Possible involvement of ecdysteroids in embryonic diapause of Locusta migratoria

In the Ibaraki population (Japan) of Locusta migratoria, adult locusts produce diapause eggs under short-day (SD) conditions and non-diapause eggs under long-day (LD) conditions. The identity and titre of ecdysteroids in the ovaries and eggs from LD and SD adult females were investigated by RIA/HPLC. Maternal ecdysteroids accumulated in the developing ovaries represented about 90% polar conjugates, 5% free ecdysteroids and 5% non-hydrolyzable metabolites. Before oviposition the quantity of ecdysteroids reached 29.8±1.85 ng 20-hydroxyecdysone equiv. per mg tissue ovaries from LD females and 13.1±3.55 ng 20E equiv./mg in ovaries from SD females. The sum of RIA-positive materials in newly laid eggs was more than three times higher in non-diapause eggs than in diapause eggs. Ecdysteroids present in egg extracts comprised about 85% polar conjugates, 5% free ecdysteroids and 10% non-hydrolyzable metabolites. On the other hand, after diapause termination the amount of ecdysteroids increased drastically. Also, the composition of ecdysteroids differed from that observed during diapause and became comparable to that of non-diapause eggs. The significant differences in the ecdysteroids between non-diapause and diapause eggs may suggest the possible involvement of these compounds in the control of embryonic diapause of this locust.     

The Insect Growth Regulator Pyriproxyfen Terminates Egg Diapause in the Asian Tiger Mosquito,Aedes albopictus

The Asian tiger mosquito, Aedes albopictus, is a highly invasive mosquito species that transmits chikungunya and dengue. This species overwinters as diapausing eggs in temperate climates. Early diapause termination may be a beneficial strategy for winter mosquito control; however, a mechanism to terminate the diapause process using chemicals is not known. We tested the hypothesis that a hormonal imbalance caused by the administration of juvenile hormone analog would terminate egg diapause in A. albopictus. We tested the insect growth regulator pyriproxyfen on all developmental stages to identify a susceptible stage for diapause termination. We found that pyriproxyfen treatment of mosquito eggs terminated embryonic diapause. The highest rates of diapause termination were recorded in newly deposited (78.9%) and fully embryonated (74.7%) eggs at 0.1 and 1 ppm, respectively. Hatching was completed earlier in newly deposited eggs (25–30 days) compared to fully embryonated eggs (71–80 days). The combined mortality from premature diapause termination and ovicidal activity was 98.2% in newly deposited and >98.9% in fully embryonated eggs at 1 ppm. The control diapause eggs did not hatch under diapausing conditions. Pyriproxyfen exposure to larvae, pupae and adults did not prevent the females from ovipositing diapausing eggs. There was no effect of pyriproxyfen on diapausing egg embryonic developmental time. We also observed mortality in diapausing eggs laid by females exposed to pyriproxyfen immediately after blood feeding. There was no mortality in eggs laid by females that survived larval and pupal exposures. In conclusion, diapausing eggs were the more susceptible to pyriproxyfen diapause termination compared to other life stages. This is the first report of diapause termination in A. albopictus with a juvenile hormone analog. We believe our findings will be useful in developing a new control strategy against overwintering mosquito populations.     

Seasonal Synchronization of Diapause Phases in Aedes albopictus (Diptera: Culicidae)

In temperate areas, population dynamics of the invasive Asian tiger mosquito Aedes albopictus are strongly affected by winter. The work we present here analyzes the adaptive synchronization of the diapause process in the wintry generation of A. albopictus, where the egg stage is exposed to adverse winter conditions. The seasonal pattern of egg laying activity of a French Mediterranean population of the Asian tiger mosquito was monitored weekly for 2 years with ovitraps. The field diapause incidence and the critical photoperiod (CPP, i.e. the maternal day length inducing diapause in 50% of the eggs), were determined by hatching experiments on the collected eggs. The period of diapause termination was estimated by a field survey of the first hatchings for both years. The CPP is equal to 13.5 hours of light and occurs in the field on the 25^th of August. Thus, it is on September 11^th, 17 days after the CPP, that 50% of the eggs are in a prediapause stage in the field. The egg diapause rate increases rapidly during September, whereas the mean number of eggs laid decreases sharply after mid-September. Surprisingly, after having reached a peak of 95% at the end of September, from mid-October the diapause incidence declined and stayed below 50%. Indeed, both years the diapause initiates before the rapid decrease of the environmental temperature. This leaves a sufficient period of time to the complete development of one generation of A. albopictus with effective induction of diapause in the laid eggs. The very first larvae hatched were sampled both years in the first half of March. With 20 to 26 weeks in the egg stage and about 7 weeks in the larval stages, the first annual generation spends a long time in immature stages. On a practical point of view, this long development time represents a wide window for eggs and larvae control in early spring.     

