Maternal thermal and photoperiodic effects on the progeny diapause in Trichogramma telengai Sorokina (Hymenoptera: Trichogrammatidae)

Although maternal photoperiodic and maternal thermal effects on the progeny diapause have been demonstrated in a number of insect species, their interaction has been rarely studied. We investigated this interaction in Trichogramma telengai. In a series of experiments, maternal females were reared at day lengths of 12–18 h and at temperatures of 17, 20, 25 and 30°C. Their progeny developed under day length of 12 h and temperatures of 13, 14 and 15°C. The experiments showed that both short day and low temperature experienced by the maternal generation significantly increased the proportion of diapausing progeny. In particular, the threshold of the maternal photoperiodic response decreased with temperature. Under combinations of photoperiod with daily thermoperiod, the role of the “night” temperature in the induction of diapause in the progeny was much more important than that of the “day” temperature. We conclude that the interaction pattern between the photoperiodic and thermal maternal effects in T. telengai is generally the same as that between the photoperiodic and thermal responses directly influencing diapause induction in other long‐day insects. The threshold temperature of the maternal thermal response of T. telengai was about 25–27°C, while diapause can be induced if larvae develop at temperatures not higher than 15–16°C. This suggests that, at least in the studied Trichogramma species, the maternal thermal effect has no ecological value. In the practice of biocontrol, however, rearing of Trichogramma wasps at high temperature can drastically reduce the proportion of diapausing progeny.     

Maternal thermal effect on diapause in Trichogramma species (Hymenoptera: Trichogrammatidae)

Both direct thermal and maternal photoperiodic effects on diapause induction have been thoroughly investigated in many insect species, while maternal thermal effects have been infrequently studied. We studied the effect of temperature during development of maternal generation on the proportion of diapausing progeny in four species of the genus Trichogramma Westw., minute egg parasitoids, widely used for biological control of lepidopteran pests. The maternal generations were reared at day lengths of 12 and 18 h and temperatures of 17, 20, 25 and 30°C, and their progeny developed under day length of 12 h and temperatures of 13 and 14°C. In T. evanescens and T. piceum, the proportion of diapausing progeny decreased with increasing temperature under all tested photoperiods and thermal regimes of progeny development; the high temperature of 30°C totally averted diapause of progeny. In T. buesi and T. principium, low temperatures of 17 and 20°C resulted in relatively high proportion of diapausing progeny only when the maternal generation developed under short‐day conditions. The threshold of the maternal thermal response varied from 17–18 to 22–23°C. Under field conditions, Trichogramma females are exposed to such high temperatures only during summer, when diapause in their progeny is in any case prevented by the maternal photoperiodic response and by the thermal response of the larvae. We conclude that the maternal thermal effect on diapause induction, although to a different extent, is inherent to Trichogramma species but, at least as suggested by laboratory experiments, it does not play any role in the regulation of seasonal development under natural conditions. However, during mass rearing of Trichogramma wasps, it should be taken into account that high temperature, even when combined with short photoperiod, can avert diapause in the next generation.     

Induction of larval diapause in the blowfly, Calliphora vicina R.-D. (Diptera, Calliphoridae) under field and laboratory conditions

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.     

On the direct influence of photoperiod on diapause in <Emphasis Type="Italic">Trichogramma embryophagum</Emphasis> (Hymenoptera,Trichogrammatidae)

A direct photoperiodic reaction of T. embryophagum larvae was investigated in the laboratory. Maternal females developed at 20°C and day length of 12 h. Progeny generation developed at 14°C and day lengths of 0, 4, 8, 12, 16, 20, and 24 h. Experiments showed that under the short days (12, 8, and 4 h) and in continuous darkness, the percentage of diapausing individuals was significantly higher than under the long days of 16 and 20 h. Under permanent light, however, the inclination to diapause also increased. The earlier investigated maternal influence on T. embryophagum progeny diapause showed a somewhat different pattern of photoperiodic reaction. These results suggest that the direct effect of the day length on Trichogramma diapause is a relatively autonomous process rather than just a side-effect of the photoperiodic reaction determining maternal influence. However, this direct effect is very weak and could possibly be a “rudimentary reaction.”     

