Establishment potential of the predatory mirid <Emphasis Type="Italic">Dicyphus hesperus</Emphasis> in northern Europe

The predatory mirid Dicyphus hesperus Knight (Hemiptera: Miridae) is native to North America. The species has been used for the control of glasshouse whitefly on aubergine in the Netherlands, and is currently being evaluated for continued and wider release in Europe. Field and laboratory studies were conducted on a population collected from southern California, USA, to assess the cold tolerance and potential for outdoor establishment under prevailing northern European climates. The supercooling points (whole animal freezing temperatures) of nymphal and adult insects were around −20°C. The lethal temperatures (LTemp₅₀) of non-diapausing nymphs and adults and diapausing adults were close to their respective freezing temperatures at −17.6, −17.6 and −19.2°C. At 5°C, the LTime₅₀ was 54, 101.7 and 117.5 days for fed nymphs, non-diapausing and diapausing adults respectively. When first instar nymphs were placed in the field in winter, starved samples died out after 70 days, but 5% of the fed nymphs survived until the end of winter (140 days) and developed to adult on return to the laboratory. After a similar 5-month field exposure, 50% of fed diapausing adults and 15% of fed non-diapausing adults were still alive at the end of winter, whereas starved diapausing adults died after 140 days. On return to the laboratory after 5 months in the field, both diapausing and non-diapausing adults mated and laid eggs, forming viable populations. Overall, the field and laboratory experiments indicate that this population of D. hesperus is able to enter diapause and that winter temperatures are not a barrier to establishment in northern Europe.     

Acclimatisation in the green spruce aphid, Elatobium abietinum

Thin layer chromatography separation of 80% ethanol extracts of adult Elatobium abietinum revealed the presence of the polyhydric alcohol mannitol in aphids overwintering outdoors but not in aphids kept permanently indoors at 15°C. After 3 days at 15°C no traces of mannitol were left in overwintering aphids. Mean freezing temperatures of outdoor, unfed instar I nymphs were about 4°C lower than those of unfed instar I nymphs produced at 15°C. Mean freezing temperatures of overwintering adults were considerably higher than those of unfed instar I nymphs and showed no changes associated with time at 15°C following transference indoors. Similarly, mean freezing temperatures of Sitka spruce needles transferred to 15°C did not change. It was concluded that, although freezing was mainly avoided by supercooling, the presence of mannitol lowered the true freezing temperature of aphid haemolymph and, consequently, the actual freezing temperatures of nymphs produced under cold conditions. However, the considerable increase in freezing point temperatures caused by imbibition of plant sap masked these acclimatisation changes in feeding aphids.     

Low temperature mortality of the peach-potato aphid Myzus persicae

ABSTRACT.

• 1 The mean supercooling points of first instar and adult Myzus persicae (Sulzer) maintained at 20°C and cooled at 1°C min^?1 were ?26.6 and ?25.0°C respectively.
• 2 The LT₅₀ (temperature) of the same age groups drawn from the same population and cooled at the same rate were ?8.1 and ?6.9°C, indicating extensive pre-freeze mortality in M.persicae under laboratory conditions.
• 3 Acclimation at 10 and 5°C did not affect supercooling but depressed the LT₅₀ of both first instars and adult aphids.
• 4 Freezing of leaves during feeding did not increase mortality above that expected from the direct effects of low temperature.
• 5 The level of cold in different winters can be expressed in terms of the total number of frost days, and the frequency of abnormally cold days. Winter temperatures differ markedly in a vertical profile from the soil to the soil or grass surface, and then to the air (and foliage) above.
• 6 The time of the first record of M.persicae in suction trap samples is correlated with January and February temperatures except in the west of England and Wales. Further north December and January temperatures are relatively more important.
• 7 Winter temperatures and the resultant aphid mortality is a primary determinant of the timing of the spring migration.

     

Effect of temperature,relative humidity and aphid developmental stage on the efficacy of the mycoinsecticide Mycotal® against Myzus persicae

The green peach aphid, Myzus persicae, is a major pest worldwide. An examination of the impact of temperature, relative humidity (RH) and developmental stages of M. persicae on the efficacy of the whitefly mycoinsecticide Mycotal®, based on Lecanicillium muscarium and the effects of infection on aphid fecundity was evaluated under controlled conditions. Although this fungus can be grown at a broad range of temperatures (15–30°C), the optimum temperature for control of M. persicae ranged between 20 and 30°C. L. muscarium had high efficacy as a microbial control agent against M. persicae between 55% and 90% RH. Total mortality of aphids treated with different spore dosages of L. muscarium varied according to the developmental stage: adults, fourth and third instar nymphs proved more susceptible than first instar nymphs. Although the fungus did not affect the rate of nymph production, the reproductive period of aphids significantly decreased with increasing the spore dosage. Thus, total fecundity of treated aphids was 22.6 ± 1.1 and 31.6 ± 2.4 offspring per adult at the medium (644 ± viable spore/mm²) and low (330 ± 40 viable spore/mm²) dosages, compared with 45.7 ± 4.3 offspring per untreated aphid. The results suggest that L. muscarium has the potential as a biological control agent of M. persicae.     