Large-Scale Modelling of the Environmentally-Driven Population Dynamics of Temperate Aedes albopictus (Skuse)

The Asian tiger mosquito, Aedes albopictus, is a highly invasive vector species. It is a proven vector of dengue and chikungunya viruses, with the potential to host a further 24 arboviruses. It has recently expanded its geographical range, threatening many countries in the Middle East, Mediterranean, Europe and North America. Here, we investigate the theoretical limitations of its range expansion by developing an environmentally-driven mathematical model of its population dynamics. We focus on the temperate strain of Ae. albopictus and compile a comprehensive literature-based database of physiological parameters. As a novel approach, we link its population dynamics to globally-available environmental datasets by performing inference on all parameters. We adopt a Bayesian approach using experimental data as prior knowledge and the surveillance dataset of Emilia-Romagna, Italy, as evidence. The model accounts for temperature, precipitation, human population density and photoperiod as the main environmental drivers, and, in addition, incorporates the mechanism of diapause and a simple breeding site model. The model demonstrates high predictive skill over the reference region and beyond, confirming most of the current reports of vector presence in Europe. One of the main hypotheses derived from the model is the survival of Ae. albopictus populations through harsh winter conditions. The model, constrained by the environmental datasets, requires that either diapausing eggs or adult vectors have increased cold resistance. The model also suggests that temperature and photoperiod control diapause initiation and termination differentially. We demonstrate that it is possible to account for unobserved properties and constraints, such as differences between laboratory and field conditions, to derive reliable inferences on the environmental dependence of Ae. albopictus populations.     

中华稻蝗长沙种群的生活史及其卵滞育的进化意义

中华稻蝗Oxya chinensis为重要的水稻害虫,在我国除青海、西藏、新疆、内蒙古等未见报道外,南起海南北至东北均有分布,在许多的分布区域1年发生1代。为探索中华稻蝗长沙种群的生活史及其季节适应特征,通过野外和实验室的研究,调查了其发生代数、孵化率和卵滞育率的季节变化及越冬卵的存活率。结果显示,中华稻蝗长沙种群为1年2代和1年1代混合发生:第1代卵产卵后大部分孵化为若虫而1年完成2代,但亦有19.4% -4.1%的卵不孵化而1年只能完成1世代。第1代成虫于6月上旬至8月上旬羽化,6月下旬至8月中旬产卵;第2代若虫于7月初开始孵化,9-10月羽化为成虫,10月上旬至11月下旬产卵。在室外自然条件下,中华稻蝗长沙种群6-8月(第1代)和10-11月(第2代)所产卵块均为部分滞育,滞育率为30%左右,皆无显著差异。然而,其卵滞育率在12月以后显著降低,仅为6.6%或以下,卵滞育快速地得以解除。因此,包括非滞育卵和滞育解除卵,中华稻蝗长沙种群的越冬卵皆以非滞育状态度过其后的寒冷季节。即使是遭遇长江流域2007年末至 2008年初异常寒冷的冬季,在长沙地区越冬后其卵的存活率亦在98%以上。非滞育状态的中华稻蝗长沙种群越冬卵完全能安全地越冬,其滞育的发生并非是为了提高其耐寒性而安全度过不适环境。并探讨了中华稻蝗长沙种群卵滞育的进化意义。     

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.     

Geographic differences on accumulation of sugars and polyols in locust eggs in response to cold acclimation

The accumulation of low molecular weight sugars and polyols is one of major mechanisms hypothesized to increase cold tolerance in overwintering insects. But little is known about whether these sugars and polyols are involved in geographic variation of cold tolerance. In this study, we investigated accumulation patterns of eight low molecular weight sugars and polyols of eggs in tropical and temperate populations of the migratory locust, which exhibits between-population variation in cold tolerance, in response to cold acclimation (5, 0 and −5 °C). Excluding erythritol, the other seven carbohydrates were identified as possible cryoprotectants in locust eggs. Basal maximal and minimal concentrations were 45 μg/g wet weight for trehalose and 0.59 μg/g wet weight for glycerol. Most sugars and polyols were elevated after a −5 °C exposure. In a tropical population, fructose, glucose, sorbitol and myo-inositol were significantly accumulated by low temperature treatments, but glycerol was not. In the temperate population, glycerol, glucose, mannitol, sorbitol, myo-inositol were significantly accumulated but trehalose did not increase. Our results suggest different accumulation patterns of these carbohydrates of locust eggs between tropical and temperate populations and highlighted possible roles for them in geographic variation of cold tolerance in the migratory locust.     