Effects of thermoperiods on the reproductive activity of females and on the maternal induction of larval diapause in the blowfly, <Emphasis Type="Italic">Calliphora vicina</Emphasis> R.-D. (Diptera,Calliphoridae)

The interaction of thermoperiod and photoperiod in their influence on the reproductive maturation of females and on the induction of the maternal effect determining larval diapause of the progeny of the blowfly, Calliphora vicina, was first investigated under laboratory conditions. Under the combination of a day length of 12 h with a thermoperiod (the alternation of 12 h long periods with temperatures of 10 and 20°C) the reproductive maturation of females was faster than at the corresponding mean constant temperature of 15°C. Under the “natural” thermoperiod, when the period with a temperature of 10°C coincided with “night-time” (the dark phase of the diurnal light-dark cycle) the maturation of females was slower than that under the “inverted” thermoperiod, when the period with a temperature of 10°C coincided with “day-time” (the light phase of the diurnal light-dark cycle). The proportion of diapausing individuals was maximal in the progeny of females kept at 20°C and decreased with the increase in temperature. Under thermoperiods (the alternations of 12 h long periods with temperatures of 20 and 26°C) the proportion of diapausing progeny was lower than that under the corresponding mean constant temperature of 23°C, but under the inverted thermoperiod with a high night temperature this effect was much stronger. In combination with the results of our previous studies, these data support the hypothesis that the effects of “night” and “day” temperatures are substantially different only when the thermal response interacts with a strong photoperiodic response.     

Multigenerational maternal inhibition of prepupal diapause in two Trichogramma species (Hymenoptera: Trichogrammatidae)

It is known that in some insect species the incidence of diapause among the progeny of females that had undergone diapause is relatively low or zero even under strong diapause-inducing conditions. Moreover, the maternal inhibition, preventing the induction of a maladaptive diapause in spring, can persist over several generations. This multigenerational effect based on hypothetical ‘interval timer’ was thoroughly studied in Aphididae. We first described a similar phenomenon in Hymenoptera: laboratory experiments demonstrated that the proportion of diapausing progeny of Trichogramma females that had undergone diapause was practically zero independently of photoperiodic and temperature conditions used (day lengths of 12 and 18 h and temperatures of 12–15 °C). Then the ability to enter diapause recovered gradually and returned to the normal level over two (in Trichogramma telengai) or even five (in Trichogramma principium) generations. We conclude that the observed effect may be based on an interval timer similar to that in aphids.     

Cumulative induction of diapause in successive generations of Trichogramma species (Hymenoptera,Trichogrammatidae)

The ability of two Trichogramma species (Trichogramma telengai Sor. and T. principium Sug. et Sor.) to accumulate the effect of the maternal photoperiodic response during 5 consecutive generations reared in the laboratory under the short day (12 h) conditions was investigated. Control individuals developed at the same temperature of 20°C, but under the long day (18 h) conditions. The tendency to diapause was estimated by the proportion of progeny that entered diapause under the short day conditions at the temperatures of 13, 14 and 15°C. Trichogramma principium manifested an evident transgenerational cumulative photoperiodic response: the development of 1, 2, and 3 consecutive generations under the short day conditions caused a gradual increase in the proportion of diapausing progeny. In T. telengai, the progeny of the females that developed under the short day conditions also entered diapause more often than the progeny of those that developed under the long day conditions, but the number of preceding generations which developed under short day had no effect on the tendency to diapause. This interspecific difference can be possibly explained by the different natural geographic ranges of the two studied species. Trichogramma principium occurs in Southern Europe, Southern Kazakhstan, and Central Asia where the autumnal decrease in temperature is very slow and thus two or even more generations can develop under the short day conditions, whereas T. telengai is distributed over Central and North-Western Europe and in Siberia, where the autumnal decrease in temperature is rather fast, the development of one more “autumnal” generation is risky, and thus even the first “short-day signal” induces a very strong tendency of the progeny to diapause.     

Effects of photoperiod and temperature on diapause induction in Conogethes punctiferalis (Lepidoptera: Pyralidae)

