected differences in the timing of establishment were not observed because ovary maturation was, surprisingly, not arrested during pre‐wintering. Prolonged pre‐wintering duration did not result in decreased life span, probably because emerging females could rapidly replenish their metabolic reserves through feeding. However, there was a very strong effect of the duration of the pre‐emergence period on the likelihood of nest establishment. 5. Longevity, the main factor determining fecundity in Osmia, is subjected to high levels of intrinsic variability, even among females of similar size exposed to identical conditions during development and nesting.     

Maternal induction of larval diapause and its sensitive stage in the blow fly Lucilia sericata (Meigen) (Diptera: Calliphoridae)

Lucilia sericata has a facultative diapause in the third larval instar after cessation of feeding. Induction of the diapause is influenced by the photoperiod and temperature conditions experienced by insects in the parental generation as well as those experienced by the larvae themselves. The sensitive stage of the parental generation for induction of diapause was examined using diapause‐averting conditions of 16 h light : 8 h darkness (LD 16:8) at 25°C and diapause‐inducing conditions of LD 12:12 at 20°C. The incidence of diapause in the progeny was predominantly determined by the conditions experienced by the parents in the adult stage. Moreover, the results of reciprocal crosses showed that only the mother's experience is involved in the induction of diapause in the progeny.     

Diapause and overwintering of the hoverfly Episyrphus balteatus

The overwintering biology of the hoverfly Episyrphus balteatus (DeGeer) (Diptera: Syrphidae) was studied and analysed. In Europe, females of this species can overwinter as adults in a facultative, reproductive diapause. The diapause phase is characterized by the ovaries ceasing to develop and by hypertrophy of the fat body. Diapause was induced during the second and third larval instars. The critical photoperiod for inducing diapause was between 11.8 and 11.9 h, corresponding to the day lengths that occur during mid‐ to late September in Hannover, Germany. When temperatures were lower, insects could be induced into diapause at longer day lengths, similar to those that occur in early September in Hannover. A semifield study was done during the winter of 1999/2000 to confirm the results obtained under laboratory conditions and to obtain additional information on the over‐wintering development and mortality of E. balteatus. The results suggested that mortality was correlated with the duration of the experiment and with humidity, rainfall, and temperature. The importance of these results for the regulation of cereal aphid populations by this hoverfly is discussed.     

Diapause termination of Rhagoletis cerasi pupae is regulated by local adaptation and phenotypic plasticity: escape in time through bet‐hedging strategies

Persistence and thriving of univoltine, herbivore insect species of the temperate zone rely on obligate diapause response that ensures winter survival and synchronization with host phenology. We used a stenophagous fruit fly (Rhagoletis cerasi) with obligate pupae diapause to determine genetic and environmental effects on diapause intensity of geographically isolated populations with habitat heterogeneity. Pupae from two Greek and one German populations with various gene flow rates were exposed at five constant chilling temperatures (0–12 °C) for different durations and then incubated at a high temperature until all adults have emerged. Pupae diapause intensity differs among Greek and German populations, suggesting an adaptive response to habitat heterogeneity (mostly differences in phenology patterns of local host cultivars). Moderately warm winter temperatures, such as 8 °C, promote diapause termination in all three populations. Insufficient chilling (short duration or warmer temperatures) regulates the expression of prolonged dormancy. Interestingly, extended chilling (longer than required for terminating diapause) ‘return’ pupae to another (facultative) cycle of dormancy enabling adults to emerge during the next appropriate ‘window of time’; a strategy first time reported for univoltine insects. Consequently, diapause duration of R. cerasi is determined both by i) the adaptive response to local climatic conditions (annual dormancy) and ii) the plastic responses to interannual climatic variability resulting in two types of long life cycles within populations, prolonged and facultative dormancy as response to insufficient chilling and extended exposure to chilling, respectively. Long life cycles are expressed as a part of dormancy bet‐hedging strategies of R. cerasi populations.     

