The seasonal cycle of Thermocyclops crassus (Fischer, 1853) (Copepoda: Cyclopoida) in a shallow,eutrophic lake

The seasonal cycle of Thermocyclops crassus was studied from 1985 to 1987, in the Gronne, a shallow, productive lake. T. crassus was present from late April to early October, while water temperature was above 10 °C. It produced three generations per year. Population peak was usually reached by the second generation, in August. Abundance was positively correlated with water temperature. Females carried 18.3 to 32.3 eggs on average. Reproduction rates were highest in July and August. Sex ratio was low, as females generally outnumbered males. Between October and April copepodites 4 went in diapause, predominantly in the deepest part of the lake and 0–4 cm deep in the bottom mud. Variations in body size were low across the year. T. crassus coexisted with the cyclopids, Cyclops vicinus and Mesocyclops leuckarti. In 1985 and 1986, abundances of M. leuckarti were low, while those of T. crassus and C. vicinus were high. In 1987 lower water temperatures, caused by cold weather, resulted in a marked decrease of the population density of T. crassus, while the abundance of M. leuckarti increased, and density of C. vicinus remained high. Comparison of the egg duration times of T. crassus, C. vicinus and M. leuckarti showed that at 15 °C and below T. crassus may be outcompeted by its comparatively longer egg development times. However, experiments in limnocorrals showed that T. crassus has an advantage over C. vicinus when fish predation is high.Department of Biology III, University of Ulm     

The effect of temperature on the development times of eggs,naupliar and copepodite stages of five species of cyclopoid copepods

The duration times of eggs, combined naupliar instars and of the different copepodite stages of five species of cyclopoid copepods — Acanthocyclops robustus, Cyclops vicinus, Diacyclops bicuspidatus, Mesocyclops leuckarti, and Thermocyclops crassus — were investigated at five different temperatures. The five species can be divided in two groups: two species, C. vicinus and D. bicuspidatus, adapted to cold water conditions and three species, A. robustus, M. leuckarti and T. crassus adapted to warm water conditions. The cold water species showed a faster egg development than M. leuckarti and T. crassus at 5–15 °C. The eggs and instars of the warm water species M. leuckarti tend to develop faster than those of the former two species at higher temperatures. A. robustus showed the shortest egg and instar development at 10–25 °C. The warm water species T. crassus produced no eggs at 10 °C and temperatures below. At higher temperatures (20, 25 °C) the egg and instar duration times were similar or longer than those of the other species. When cultured in total darkness a great part of the C_IV respectively C_V copepodites of the summer forms entered arrest and the percentage of copepodites that showed an arrest of development was highest at lowest temperatures. The present results are compared with data from literature and differences are discussed.     

Survival and development of five species of cyclopoid copepods in relation to food supply: experiments with algal food in a flow-through system

1. Cyclops spp. generally develop and grow during favourable food conditions in spring and undergo a diapause in summer, while Acanthocyclops robustus, Mesocyclops leuckarti and Thermocyclops crassus develop and grow in summer when they face poorer food conditions and more competition from Cladocera. Since nauplii are the bottleneck in copepod development, we tested the hypothesis that Cyclops abyssorum and C. vicinus nauplii have higher food requirements for survival and development than the nauplii of A. robustus, M. leuckarti and T. crassus. We also tested survivorship and development from hatching to adulthood. 2. Survivorship and development of the copepods was studied in a flow‐through system using five concentrations of the phytoflagellate Chlamydomonas reinhardtii in the range from 1 × 10⁴ to 4.5 × 10⁵ cells mL^?1 (approximately 0.5–22.5 mg C L^?1). 3. Nauplii of both species of Cyclops died at intermediate to low (C. abyssorum) and low (C. vicinus) food concentrations, while nauplii of A. robustus, M. leuckarti and T. crassus survived at all concentrations. 4. The negative effects of low food concentration were also reflected in development. In C. abyssorum and C. vicinus, development duration increased at low food concentration while development was much less affected in A. robustus and T. crassus. Mesocyclops leuckarti was intermediate between Cyclops spp. and A. robustus/T. crassus, with an increase in development duration at the lowest food concentration. 5. Our results support the hypothesis that summer diapause in Cyclops spp. has developed as a strategy to avoid a food bottleneck for nauplii.     