The yellow peach moth, Conogethes punctiferalis (Guenée), a multivoltine species that overwinters as diapausing larvae, is one of the most serious insect pests on maize in China. Effect of photoperiod and temperature on larval diapause was examined under empirical laboratory conditions. Short‐day treatments caused larval diapause at 25°C, and the critical photoperiod was between 12 and 13 h (or 12 h 51 min) light per day. No sensitive instar was identified for diapause induction under alternated short‐ (L : D 11 : 13 h) and long‐day (L : D 14 : 10 h) treatments at different larval stages. However, accumulative treatment of three instars and 10 d under short‐day treatment was required for the induction of 50% larval diapause. All larvae entered diapause at 20°C, whereas less than 3% did so at 30°C, irrespective of the long‐ or short‐day treatment. Furthermore, under the short‐day treatment, more than 90% of larvae went into diapause with temperatures ≤ 25°C, but less than 17% did so at 28°C. In contrast, under the long‐day treatment, less than 19% of larvae went into diapause with temperatures ≥ 23°C. The forward shift (5°C) of critical temperature under the long‐day regime demonstrated the compensatory effect of temperature and photoperiod on diapause induction. In conclusion, C. punctiferalis had a temperature‐dependent type I photoperiodic diapause response; there was no sensitive instar for diapause determination, but the photoperiodic accumulation time countermeasures both of the short‐day cycles and the number of instars exposed, and the photoperiodic diapause response, was a temperature‐compensated phenomenon.     

Effects of photoperiod and temperature on diapause induction and termination in the swallowtail,Sericinus montelus

Abstract Sericinus montelus overwinters as diapausing pupae. In the present study, the effects of photoperiod and temperature on diapause induction and termination of diapause are investigated. The results obtained demonstrate that high temperature can reverse the effect of short day‐lengths on diapause induction. Under an LD 12 : 12 h photoperiod, all pupae enter diapause at 15, 20 and 25 °C, whereas all pupae develop without diapause at 35 °C. No pupae enter diapause under an LD 14 : 10 h photoperiod when the temperature is above 20 °C. Photoperiodic response curves obtained at 25 and 30 °C indicate that S. montelus is a long‐day species and the critical day‐length is approximately 13 h at 25 °C. At 25 °C, the duration of diapause is shortest when the diapausing pupae are maintained under an LD 16 : 8 h photoperiod and increases under LD 14 : 10 h and LD 12 : 12 h photoperiods. Under an LD 16 : 8 h photoperiod, the duration of diapause is shortest when the diapausing pupae are maintained at 25 °C, followed by 20 and 30 °C, and then at 15 °C. These results suggest that a moderate temperature favours diapause development under a diapause‐averting photoperiod in this species. The duration of diapause induced by an LD 12 : 12 h photoperiod is significantly longer at 25 °C than those at 15, 20 and 30 °C, and is shortest at 15 °C. At 25 °C, the duration of diapause induced by LD 6 : 18, LD 12 : 12 and LD 13 : 11 h photoperiods is similar and longer than 90 days. Thus, the diapause‐inducing conditions may affect diapause intensity and a photoperiod close to the critical day‐length has significant influence on diapause intensity in S. montelus.     

Inheritance of photoperiodic induction of larval diapause in the Asian corn borer Ostrinia furnacalis

The Asian corn borer Ostrinia furnacalis (Guenée) enters facultative diapause as fully‐developed larvae in response to short‐day conditions. As a consequence of geographical variation in photoperiodic response, moths from Nanchang (28°46′N, 115°50′E) enter diapause in response to short day‐lengths (D strain), even at the high temperatures whereas moths from Ledong (18°47′N, 108°89′E) exhibit almost no diapause under the same conditions (N strain). In the present study, crosses between the two strains are used to evaluate the inheritance of diapause under different photoperiods at temperatures of 22, 25 and 28 °C. The moths, both reciprocal crosses and backcrosses, show a clear long‐day response, similar to that of the D strain, suggesting that the photoperiodic response controlling diapause in this moth is heritable. However, the critical day‐length for induction of diapause is shorter in hybrids than in the D strain. The N strain also shows a short‐day photoperiodic response at the lower temperature of 22 °C, indicating that the N strain still has the capacity to enter a photoperiodically‐induced diapause, depending on the rearing temperature. The incidence of diapause in all crosses is highest with D strain fathers or grandfathers and lowest with N strain fathers or grandfathers, indicating that the male parent has significantly more influence on the incidence of diapause of subsequent progeny than the female. The results obtained from all crosses under LD 12 : 12 h or LD 13 : 11 h photocycles at 25 °C show that inheritance of diapause in O. furnacalis does not fit an additive hypothesis and that the capacity for diapause is transmitted genetically in the manner of incomplete dominance.     