Long life cycles in insects

Long life cycles covering more than one year are known for all orders of insects. There are different mechanisms of prolongation of the life cycle: (1) slow larval development; (2) prolongation of the adult stage with several reproduction periods; (3) prolongation of diapause; (4) combination of these mechanisms in one life cycle. Lasting suboptimal conditions (such as low temperature, low quality of food or instability of food resources, natural enemies, etc.) tend to prolong life cycles of all individuals in a population. In this case, the larvae feed and develop for longer than a year, and the active periods are interrupted by dormancy periods. The nature of this dormancy is unknown: in some cases it appears to be simple quiescence, in others it has been experimentally shown to be a true diapause. Induction and termination of these repeated dormancy states are controlled by different environmental cues, the day-length being the principal one as in the case of the annual diapause. The long life cycles resulting from prolonged adult lifespan were experimentally studied mainly in beetles and true bugs. The possibility of repeated diapause and several periods of reproductive activity is related to the fact that the adults remain sensitive to day length, which is the main environmental cue controlling their alternative physiological states (reproduction vs. diapause). Habitats with unpredictable environmental changes stimulate some individuals in a population to extend their life cycles by prolonged diapause. The properties of this diapause are poorly understood, but results of studies of a few species suggest that this physiological state differs from the true annual diapause in deeper suppression of metabolism. Induction and intensity of prolonged diapause in some species appear to be genetically controlled, so that the duration of prolonged diapause varies among individuals in a group, even that of sibles reared under identical conditions. Thus, long life cycles are realized due to the ability of insects to interrupt activity repeatedly and enter dormancy. This provides high resistance to various environmental factors. Regardless of the nature of this dormancy (quiescence, annual or prolonged diapause, or other forms) and the life cycle duration, the adults always appear synchronously after dormancy in the nature. The only feasible explanation of this is the presence of a special synchronizing mechanism, most likely both exo- and endogenous, since the adults appear not only synchronously but also in the period best suited for reproduction. As a whole, the long life cycles resulting from various structural modifications of the annual life cycle, are typical of the species living under stable suboptimal conditions when the pressure of individual environmental factors is close to the tolerance limits of the species, even though it represents its norm of existence. Such life cycles are also typical of the insects living in unstable environments with unpredictable variability of conditions, those developing in cones and galls, feeding on flowers, seeds, or fruits with limited periods of availability, those associated with the plant species with irregular patterns of blossoming and fruiting, and those consuming low-quality food or depending on unpredictable food sources (e.g., predators or parasites). Long cycles are more common in: (1) insect species at high latitudes and mountain landscapes where the vegetation season is short and unstable; (2) species living in deserts or arid areas where precipitation is unstable and often insufficient for survival of food plants; (3) inhabitants of cold and temporary water bodies that are not filled with water every year. At the same time, long life cycles sometimes occur in insects from other climatic zones as well. It is also important to note that while there is a large body of literature dealing with the long life cycles in insects, it mostly focuses on external aspects of the phenomenon. Experimental studies are needed to understand this phenomenon, first of all the nature of dormancy and mechanisms of synchronization of adult emergence.     

Prepupal diapause and instar IV developmental rates of the spruce beetle, Dendroctonus rufipennis (Coleoptera: Curculionidae, Scolytinae)

The spruce beetle, Dendroctonus rufipennis (Kirby), is an important mortality agent of native spruces throughout North America. The life-cycle duration of this species varies from 1 to 3 years depending temperature. The univoltine cycle (one generation per year) is thought to maximize outbreak risk and accelerate host mortality in established outbreaks. Prepupal diapause is associated with the semivoltine cycle (one generation per 2 years) and we investigated thermal conditions that result in diapause induction. Preliminary experiments used respirometry in an attempt to distinguish the diapause state of experimental insects but the technique was apparently confounded by low respiration before and during pupation, regardless of diapause status. Therefore, diapause induction was deduced using developmental delays. The observed developmental response was not a “switch”, with developmental delay either present or absent, but instead varied continuously. We found that temperatures <15 °C from instar III through mid-instar IV were associated with developmental delays beyond that expected from cool temperatures. Moreover, the duration of exposure to cool temperatures was important in determining the degree of developmental delay. Small, if any, delays were observed if the cumulative exposure to <15 °C was <20 d whereas >40 d cumulative exposure was associated with distinct developmental suppression. Intermediate exposure to cool temperatures resulted in minor developmental delays. We used our results to parameterize a maximum likelihood estimation model of temperature-dependent instar IV developmental rates, including the effect of diapause. This model can be included as part of a spruce beetle phenology model for predicting population dynamics.     