Reproduction and adult longevity of five species of planktonic cyclopoid copepods reared on different diets: a comparative study

1. The nutritional value of a pure algal food, the phytoflagellate Chlamydomonas reinhardtii, and a mixed diet was tested for five planktonic cyclopoid copepods (Acanthocyclops robustus, Cyclops vicinus, Cyclops abyssorum, Mesocyclops leuckarti and Thermocyclops crassus). The algae were offered at high density (4.5 × 10⁵ cells ml^–1; 22.5 mg C l^–1) in a flow-through system. The mixed diet consisted of concentrated natural plankton (rotifers, copepod nauplii, small copepodites and large phytoplankton forms) in the size range 50–150 μm and with a dry mass > 20 mg l^–1. Reproductive parameters (clutch size, interclutch periods, number of clutches produced) and adult longevity were monitored as indicators of nutritive value. 2. All species had a significantly lower reproductive output and a shorter or unchanged adult lifespan on the algal compared with the mixed diet. 3. The species differed considerably in their ability to use algae. Mesocyclops leuckarti produced no clutches with algae, and females died earlier than with the mixed food. Acanthocyclops robustus and C. vicinus produced smaller and fewer clutches, displayed a longer interclutch period and shorter (A. robustus) or similar (C. vicinus) lifespan on the algal food than on the mixed food. Thermocyclops crassus and C. abyssorum produced smaller clutches with the algal food, but interclutch period was shorter with the algal than with the mixed diet (T. crassus) or of the same length with both diets (C. abyssorum). Adult lifespan was the same under both food regimes. Unfed females produced no eggs. 4. The ability to utilize algae, the reproductive output and the reproductive allocation were not related to body size. Acanthocyclops robustus, a species of intermediate size, produced by far the largest clutches and the most eggs per lifetime under both food regimes and invested more in reproduction than the other species. 5. Reproduction was costly. Unfed and non-reproducing females of C. vicinus and C. abyssorum reached the same age or lived longer than fed and reproducing ones. 6. The adult cyclopoids tested are primarily omnivorous, but utilize algae which are frequently sufficient for egg production.     

An example of niche partitioning in three coexisting freshwater cyclopoid copepods

Vertical distributions at noon and midnight of copepodites,males and females of three coexisting species of freshwatercyclopoid copepods, Cyclops vicinus (a cold-water form), Thermocyclopscrassus and Mesocyclops leuckarti (warm-water forms), were studiedin Dreiangel Lake, a small eutrophic gravel pit. The largestspecies, C. vicinus, inhabits deep water layers, both at noonand midnight, while the smaller species, T.crassus and M.leuckarti,inhabit upper water layers. In all three species, copepoditesand males live closer to the water surface than females. Insummer, abundance of the smallest species (T.crassus) is highest,although finite birth rates are lowest. The distribution patternsand population characteristics suggest that the vertical distributionof cyclopoid copepods in Oreiangel Lake is mainly governed byfish predation; different temperature tolerances and preferencesof the three species may also be of significance.     

Implication of the feeding limb morphology for herbivorous feeding in some freshwater cyclopoid copepods

1. The mouthparts of five species of adult planktonic cyclopoid copepods (Cyclops vicinus, C. abyssorum, Acanthocyclops robustus, Mesocyclops leuckarti and Thermocyclopscrassus), in particular the distance between setae and setules of the maxilliped (which can indicate the ability to retain small particles), were compared using electron and light microscopy. 2. The mesh‐sizes of the food‐collection grid formed by these setae and setules ranged between 4.6 and 13.2 μm; the area covered by the grid ranged between 6000 and 32 000 μm². 3. Mesh‐size was not simply correlated with body size. Cyclops abyssorum and M. leuckarti have the coarsest meshes and T. crassus the finest, while C. vicinus and A. robustus were intermediate. 4. The results suggest that cyclopoid copepods are able to retain particles in the size range of nanoplankton and that differences in mesh‐sizes between species may explain differences in the ability to subsist and reproduce on a diet of small algae.     