The seasonal development cycle of <Emphasis Type="Italic">Aphantopus hyperantus</Emphasis> (L.) (Lepidoptera,Nymphalidae: Satyrinae) in Leningrad Province

The simultaneous influence of the photoperiodic and temperature conditions on pre-diapause and postdiapause larval development of the ringlet Aphantopus hyperantus was studied. At the short day (12 h of light a day) all larvae reached the III instar and entered diapause at all the temperatures tested (18–24°C). At the long day (22 h) the number of diapausing larvae increased with decreasing temperature, and the larvae diapaused at the III and IV instars. The I and II instar larvae (before winter) developed and grew faster under short-day, and the hibernated larvae, under long-day conditions. At the short day the growth rate of the I and II instar larvae did not depend on the temperature, and at the long day it decreased as the temperature increased. After hibernation the growth rate of the IV instar larvae increased with the temperature under long-day conditions, and did not depend on the temperature under short-day ones. Thus, acceleration or deceleration of development, depending on the photoperiodic and temperature conditions, regulated the timing of diapause in the overwintering stage (the III instar larva) and maintained the univoltine seasonal cycle. The males of A. hyperantus developed and grew faster than females while their weight at the larval and pupal stages was smaller. The thermal reaction norms for development (the lower temperature threshold and the coefficient of thermal sensitivity of development) were calculated for the eggs, IV and V instar larvae, and pupae. The coefficient of thermal sensitivity of development for the IV and V instar larvae taken together was lower, and the threshold was higher than the corresponding values for eggs and pupae. The thermal reaction norms for development of the latter stages were the same. The larvae of A. hyperantus retained their sensitivity to the photoperiod after hibernation. Resumption of development after cold reactivation occurred only under long-day conditions, whereas under short-day ones the overwintered larvae entered a repeated diapause. The possibility of photoperiodic reactivation and the absence of spontaneous resumption of development in these larvae were demonstrated.     

Environmental induction of larval diapause and life-history consequences of post-diapause development in the Large Copper butterfly,Lycaena dispar (Lepidoptera: Lycaenidae)

Many insects in temperate zones withstand the adverse conditions of winter through entering diapause and the two most important environmental stimuli that induce diapause are photoperiod and ambient temperature. The Large Copper butterfly, Lycaena dispar Haworth (Lepidoptera: Lycaenidae), is a Palearctic butterfly that hibernates as larvae. Since this butterfly is a near threatened species in some regions, there has been a growing need for a standardized protocol for mass rearing of this butterfly based on the adequate knowledge of its ecology. In the present study, we first identified that L. dispar larvae were sensitive to the photoperiodic induction of diapause during their first larval instar. We then investigated to what extent the diapause-inducing effects of photoperiod could be modified by ambient temperatures in L. dispar larvae by exposing them to the range of day-lengths (L:D 14:10, 12:12, 10:14 and 8:16) at three different temperatures (15, 20 and 25 °C). All larvae were induced to enter diapause at low ambient temperature (15 °C) regardless of photoperiod, whereas most of them (86 %) exhibited direct development when temperature was high (25 °C). The photoperiodic induction of diapause was evident when day-length was shorter than 14 h at intermediate temperature (20 °C). Pre-diapause development was prolonged at low temperatures. Finally, we found that post-diapause development of L. dispar larvae was determined by both the chilling temperature experienced by diapausing larvae and the duration of larval diapause. Adult emergence was enhanced when larvae were chilled at 8 °C and when they had been under the state of diapause for 20 days before they were treated to terminate diapause.     

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, from tropical and temperate regions

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, obtained from south-central Mexico (19°N latitude) and southeast Missouri (37°N latitude) were compared. Day length and temperature were found to serve as environmental cues to programme the larval diapause of both populations, but different critical values were observed. The critical day length for diapause induction was about 13 hr light/day for Mexican larvae and about 15 hr light/day for Missouri larvae, and was relatively stable at 20 to 30°C. Mexican larvae displayed a less-intense diapause than did Missouri larvae. Some diapausing Mexican larvae maintained at 25 or 30°C pupated in about 15 days, regardless of the day length to which they were exposed. The rate of diapause development of Mexican larvae was high at day lengths between 14 hr and 16 hr, whereas that of Missouri larvae was accelerated at day lengths of 16 hr at 25 and 30°C. Diapause development of Mexican larvae was virtually unaffected by chilling at 10°C, whereas that of Missouri larvae continued at a low rate at 10°C. Selection of Mexican larvae for diapause showed that only four generations were needed to significantly increase the incidence of diapause.     