Very long diapause and extreme resistance to population disturbance in a galling insect

1. Some insects have a prolonged diapause – a dormancy that extends over more than 1 year. In most species prolonged diapause involves one or a few extra years, but in extreme cases diapause may surpass 10 years. Few cases of very long diapause have been described, and very little is known about the population consequences of the temporal refuge formed by the diapausing individuals. 2. The gall midge Contarinia vincetoxici Kieffer galls the flowers of a long‐lived herb Vincetoxicum hirundinaria Med. After completing development, larvae leave the galls for the ground where they enter diapause. Extending an earlier published inoculation experiment, we show that the diapause may last up to at least 13 years, with a median duration of at least 6 years. 3. The gall midge is attacked by two parasitoid species. Dissections of gall midge larvae for presence of parasitoids revealed that Omphale salicis Haliday had a maximum 2 year diapause and Synopeas acuminatus Kieffer a maximum 4 years. The very long diapause of the gall midge may thus provide a temporal refuge from these enemies. 4. In a 15‐year field experiment all galls were removed every year from six isolated habitat patches. Density changes in experimental populations were not statistically different from control populations for over a decade. After 14–15 years a modest decline could be observed. This slow response illustrates that prolonged diapause in C. vincetoxici provides a very strong population buffer against mortality during the galling stage.     

Coping with drought: diapause plasticity in katydid eggs at high,mid-, and low elevations

1. Semi-arid rangeland productivity is limited by precipitation, and yet droughts are projected to increase in frequency and duration with unknown impacts on insect populations. As some katydids prolong diapause and remain in an egg bank as a blastoderm for multiple growing seasons, is it possible that drought could prolong diapause and promote outbreaks by synchronising embryonic development and hatching of Mormon crickets, Anabrus simplex, after moisture is restored? 2. In this study, a high-elevation Wyoming population (WY) was compared with a mid-elevation Idaho (ID) and a low-elevation Oregon population (OR). It was predicted that eggs from the drier ID and OR habitats would be more tolerant of desiccation. Developmental state and water loss of eggs were measured after drought treatments, and when moisture was restored. 3. The two drier treatments had significantly more WY eggs prolonging diapause until after drought ended compared with the two wetter treatments. Whether WY eggs developed in the second or subsequent warm periods was independent of drought treatments. Significantly fewer OR embryos developed in the driest treatment compared with the others, whereas almost all ID eggs developed irrespective of the drought treatment. 4. In conclusion, Mormon crickets can delay embryonic development to improve drought tolerance. Although drought did not synchronise development and hatching, diapause plasticity allowed insects to cope and await more favourable conditions. 5. Unexpectedly, eggs from WY (the highest, wettest site) were more tolerant, because postponing development resulted in less water loss than in developed embryos. OR egg loss was also reduced by prolonging diapause, relative to ID, which developed in even the driest conditions.     

Strategies of emergence in the chestnut weevil Curculio elephas (Coleoptera: Curculionidae)

In the chestnut weevil Curculio elephas, adult emergences spread over 3 or 4 years due to prolonged larval diapause in some individuals. Weevils with an extended diapause emerge, on the average, 1–10 days before those with simple diapause, but whatever the age of insects, emergences occur always from mid-August to early October. When the summer is dry, some adults cannot emerge because of the hardness of the soil. Emergence sucess of adults is smaller in females than in males. The result is that the sex ratio is female-biased before emergence and male-biased after. Summer drought cannot be predicted by the chestnut weevil, and when the soil is dry 27–78% of females cannot emerge and do not reproduce. The year after a summer drought, many reproducing females may emerge from larvae with prolonged diapause. These results suggest an evolutionary influence on the variability in diapause duration. Computer simulations and observations do not support the hypothesis that the main cause of variation in diapause length is the existence of several distinct genotypes within populations. On the contrary, our data strengthen the hypothesis for coin-flipping plasticity discussed in a previous paper.     