Inheritance of the photoperiodic response controlling imaginal summer diapause in the cabbage beetle, Colaphellus bowringi

Adults of the cabbage beetle Colaphellus bowringi display a summer diapause in response to the exposure of their larvae to long photoperiods. In the present study, the inheritance of the photoperiodic response controlling summer diapause in C. bowringi by crossing a high diapause strain (D strain) with a laboratory selected nondiapause strain (N strain) was investigated under different photoperiods at 22, 25 and 28 °C. The beetles in both reciprocal crosses and backcrosses showed a clear short-day response for the induction of diapause at all temperatures, similar to that of the D strain, suggesting that photoperiodic response of this beetle is heritable. The diapause incidences in the progeny from all the crosses under LD 15:9 or LD 12:12 at 25 °C suggest that genetic and genetic-environmental interactions are involved in diapause induction. The incidence of diapauses in F₁ progeny was significantly lower than that in the D × D strain but significantly higher than that in the N × N strain, indicating that the diapause capability is inherited in an incomplete dominant manner. The incidence of diapause was greater among the offspring of hybrid females when those females had a D strain mother or grandmother rather than a N strain mother or grandmother, indicating that maternal effects on diapause induction are stronger than paternal effects. The laboratory selected nondiapause strain also showed a short-day photoperiodic response at a low temperature of 22 °C, indicating that the photoperiodic photoreceptor and photoperiodic clock still function in the nondiapause strain.     

Nutritional and photoperiodic control of the seasonal reproductive cycle in Chrysopa mohave (Neuroptera)

Chrysopa mohave adults, which are predaceous, undergo a facultative reproductive diapause which can be induced, averted, and terminated in the laboratory by manipulating either photoperiod or diet. Photoperiods of LD 14:10 or shorter evoke a photoperiodically controlled diapause which breaks in long day (LD 16:8) conditions. Withholding prey induces the diapause syndrome in animals experiencing long day lengths, whereas supplying prey terminates this diapause; thus food constitutes a major factor in diapause induction and termination.Under short day regimens, with prey continuously present, diapause persists for approximately 60 days at 24±1°C; however, protein-fed, short day animals retain the diapause characteristics until they receive prey.Our experimental results in combination with field observations and examination of collected specimens, indicate four periods in the annual reproductive cycle as follows. The animals reproduce during April, May, and June when photoperiods are long and prey abundant. When prey become scarce during the dry months of the California summer, part of the population enters a food mediated diapause. During October, November, and December a short day diapause occurs in the population. Although the photoperiodic maintenance of diapause probably ends during early winter, the insects retain the diapause symptoms until prey becomes abundant at the end of March.     

Temperature and photoperiod effects on dormancy status and life cycle parameters in Aedes albopictus and Aedes aegypti from subtropical Argentina

Aedes albopictus (Diptera: Culicidae) distribution is bounded to a subtropical area in Argentina, while Aedes aegypti (Diptera: Culicidae) covers both temperate and subtropical regions. We assessed thermal and photoperiod conditions on dormancy status, development time and mortality for these species from subtropical Argentina. Short days (8 light : 16 dark) significantly increased larval development time for both species, an effect previously linked to diapause incidence. Aedes albopictus showed higher mortality than Ae. aegypti at 16 °C under long day treatments (16 light : 8 dark), which could indicate a lower tolerance to a sudden temperature decrease during the summer season. Aedes albopictus showed a slightly higher percentage of dormant eggs from females exposed to a short day, relative to previous research in Brazilian populations. Since we employed more hours of darkness, this could suggest a relationship between day‐length and dormancy intensity. Interestingly, local Ae. aegypti presented dormancy similar to Ae. albopictus, in accordance with temperate populations. The minimum dormancy in Ae. albopictus would not be sufficient to extend its bounded distribution. We believe that these findings represent a novel contribution to current knowledge about the ecophysiology of Ae. albopictus and Ae. aegypti, two species with great epidemiological relevance in this subtropical region.     