Diapause in the egg parasitoid Trichogramma cordubensis: role of temperature

The role of temperature in the induction of diapause in Trichogramma cordubensis (Hymenoptera: Trichogrammatidae), under controlled laboratory conditions, was investigated using Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) eggs as hosts. Results indicate that prestorage temperatures and the duration of exposure of the parasitoids to these temperatures affected the induction of diapause. It was possible to induce diapause in prepupae of T. cordubensis by exposing the preimaginal stages (prior to the prepupal stage) to 10°C for at least 30 days, but adults emerged without diapause when the duration of exposure was of only 10 or 20 days. Parasitoids failed to enter diapause when prestorage temperatures were 7 or 12°C, regardless of the duration of exposure. However, at these two temperatures, preimaginal development of T. cordubensis was delayed, allowing short-term storage (40 days at induction temperatures followed by 30 days at 3°C) by keeping parasitoids in quiescence without reducing the percentages of adult emergence. Good percentages of adult emergence after long-term low-temperature storage (30 or 40 days at 10°C followed by six months at 3°C) occurred only when T. cordubensis was in diapause. The long-term storage of parasitoids in diapause allows an enlargement in the mass rearing potentialities of this species for future biological control releases by allowing producers to stockpile the parasitoids for release in the field season.     

Life cycles of two limnetic cyclopoid copepods, Cyclops vicinus and Thermocyclops crassus, in two different habitats

The life cycles of Cyclops vicinus and Thermocyclops crassusin two shallow eutrophic habitats, Junsainuma and Naganuma Ponds,Hokkaido, Japan, were investigated. Both ponds exhibited similarseasonal patterns of temperature, oxygen levels and pH duringice-free periods; however, oxygen levels were extremely lowerunder the ice in Naganuma Pond. Cyclops vicinus showed differentlife cycles in the two ponds; in Junsainuma Pond, it reproducedin winter and spring (January-May) and entered diapause duringsummer and autumn (June-October) as copepodite IV stage, whileit reproduced in autumn (October-November) and spring (April-May),and entered diapause in summer (June-September) and winter (Januaryand February) as copepodite V stage in Naganuma Pond. Thermocyclopscrassus entered diapause during winter (December-April) as copepoditeIV and V stages in both ponds, and egg-bearing females appearedonly during the warmseason, from early May to late October,when water temperatures were >10°C. Summer diapause inC.vicinus was suggested to be an adaptation against fish predation,whereas C.vicinus entered winter diapause in Naganuma Pond probablyto avoid low oxygen levels. Thermocyclops crassus entered diapausein both ponds to avoid low water temperature. These resultssuggest that biotic and abiotic factors are important for leadingto specific life cycles of cyclopoid copepods in small waterbodies.     

Mating frequency and interspecific matings in some freshwater cyclopoid copepods

Laboratory investigations with Cyclops vicinus and Metacyclops minutus documented that mating frequency decreased significantly after the initial mating, i.e. re-mating is relatively rare in cyclopoid copepods. Re-mating is unnecessary because females are able to fertilize multiple clutches of viable eggs from one insemination. Similar sized Cyclops vicinus and Cyclops furcifer interbreed frequently. Interbreeding does not occur when size differences and taxonomic differences are as great as between C. vicinus and M. leuckarti. In C. vicinus and M. leuckarti, the duration of the last mating phase (spermatophore transfer until release of the female) and consequently the duration of the entire mating process, is different. I conclude that the low re-mating frequency in cyclopoid copepods probably evolved to accelerate clutch production in unpredictable environments and to reduce predation risk. The occurrence of interspecific mating discourages the co-existence of similarly sized, related species. Mating behaviour may have a considerable influence on zooplankton community structure.     

Variable life history of a cyclopoid copepod: the role of food availability

Populations of the copepod Cyclops vicinus in two Danish lakes differed in their life cycles. In Lake Væng C. vicinus was absent during summer, whereas in Lake Søbygård it continued producing distinct cohorts throughout summer. A comparison between the lakes showed that in the lake where C. vicinus was absent during summer, food limitation generally existed. In contrast, abundance of C. vicinus during summer in Lake Søbygård always coincided with available resources sufficient for the whole life cycle. Thus, summer diapause of C. vicinus is suggested to be a strategy to avoid food limitation. Avoidance of fish predation does not seem substantially to influence life cycles.     