Diapause strategy in the parasitoid Psyllaephagus pistaciae

The processes involved in the induction and termination of diapause in the parasitoid Psyllaephagus pistaciae Ferrière (Hymenoptera: Encyrtidae) were investigated under controlled laboratory conditions. Results indicated that the parasitoid P. pistaciae is able to overwinter successfully as a pupa within the mummified psyllid host Agonoscena pistaciae Burckhardt and Lauterer (Homoptera: Psylloidea), and the parasitoid clearly responded to a short photoperiod by entering diapause. The incidence of diapause increased significantly with decreasing temperature, indicating the existence of an interaction between low temperature and short‐day photoperiod that resulted in a 100% diapause. However, low temperature alone partly stimulated P. pistaciae to enter diapause. The incidence of diapause was likely to be independent from psyllid nymphal instar and the condition of its mother. This investigation showed that the termination of diapause in the overwintering parasitoid pupae and the duration of diapause completion or adult emergence is photoperiod independent, but dependent on temperature. However, chilling treatment was found to be unnecessary for diapause termination in P. pistaciae, although it decreased the duration of diapause maturation. The positive responses of parasitoids to environmental factors appears to act as a basic strategy to induce diapause in the parasitoid P. pistaciae against extreme climatic conditions.     

Environmental control of the seasonal life cycle of a zoophytophagous mirid, <Emphasis Type="Italic">Adelphocoris triannulatus</Emphasis> (Hemiptera: Miridae)

Adelphocoris triannulatus (Stål) (Heteroptera: Miridae) is a zoophytophagous bug that inhabits grasslands and crop fields, and is widely distributed in eastern Asia. In this study, the seasonal occurrence and environmental factors controlling the seasonal life cycle of this bug were investigated in a local population in southern Kyushu, Japan. When insects were reared under different photoperiods at 20 and 25 °C, they produced diapause or nondiapause eggs depending on the photoperiod: when reared under short-day photoperiods (≤?13 h/day), females laid diapause eggs, whereas they laid nondiapause eggs when reared under long-day photoperiods (≥?14 h/day). Because the developmental duration of the pre-adult stages and maturation (minimum duration of one generation) is estimated to be approximately 30 days at summer temperatures, these results suggest that this bug has a multivoltine seasonal life cycle. In agreement with the laboratory results, only nondiapause eggs were produced until mid-August in the field, and an increasing proportion of diapause eggs were produced from early September. In addition to these findings, field sampling from spring to autumn in 2009 indicated that three or four generations were produced in one year. These results suggest that egg diapause plays a role in preventing nymphs of A. triannulatus from developing in winter, and thereby ensures the survival of the population in adverse seasons.     

A trade-off between diapause duration and fitness in female parasitoids

Abstract 1. Diapause allows insects to cope with adverse weather conditions but also poses substantial fitness costs, e.g. through reduced survival of the diapausing stage or sublethal effects in the following season.
2. The energetic costs of diapause in females of the parasitoid Asobara tabida Nees (Hymenoptera: Braconidae) were assessed using experimental manipulation of diapause duration.
3. An increase in diapause length not only led to higher mortality among diapausing pupae but also caused a significant decrease in egg load, fat reserves, and dry weight of the emerging adult females.
4. Only larvae with sufficient resources were able to survive the entire diapause period and there was a trade-off between the metabolic costs of diapause and adult fitness components.
5. In contrast, the size (tibia length) of emerging females increased with increasing diapause duration. It is proposed that this may be due to a genetic correlation between larval dry weight and adult size, so that selection for heavier larvae would result in larger adults even when resources are limited.
6. Ultimately, the evolution of diapause duration and its consequences for adult performance will depend on the costs of overwintering and the ecological conditions in the habitat.     