Photoperiodic Control of Pre-Adult Development and Adult Diapause Induction in Zoophytophagous Bug Dicyphus errans (Wolff) (Heteroptera,Miridae)

Zoophytophagous plant bugs (Heteroptera, Miridae) increasingly attract interest as agents of biological plant protection. In the laboratory experiment, the effects of the day length and temperature on the duration of the pre-adult period and on induction of facultative winter adult diapause were studied in Dicyphus errans (Wolff, 1804) collected in Italy. The experiment demonstrated that at 20°C the duration of the pre-adult period of D. errans significantly depended on the day length. On average, females developed 1.3 days longer than males and, at the same time, the day length equally influenced the duration of the pre-adult period in both sexes. The pre-adult period was the shortest under short-day conditions (10 to 12 h of light per day), reached its maximum at day length of 14 h, but then decreased at 15 h, and at day length of 16 h it was as short as under short-day conditions. Also, a pronounced long-day type photoperiodic response of adult diapause induction was recorded in females of D. errans at 20°C: under short-day conditions (10 to 14 h of light per day) almost all females entered diapause, whereas under long-day conditions (15 and 16 h of light per day) about 90% of females were mature. The threshold of this photoperiodic response was close to 14 h 30 min. The mean (± S.D.) egg load of mature females was 6.3 ± 4.0 eggs per female and did not depend on the day length at which the female was reared before and after the final molt. When photoperiodic response of adult diapause induction was observed at two constant temperatures (20 and 25°C), the proportion of mature females depended significantly on the day length but not on the temperature: the shapes of the photoperiodic response curves of diapause induction were almost the same within the near-threshold zone at 20 and 25°C, i.e., the photoperiodic response was thermostable. The set of two photoperiodic responses manifested at different stages of the species’ life cycle has an obvious adaptive significance. In Central Europe, D. errans has 2 or 3 generations per year and hibernates at the adult stage. Due to the thermostable photoperiodic response, females enter diapause always at the same time at the end of summer, regardless of the weather conditions of a particular year. When oviposition and pre-adult development are extended over a prolonged period in summer, nymphs from the later eggs might not be able to molt to adults in due time and then fully prepare for stable winter diapause. Under such circumstances, the photoperiodic response controlling the rates of pre-adult development acquires apparent adaptive meaning: with an autumnal shortening of the day length to 10–12 h, even under conditions of seasonal decrease in temperature, the rates of nymphal development increase and, thus, the chances of nymphs from the later eggs to molt to adults and properly prepare for overwintering also increase. The new data should be taken into account when analyzing the seasonal cycle of D. errans and developing the programs of mass rearing of this zoophytophagous mirid as an agent of biological plant protection.

     

Effect of photoperiod and temperature on diapause in a Florida strain of the tropical warehouse moth Ephestia cautella

A photoperiodically-controlled diapause of the long-day, short-day type was identified in a brown-winged, yellow-eyed strain of Ephestia cautella (Walker). The proportion of larvae diapausing in very long photoperiods was less than in short photoperiods. The mean critical photoperiod, here defined as that photoperiod giving half the maximum percentage of insects that diapause in response to photoperiod at a given temperature, was between 12 and 13 hr for the long-day reaction at both 20 and 25°C. The principal sensitive phase occurred near the time of the last larval moult. The mean duration of diapause was 2–3 months at 20°C and slightly longer at 25°C. The optimum temperature for diapause development was near 15°C, all larvae pupating within 24 days after a 45-day exposure at this temperature. Diapause could be terminated whenever larvae diapausing at 20°C were exposed to as few as five long (15 hr) photoperiods at 25°C. Long photoperiods at 20°C, or short photoperiods (9 hr) at 25°C were less effective in terminating diapause.     

Diapause‐inducing signals prolong nymphal development in the two‐spotted spider mite Tetranychus urticae

Female two‐spotted spider mite Tetranychus urticae are grown under different photoperiods and the photoperiodic regulation of diapause is examined. The photoperiodic response curve for diapause induction was of the long day–short day type, with critical day lengths (CDLs) of 2 and 12.5 h; diapause was induced between these CDLs. The preimaginal period is significantly longer in diapausing females than in non‐diapausing females; moreover, a significant positive correlation is detected between diapause incidence and deutonymphal period. Diapause incidence is high when long‐night photoperiods are applied against a background of continuous darkness in the stages including the deutonymph; this stage appears to be the most sensitive to photoperiod. These observations suggest that diapause‐inducing conditions inhibit nymphal development, particularly in the deutonymphal stage when photoperiodic time measurement for determination of reproduction or diapause is carried out.     