The freeliving freshwater cyclopoid copepoda (crustacea) of Malaysia and Singapore

A study of freeliving freshwater Cyclopoida of Malaysia and Singapore yielded 15 species. Some remarks are made on the morphology including a species belonging to the schmeili group of Thermocyclops. Shallow and vegetation-bearing habitats had the largest numbers of species. Most of the species are cosmopolitan. The three common species, Mesocyclops leuckarti, Thermocyclops crassus and Microcyclops varicans are eurytopic. M. leuckarti and T. crassus are the only species occurring commonly in limnetic situations in contrast to a much wider limnetic Cyclopoida species spectrum in temperate lakes. There appears to be a reduction in Cyclopoida from temperate to tropical latitudes, but the data on which this are based are at present fragmentary.     

Experimental studies on the development of Lernaeocera branchialis (Copepoda: Pennellidae): population processes from egg production to maturation on the flatfish host

Quantitative population dynamical information derived from laboratory- and field-based experiments is provided for the fish-parasitic copepod, Lernaeocera branchialis, infecting flounder (Platichthys flesus) and whiting (Merlangius merlangus). Adult, post-metamorphosis females from whiting can produce more than one set of egg-strings. The mean number of eggs in each egg-string pair was 1445. At 10 °C these eggs took about 12.7 d after extrusion before hatching of NI nauplii began. Hatching took up to 12 days to be completed with an exponentially declining pattern of output over this period. In the laboratory about 44% of the egg-string egg population successfully passed through the NI to NII nauplius moult and the NII to copepodid moult to produce infective copepodids, a process lasting about 2 d. The non-feeding copepodids had a maximum survival time at 10 °C of 18 d, with a time to 50% survival of 7.5 d. In laboratory infection experiments at 10 °C, copepodids infected flounder and passed through all their developmental stages to adulthood and copulation in a minimum of 25 d. Field experiments on the seabed off Lowestoft in June 1987 with a sea temperature of about 16 °C suggested that the developmental period in those conditions could be as short as 11 d. Previously uninfected flounder in the field experiments became naturally infected with copepodids at a mean rate of not less than 30 parasites per fish d^–1.     

Diapause and polyphenism of life-history of Lagria hirta

This paper is focused on diapause and polyphenism of development of Lagria hirta L. and tries to unravel its mechanism of life-history adaptation. Lagria hirta, distributed widely in Europe, has a strictly univoltine life cycle. The results showed that larval diapause and moulting polymorphism were the deciding factors that made L. hirta maintain its univoltinism and keep a flexible relation between seasonal changes and life-history phases. In the laboratory, larvae of this species were not able to pupate if kept at constant temperatures of 5 °, 10 °, 15 °, 20 °, 25 ° or 30 °C combined with a photoperiod of either LD (L16:D8) or SD (L8:L16). Pupation only occurred if larvae were reared at 15 °–25 °C when intervened by a three-month chilling at 5 °C in stages L₃, L₄, L₅ or L₆. A chilling treatment was shown to be obligatory for the termination of its larval diapause and had an accelerating and synchronizing effect on larval development. Larval diapause of L. hirta was characterized by no pupation and more moulting in advanced instars, longer duration of each single stage, and moulting desynchronization. Larval development was found to be variable with respect to the total number of instars: most larvae underwent a total of seven or eight moults; some larvae might even moult once or twice more, but they seldom pupated. It seemed that the choice for the 7-instar or the 8-instar development did not directly relate to any of the external conditions, such as temperature, photoperiod, and stages with chilling treatment. This polyphenism was observed in the same group under identical conditions and even in a single egg clutch. In L. hirta, overwintering in different stages of L₃–L₆, and choosing the 7- or 8-instar pathway of development are two features that increase the plasticity and flexibility in coordinating its life cycle with seasonal change, that varies unpredictably from year to year.     