Activity enhancement of Zygogramma bicolorata,a biocontrol agent of Parthenium hysterophorus,by temperature regulated diapause aversion

Diapause is a unique strategy of dormancy in insects to avoid unfavourable conditions. The exotic beetle Zygogramma bicolorata, is an effective biological control agent of Parthenium hysterophorus in India, Adults diapause in soil during December to May. As a result, there is delay in its effectiveness on the plant that reaches to flowering and seed production by the time the beetle is able to build up its population after emerging from diapause. Therefore, a study was conducted to explore possibilities of diapause aversion by temperature regulation. Results indicated that exposure of newly emerged adults to heat treatment of 35°C and to low temperature of 10°C could reduce diapause in Z. bicolorata. The low temperature can also be used as a medium for the storage of the mass reared beetles for a long time without having negative effect on their longevity and fecundity.     

Dispersion of flightless adults of the Asian lady beetle, Harmonia axyridis, in greenhouses containing cucumbers infested with the aphid Aphis gossypii: effect of the presence of conspecific larvae

Nezara viridula (L.) (Heteroptera: Pentatomidae) recently expanded its distribution range in Japan and reached Osaka. In the southern temperate zone, the species overwinters in the adult stage and reproductive diapause is associated with a body colour change from green to russet. In Osaka, the reproductive diapause is only induced in September–October and nymphs from late egg masses are destined to die during winter. However, the fate of adults emerging late in the season remained unknown. Survival, body colour change, and post‐diapause reproduction were studied under quasi‐natural conditions in Osaka in 1999–2000 in those adults that attained adulthood as late as in November. Two experimental cohorts were used: in the Outdoor cohort, insects were reared outdoors starting in their second instar (28 September); in the Laboratory cohort, nymphs and subsequently adults were reared from the same day under diapause‐inducing conditions (L10:D14 at 25 °C), then acclimatized (5 days at 20 °C and 5 days at 15 °C; L10:D14) and transferred outdoors on 1 December. Adults in both cohorts did not reproduce in autumn and survived the winter with a low mortality. More than 20% of adults in the Outdoor cohort failed to change body colour from green to russet during winter, apparently because of the low ambient temperature, suggesting that the environmental conditions required for colour change do not completely coincide with those required for diapause induction, and that the colour of the adults is not always a reliable indicator of diapause in this species. After overwintering, females from the Outdoor cohort produced significantly fewer egg masses and eggs and had a significantly shorter period of oviposition than females that entered diapause under short‐day conditions in the Laboratory cohort. Thus, if progeny from the late egg masses attain adulthood late in the autumn, these adults have high chances of successful overwintering, but their reproductive output after the winter diapause is significantly reduced.     

The roles of juvenile hormone and biogenic amines on pheromone response plasticity and diapause termination in male Caloptilia fraxinella

In insects that exhibit a period of delayed reproduction, the timing of mating and reproduction is controlled by environmental conditions that regulate endogenous factors such as hormones and biogenic amines (BAs). Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae) undergoes a 9‐month reproductive diapause from adult eclosion in the summer until diapause termination the following spring when adults mate. Male response to female sex pheromone is plastic, and is most acute when moths are reproductively active. The aim of this study is to further elucidate the mechanisms involved in the regulation of male response to pheromone in C. fraxinella, and to test whether the application of BAs with and without a juvenile hormone analogue (JHA) to males in different physiological states impacts pheromone responsiveness, as measured by electroantennogram and wind tunnel bioassays. Treatment of male C. fraxinella in reproductive diapause with one application of a JHA induces the highest subsequent pheromone response in the fall, but does not alter pheromone response earlier in reproductive diapause in the summer. The JHAs methoprene and pyriproxyfen similarly enhance pheromone response in the fall. Treatment with methoprene alone or in combination with one of the BAs octopamine, dopamine or serotonin increases male pheromone responsiveness in the fall. The increase in pheromone response can be attributed to methoprene only, as treatment with any of the BAs alone does not enhance male response to pheromone. Biogenic amine treatment lowers male responsiveness to pheromone in some experiments, indicating that there may be a role for BAs in maintaining low pheromone response during reproductive diapause in this species.     