On the factors inducing the inhibition of diapause in the progeny of diapause females of Trichogramma telengai

The ability to enter diapause in the progeny of Trichogramma Westw. (Hymenoptera: Trichogrammatidae) females that have undergone diapause is markedly reduced or even completely absent. This maternal inhibition of diapause can result from three types of factors: (i) environmental factors that induce maternal diapause; (ii) maternal diapause itself; and (iii) diapause‐terminating environmental factors. The present study aims to determine the factors that prevent diapause in the progeny of a Trichogramma telengai Sor. generation that has undergone diapause. Different tendencies to enter diapause in the maternal generation are induced by different photoperiods (LD 12 : 12 h, 16 : 8 h and 18 : 6 h) during development of the grandmaternal generation and different temperatures (from 10 to 15 °C) during larval development of the maternal generation. To terminate diapause, prepupae of the maternal generation are kept at 4–5 °C in the dark. To estimate the ability to enter diapause, the progeny generation is incubated at 14 °C. The results suggest that the inhibition of the tendency to enter diapause in the progeny of T. telengai females that have undergone diapause is caused by diapause itself rather than by diapause‐inducing or diapause‐terminating environmental factors. This result can be used to clarify the mechanism of the inhibition of diapause in the progeny of females that have undergone diapause.     

Geographical variation and inheritance of the photoperiodic response controlling larval diapause in two distinct voltine ecotypes of the Asian cornborer Ostrinia furnacalis

The Asian corn borer Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) enters facultative diapause as fully‐grown larvae in response to short day lengths during autumn. As a result of geographical variations in photoperiodic response, the moths from Nanchang (28.8°N, 115.9°E; NC strain) judge both LD 14 : 10 h and LD 15 : 9 h photocycles as long days and develop directly, whereas moths from Haerbin (44.9°N, 127.2°E; HB strain) judge the same photocycles as short days and enter diapause. Crosses between the two strains are used to evaluate the inheritance of diapause. The critical day lengths for diapause induction in the HB strain are significantly longer than those in the NC strain at all temperatures. The critical day length of F₁ progeny is intermediate between the two strains. However, the critical day length in all crosses is significantly longer with HB strain fathers or grandfathers than with NC strain fathers or grandfathers, indicating that the male parent has significantly more influence on the critical day length of subsequent progeny than the female. The results from all crosses under LD 14 : 10 h or LD 15 : 9 h photocycles at 25 °C show that the inheritance of diapause in O. furnacalis does not fit a purely additive hypothesis and that the capacity for diapause is transmitted genetically in the manner of incomplete dominance. The incidence of diapause for F₁ progeny under an LD 14 : 10 h photocycle is significantly higher than that under an LD 15 : 9 h photocycle, suggesting that the induction of diapause can be influenced by interactions between the F₁ genotype and photoperiod.     

Photoperiodic response in the subtropical and warm‐temperate zone populations of the southern green stink bug Nezara viridula: why does it not fit the common latitudinal trend?

The photoperiodic response of diapause induction is studied in females of five subtropical and warm‐temperate zone populations of Nezara viridula (L.) (Heteroptera: Pentatomidae) in Japan (26.4–34.7°N; 127.4–135.7°E). Laboratory tests at 25 °C demonstrate that both warm‐temperate and subtropical populations have pronounced photoperiodic responses of adult diapause induction. Under short‐day conditions (LD 10 : 14 h), 73–100% of females enter diapause, whereas, under long‐day conditions (LD 15 : 9 h), 87–100% of females are nondiapause and reproduce. When the critical photoperiod for diapause induction is plotted against the latitude of origin of each population, the data points do not show the expected tendency of increasing critical photoperiod northwards but, instead, vary between 12 h 15 min and 13 h 30 min. It is suggested that adults from different populations of such a highly migratory species move often among subtropical islands (by themselves or being assisted by typhoons), thus constantly destabilizing the photoperiodic responses. Although important for general synchronization of seasonal development in the tropics and subtropics, winter diapause might not be so critical for survival in mild winters in these zones. Such circumstances might make the selective pressure over the photoperiodic response less severe in the tropics and subtropics than in the temperate zone. It is suggested that the current climate warming might contribute to this situation as well by promoting polewards migrations and lessening the selective pressure of overwintering conditions.     

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.