Influences of daylength and temperature on the period of diapause and its ending process in dormant larvae of burnet moths (Lepidoptera,Zygaenidae)

The onset of larval diapause in the burnet moth Zygaena trifolii is clearly characterized by the larva molting into a specialized dormant morph. In a potentially bivoltine Mediterranean population (Marseille) two types of diapause can occur within 1 year: firstly, a facultative summer diapause of 3–10 weeks, and secondly, an obligate winter diapause, which can be lengthened by a period of thermal quiescence to several months in temperatures of 5°C. For the first time, three successive physiological periods have been experimentally distinguished within an insect dormancy (between onset of diapause and molting to the next non-diapause stage), using chilling periods of 30–180 days at 5°C, and varying conditions of photoperiod and temperature. These stages are: (1) a continuous Diapause-ending process (DEP); (2) thermal quiescence (Q); and finally, (3) a period of postdiapause development (PDD) before molting to the next larval instar. The result of transferring dormant larvae from chilling at 5°C to 20°C depended on the length of the chilling period. After chilling for 120–180 days, molting to the next instar occurred after 6–10 days, independent of daylength. This period corresponds with the duration of PDD. After shorter chilling periods (90, 60, 30 days and the control, 0 days) the period to eclosion increased exponentially, and included both the latter part of the previous diapause process and the 6–10 day period of PDD. However, photoperiod also influences the time to eclosion after chilling. Short daylength (8 h light / 16 h dark: LD 8/16) lengthened the diapause in comparison to long daylength (16 h light / 8 h dark: LD 16/8). Short daylength had a similar effect during chilling at 5°C, as measured by the longer time to eclosion after transfer. The shorter time to eclosion resulting from longer chilling periods (30–90 days) demonstrates that the state of diapause is continuously shortened at 5°C, and corresponds to the neuroendocrine controlled DEP. Presumably the DEP has already started after the onset of diapause. When chilling was continued after the end of the DEP, which ranged between 90 and 120 days, thermal quiescence (Q) followed (observed maximum 395 days). Different photoperiodic conditions during the pre-diapause inductive period modified diapause intensity (measured as the duration of diapause), in that a photoperiodic signal just below the critical photoperiod for diapause induction (LD 15/9) intensified diapause. Experiments simulating the summer diapause showed that PDD occurred in the range of 10–25°C. Higher temperatures (15 and 20°C) shortened the DEP at LD 16/8, so that at 20°C many individuals had already terminated diapause after 10–40 days and had molted after the 6–10 days of PDD. A temperature of 25°C unexpectedly lengthened the DEP to 110 days in several individuals. The ecological consequences and the adaptive significance of variation in the duration of the diapause are discussed in relation to the persistence of local populations predictably variable and rare climatic extremes throughout the year.     

Temperature dependent sorbitol utilization in diapause eggs of the silkworm,Bombyx mori

Summary A series of experiments was conducted to determine whether the interconversion of glycogen and sorbitol at the initiation and termination of diapause in eggs of the silkworm,Bombyx mori was influenced by low temperatures (5°C and 1°C). The conversion of glycogen to sorbitol and glycerol at the initiation of diapause was not affected by exposure to 5°C and 1°C. Chilling diapause eggs at 5°C for over 70 days resulted in a progressive conversion of sorbitol to glycogen. Transfer to 1°C after chilling to 5°C led to a decrease in sorbitol and glycerol conversion. While continuous chilling at 1°C completely inhibited sorbitol utilization for at least 400 days, sorbitol began to decrease immediately following transfer to 5°C. In contrast, glycerol utilization was not prevented by acclimation to 1°C, but a delayed decrease occurred. Continuous exposure to 1°C delayed hatching for a period of 440 days, whereas acclimation to 5°C resulted in 100 per cent hatching by about 100 days. A low, or high acclimation temperature 1°C or 5°C produce different effects on sorbitol utilization and termination of diapause in silkworm eggs.     