Comparative Morphometrics of Eggs and Second-Stage Juveniles of Heterodera schachtii and a Race of H. trifolii Parasitic on Sugarbeet in The Netherlands

Measurements of second-stage juveniles of Heterodera schachtii from California and The Netherlands and a race of H. trifolii from The Netherlands were obtained and compared to determine if these populations can be differentiated by morphometrics. Juvenile lengths of 10 specimens from each of 10 cysts of each population were measured. Dimensions of tail regions of 20 juveniles from individual cysts of H. schachtii (California) and a like number of juveniles of H. trifolii (The Netherlands) were also obtained. The mean lengths of juveniles of H. schachtii from California and The Netherlands were not significantly different, but similar measurements of H. schachtii and H. trifolii were different (P = 0.05). Mean dimensions of tail lengths, tail widths, tail hyaline lengths, and tail length/tail width were significantly greater for H. trifolii than for H. schachtii. Also, dimensions of eggs of H. trifolii were significantly greater than dimensions of H. schachtii eggs. The investigations established that H. schachtii can be readily differentiated from H. trifolii by morphometrics of eggs and juveniles, Minimum sample sizes required for specified confidence intervals for each criterion measured are provided.     

Effect of daylength and temperature on the larval diapause of the sunflower moth, Homoeosoma electellum

The effect of daylength and temperature on the regulation of the larval diapause of a central Missouri population of the sunflower moth, Homoeosoma electellum, was examined. Fully grown fourth-instar larvae exhibit a facultative diapause. Measurements of the effect of photoperiod on diapause induction revealed critical photoperiods of about 13 h 30 min light/day at 20°C, and between 11 h 45 min and 12 h light/day at 23°C. Third and fourth-instar larvae were shown to be the main sensitive stages for diapause determination. Daylength was also shown to be an important regulator of the rate of diapause development. A short day of LD 10:14 h permitted only a low rate of diapause development, whereas long days of LD 14:10 h and LD 16:8 h accelerated diapause development at 25 and 30°C. When long days were alternated with short days at 30°C the accelerating effect of long days on diapause development was not found. Systematic transfers of chilled diapausing larvae revealed an accelerated diapause development in groups transferred from 10 to 30°C LD 10:14 h, but diapause development was not accelerated in groups transferred from 10 to 30°C LD 16:8 h.     

Satyrization without evidence of successful insemination from interspecific mating between invasive mosquitoes

Previous research has documented low frequencies of interspecific mating in nature between the invasive vectors Aedes aegypti and Aedes albopictus. It is also known that heterospecific male accessory gland substances transferred during mating sterilize A. aegypti but not A. albopictus females, leading to satyrization, a form of reproductive interference. This paper demonstrates that satyrization of A. aegypti by A. albopictus may occur without evidence of successful insemination. Our results show that A. aegypti females, previously exposed to A. albopictus males, are rendered refractory to subsequent conspecific mating even though their spermathecae contain no heterospecific sperm. Additional experiments demonstrating transfer of labelled semen from A. albopictus males to A. aegypti females and low production of viable eggs of females housed with conspecific males, following exposure to A. albopictus males, confirm higher incidences of satyrization than expected, based on heterospecific insemination rates. We conclude that frequencies of satyrization based on detection of interspecific sperm in spermathecae may underestimate the impact of this form of reproductive interference.     

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.     

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.     

Balancing insect energy budgets

Summary Breeding experiments with a temperate zone population of the marine insect Clunio marinus (stock Helgoland-North Sea from 54° N) demonstrated an onset of larval diapause under combined short day and low temperature conditions only (examined LD 8:16 with 7 or 10°C resp.). Long photoperiods (LD 16:8) at 7–10° C or higher temperatures in shorter photoperiods completely prevented any dormancy response. The diapausing larvae were still locomotory active and fed in their benthic environment; their metamorphosis, however, stopped during a specific stage of the early imaginal disc formation in the last instar. In the experiments, the diapause was maintained up to 5.5 months and longer. In nature, the termination may be mainly controlled by temperature rises during low tide and fine weather under long day conditions, at Helgoland generally in late April or early May. The dormancy response can be classified as a larval oligopause (sensu H.J. Müller) of a potentially multivoltine species. It is supposed that this kind of diapause control is more widely distributed in aquatic chironomids.