Host‐plant quality adaptively affects the diapause threshold: evidence from leaf beetles in willow plantations

1. Voltinism of herbivorous insects can vary depending on environmental conditions. The leaf beetle Phratora vulgatissima L. is univoltine in Sweden but will sometimes initiate a second generation in short‐rotation coppice (SRC) willow plantations. 2. The study investigated whether increased voltinism by P. vulgatissima in plantations can be explained by (i) rapid life‐cycle development allowing two generations, or (ii) postponed diapause induction on coppiced willows. 3. In the field, no difference was found in the phenology or development of first‐generation broods between plantations (S. viminalis) and natural willow habitats (S. cinerea). However, the induction of diapause occurred 1–2 weeks later in SRC willow plantations. 4. Laboratory experiments indicated no genetic difference in the critical day‐length for diapause induction between beetles originating from plantations and natural habitats. Development time was unaffected by host‐plant quality but critical day‐length was prolonged by almost an hour when the beetles were reared on a non‐preferred willow species (S. phylicifolia). When reared on new leaves from re‐sprouting shoots of recently coppiced willow plants, diapause incidence was significantly less than when the beetles were reared on mature leaves from uncoppiced plants. 5. The study suggests that P. vulgatissima has a plastic diapause threshold influenced by host‐plant quality. The use of host‐plant quality as a diapause‐inducing stimulus is likely to be adaptive in cases where food resources are unpredictable, such as when new host‐plant tissue is produced after a disturbance. SRC willows may allow two beetle generations due to longer growing seasons of coppiced plants that grow vigorously.     

The effects of temperature and moisture on the inception of diapause in eggs of the Australian plague locust,Chortoicetes terminifera Walker (Orthoptera: Acrididae)

Chortoicetes terminifera lays a mixture of diapause and non-diapause eggs during the autumn months of March and April. Diapause eggs cease development in late anatrepsis when kept in moist soil at 32.5°C. These conditions favour rapid embryogenesis and hatching in non-diapause eggs. Significant reductions in the incidence of diapause were obtained when newly-laid eggs were kept for 40 days at either 15.5°C or 32.5°C in conditions that prevented the uptake of water. In eggs held for a similar period in moist soil at 15.5°C, diapause was virtually eliminated. These eggs did not develop to the stage at which water uptake occurs. It is suggested that the inception of diapause is dependent upon the occurrence of warm, moist conditions at the time of laying and that these requirements are an integral part of the overall inductive process, which also involves the temperature and photoperiod at which the parent insects are reared. A flow-diagram illustrating the development of diapause eggs in relation to temperature and moisture is discussed in relation to the survival of the species.     

Maternal influence on diapause induction in Trichogramma (Hymenoptera: Trichogrammatidae): the dynamics of photosensitivity

It is known that the prepupal diapause in Trichogramma is dependent on temperature conditions of embryonic and larval development and on the photoperiodic conditions of preimaginal development of maternal females. However, the relative photosensitivity of different preimaginal stages has been never investigated. We studied the position of the photosensitive period over the preimaginal development of maternal females in T. embryophagum Htg. and T. principium Sug. et Sor. by transferring preimaginal stages between diapause‐averting ‘long’ day (L : D = 20 : 4) and diapause‐inducing ‘short’ day (L : D = 12 : 12). Results showed that the influence of maternal photoperiod on progeny diapause was determined during the late pupal stage (last days of preimaginal development at 20°C). During this time, the critical duration of the photoperiodic induction was extremely small: one short or long day caused almost the same effect as the permanent development under these conditions. As a whole, photoperiodic response in Trichogramma was found to be unusually rapid, labile and easily reversible which is probably explained by extremely small size and fast development of these egg parasitoids. The results of this study could be used for elaboration of optimal methods for Trichogramma mass rearing and storage and for prediction of its seasonal cycles under natural conditions.