An analysis of diapause and resistance in the egg stage of Floronia bucculenta (Araneida: Linyphiidae)

Summary Floronia bucculenta hibernates in the egg stage; the egg sacs are deposited on the leaves of grass tussocks without any shelter. The morphogenesis of the eggs was divided into 10 arbitrarily chosen stages, in order to test the dependence of embryonic development on temperature in the laboratory. The eggs developed slowly at 23°C, 16°, 12.5°; embryogenesis stopped after 70–45 days, when prosomal appendage rudiments began to form. At 10°, 7.5°, 5°, 0° complete embryogenesis was possible until the emergence of the first complete stage. The eggs developed most rapidly at 5° (mean developmental time 203 days). The egg development was normal at 5° and 0°, when compared with the timetable of the embryogenesis of the linyphiid Bathyphantes gracilis, a species which has no egg diapause. At 7.5° and 10° the embryogenesis was strongly delayed during the median phases of development (elongation of the germ band, formation of prosomal appendages); after reversion the development was accelerated (postdiapause phase). After long exposure to low temperatures (-10° to +10°) the diapause was terminated. A temperature of 0° was optimal (minimal time of exposure 8–9 weeks). The time required for embryonic development of postdiapause eggs decreased hyperbolically with increasing temperature. In the field the median phases of embryogenesis were retarded by low ambient temperatures; diapause was terminated from late December to mid-January. The spread of hatching in spring was 7–15 days.During the diapause phase the O₂-consumption of the eggs at 25° was depressed. It rose from 1.55 (in late diapause) to 4.21 ml/100 eggs·h at the onset of postdiapause, whereas O₂-utilization did not change significantly at 5° (from 0.54 to 0.61 ml/100 eggs·h just after the termination of diapause).The diapause phase was not characterized by higher resistance to cold, drought, or flooding. As compared with single eggs removed from the cocoon, the silken wall of the intact egg sac did not affect the survival of postdiapause eggs exposed to-15° (LD₅₀=28 days); it raised, however, the survival time of eggs exposed to a R.H. of 32% (at 5°) or flooding by distilled water (at 5°): from LD₅₀=37 to 68 days at drought, from LD₅₀=30 to 92 days at flooding.Diapause is important for synchronizing the life-cycle of F. bucculenta with the seasonal fluctuations of environment. The egg stage is highly tolerant to the extreme factors of the winter. Some implications of the relation of the studied spider to its habitat are discussed.     

Changes in the abundance and composition of cyclopoid copepods following fish manipulation in eutrophic Lake Væng, Denmark

1. The seasonal cycle of cyclopoid copepods during and following an approximately 50% reduction in planktivorous fish biomass was studied in shallow, eutrophic Lake Vaeng, Denmark, from 1986 to 1990. 2. The dominant cyclopoid copepods changed from Cyclops vicinus and Mesacyclops leuckarti during 1986–1989 to M. leuckarti and Megacyclops viridis in 1990. The abundance of cyclopoid copepods gradually increased from 1986 to 1988–89, decreased in autumn 1989 and markedly decreased in 1990. 3. The increase in the abundance of cyclopoid copepods from 1986 to 1988 is attributed mainly to the reduction in fish predation pressure, there being no concomitant increase in edible phytoplankton. The appearance of M. viridis in 1990, and the general decrease in cyclopoid copepod density in autumn 1989 and in 1990, are attributed to the appearance of submerged macrophytes. 4. Temperature, predation and availability of edible phytoplankton appear to determine whether C. vicinus or M. leuckarti dominates the cyclopoid copepod population of eutrophic Lake Væng.     

Diapause termination and thermal requirements for postdiapause development inAphelinus mali at constant and fluctuating temperatures

Diapause termination under natural and simulated overwintering conditions, the effect of subzero temperature on postdiapause development and the relationship between postdiapause development rate and constant and fluctuating temperatures was studied in a Dutch population ofAphelinus mali Hald. (Hymenoptera: Aphelinidae).The rate of diapause termination was similar in larvae overwintering under natural and simulated conditions. Most larvae had terminated diapause by the last week of February. Some female larvae may have remained in diapause until the end of March. The exposure of postdiapause larvae to –10°C for two weeks did not affect their survival or postdiapause development rate.PostdiapauseA. mali larvae could complete development and the adults emerge from their mummified aphid hosts at constant temperatures from 12 to 24°C. Although some larvae completed postdiapause development at 10°C, few emerged. The theoretical threshold temperature (t_o) for postdiapause development was 9.4°C and the thermal constant (K) 136.4 degree-days. K was 121.4 and 134.8 for first and 50% emergence, respectively.The number of heat units accumulating above 9.4°C to 1st and 50% emergence was similar under constant and fluctuating temperatures.

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Fin de la diapause et exigences thermiques pour le développement après la diapause d'Aphelinus mali soumis à des températures constantes ou à des thermopériodes

Résumé L'achèvement de la diapause en conditions naturelles ou simulant l'hiver, les effets des températures inférieures à zéro sur le développement après la diapause et les relations entre la vitesse de développement après la diapause et les températures constantes ou en thermopériodes ont été examinés sur des populations néerlandaises d'A. mali (Hymenop.; Aphélinidae).Les taux d'achèvement de la diapause de larves hivernantes étaient semblables en conditions naturelles ou simulées. La plupart des larves ont terminé leur diapause la dernière semaine de février. Quelques larves femelles sont restées en diapause jusqu'à fin mars. L'exposition pendant 2 semaines des larves sorties de diapause à –10 °C ne compromet pas leur survie ou leur taux de développement après la diapause.Les larves ayant diapause peuvent terminer leur développement et les adultes émerger des pucerons momifiés aux températures constantes comprises entre 12 et 24 °C. Bien que quelques larves achèvent leur développement à 10 °C, peu émergent. La température seuil théorique de développement après la diapause (t_o) a été de 9,4 °C et la constante thermique (K), 136,5 degrés-jours. Pour la première émergence et pour 50% d'émergences, les valeurs de K étaient respectivement: 121,4 et 134,8.Le nombre d'unités thermiques pour la première émergence et pour 50% d'émergences était le même à température constante ou avec une thermopériode.

     

Thermal response and reversibility of prolonged larval diapause in the chestnut weevil, Curculio sikkimensis

Curculio sikkimensis undergoes prolonged larval diapause that is terminated by chilling and warming cycles. To examine the effects of warming temperatures and their duration on diapause termination, we exposed diapause larvae that had not been reactivated after chilling at 5 °C to 20 or 25 °C and chilled them again before incubation at 20 °C. With increasing warming duration at 20 °C, diapause termination after chilling increased and shorter chilling durations became effective. In contrast, few or no larvae warmed at 25 °C terminated diapause after chilling, irrespective of the warming duration. To investigate the effect of warming temperature on diapause intensity, larvae with diapause weakened by initial incubation at 20 °C after the first chilling were subsequently incubated at 15, 20, or 25 °C, then chilled at 5 °C before incubation at 20 °C. Diapause termination increased significantly after the larvae were treated at 15 or 20 °C but decreased significantly after they were treated at 25 °C. The intensification of prolonged diapause at 25 °C was reversed when the larvae were transferred to 20 °C. Diapause intensity in C. sikkimensis therefore decreases at 20 °C, increases at 25 °C, and can be reversed by alternately exposing diapause larvae to 20 and 25 °C. In C. sikkimensis, prolonged diapause does not always proceed in one direction, and its intensity fluctuates in response to ambient temperature conditions.     

Diapause induction in the codling moth, Cydia pomonella: effect of prediapause temperatures

The effect of four prediapause temperatures (18, 22, 26 and 30°C) on the photoperiodic response of the codling moth, Cydia pomonella (L.), was studied under controlled conditions. The highest rates of diapause were recorded, for all day-lengths, at temperatures of 22 and 26°C while relatively lower rates of diapause were elicited at 18 and 30°C. The same trend was demonstrated by projecting the values of the critical photoperiod which induces 50% diapause (=CPhP₅₀) over the prediapause temperature. The change in diapause incidence as a function of photoperiod, at all prediapause temperatures, exhibited a response characteristic of long-day insects, i.e. high rates of diapause at short days (12–13.5 h) and a decrease in diapause incidence at long days (14–15 h). The results for temperatures 22, 26 and 30°C support the view that lower prediapause temperatures enhance diapause induction, at a give photoperiod, while higher temperatures tend to avert or diminish the process. On the other hand, the low rates of diapause obtained at 18°C contradict this view. Nevertheless, high correlation was found between the laboratory evidence and field data, indicating the adaptability of the Israeli codling moth to subtropical climate.