Insect growth regulating effects of neem extract and azadirachtin on aphids

Neem,Azadirachta indica (A. Juss.), seed oil (NSO) applied to leaf discs at a concentration of 1.0% resulted in 94% to 100% mortality of second instar nymphs of currant-lettuce aphid,Nasonovia ribis-nigri (Mosley), and green peach aphid,Myzus persicae (Sulzer), after nine days. The equivalent amount of pure azadirachtin (AZA) (≈40 ppm), the principle active ingredient of neem, was as effective as NSO. The survival of adult aphids was unaffected by NSO or AZA, but the survival of offspring from treated adultM. persicae andN. ribis-nigri was reduced significantly. The lethal concentration of AZA resulting in 50% mortality of second instar nymphs of nine species of aphids ranged from 2.4 ppm forM. persicae on pepper to 635.0 ppm for the strawberry aphid,Chaetosiphon fragaefolii (Cockerell), on strawberry. ForM. persicae, the growth regulating effect of AZA was influenced by the host plant and the nymphal instar treated.     

麦长管蚜的低温适应性及陕西杨凌小麦田春季虫源分析

【目的】本研究旨在探明低温条件下麦长管蚜Sitobion avenae的存活率和快速冷驯化反应,以期为该虫耐寒性的研究和准确预测预报提供依据。【方法】测定麦长管蚜实验室种群各发育阶段的过冷却点和结冰点;测定1龄若蚜和未产仔成蚜分别在-7.0～-11.0℃极端低温下暴露3 h和在0℃冷驯化1～5 h后暴露于致死温度3 h,再转移至15℃72 h后的存活率;调查自然条件下陕西杨凌小麦整个生育期麦长管蚜的种群动态。【结果】麦长管蚜1龄和2龄若蚜的过冷却点波动范围较小,分别为-27.4～-19.2℃和-27.3～-18.3℃;3龄若蚜、4龄若蚜和成蚜的过冷却点波动范围较大,分别为-27.4～-10.7℃,-26.7～-12.5℃和-26.7～-11.2℃。麦长管蚜的过冷却点和结冰点随龄期增加均显著升高,其中成蚜的过冷却点显著高于1龄和2龄若蚜。3龄若蚜、4龄若蚜和成蚜的过冷却点在不同翅型之间不存在显著性差异。低温存活率分析表明,麦长管蚜1龄若蚜和无翅成蚜的致死温度(80%死亡率)分别在-10.5℃和-8.1℃左右。0℃快速冷驯化显著提高了麦长管蚜1龄若蚜和无翅成蚜在极端低温下的存活率,其中冷驯化2 h时的存活率最高。2018-2019年小麦生育期田间调查结果表明,麦长管蚜能以孤雌生殖若蚜和成蚜在陕西杨凌越冬。【结论】麦长管蚜具有较强的低温适应能力,在陕西杨凌能以孤雌生殖蚜成功越冬。因此,其本地越冬存活个体是陕西杨凌小麦田的早春虫源之一。     

Effects of brief exposures to low temperature on the development, longevity and fecundity of the grain aphid Sitobion auenae (Hemiptera: Aphididae)

First instar nymphs and adults of the grain aphid Sirobion auenae that had been reared at 10°C and 20°C over a number of generations, were cooled to -5°C and -10°C for 1 h and 6 h and returned to 20°C to assess the effects of brief exposures to low temperatures (cold-pulses) on their survival. rate of development, longevity and fecundity. A strong acclimation response was observed in first instar nymphs, with significantly less mortality in groups reared to 10°C compared to 20°C. Mean development time from first instar to adult was not significantly affected by low temperature exposure at the first nymphal stage. Longevity in all groups cooled as first instars was reduced by the sub-zero cold-pulses, and was also dependent on temperature and exposure time. Acclimated aphids survived longer than non-acclimated individuals. Reproductive rate, in terms of the number of nymphs born per aphid per day, was unaffected by cold stress applied at the first instar stage. Total fecundity was however reduced, being a function of the number and longevity of the survivors. Adult aphids were less cold hardy than nymphs; mortality was higher at -10°C than -5°C increasing with duration of exposure from 1 h to 6 h. Mean fecundity was reduced significantly in aphids cooled at the adult stage, the number of aphids born per day decreasing as the exposure period of the cold-pulse increased, suggesting that low temperature had affected embryogenesis. All the nymphs born to adults surviving exposure to -5°C for 6 h died within 48 h of birth, indicating that low temperature has a pre-natal effect on mortality.     

Antibiotic resistance in potato cultivars to the aphid Myzus persicae

Commercially-grown potato cultivars were assessed for antibiotic resistance to Myzus persicae in a culture room at 19°C with a 2°C range and a 16 h photoperiod. The resistant rankings obtained were consistent for the following aphid metrics: maturation time, teneral adult weight, embryo complement and the production of nymphs. Intrinsic rates of increase (r_m) correlated with the above parameters of aphid growth and reproduction but this was not always the case for nymphal survival rate. The cultivars Ulster Tarn, Record and Maris Piper were relatively resistant to M. persicae and cultivars Desirée and King Edward were the most susceptible. When the aphid clone or the culture plants were changed there was no significant effect on the relative resistance of the potato cultivars under test. The relationship between the number of embryos in teneral adults and the production of nymphs by aphids was examined by grouping aphids in weight batches. There was a strong relationship between these two variables.     

Differential cold hardiness in adults and nymphs of the peach-potato aphid Myzus persicae

The LT₅₀ (lethal temperature) of first instar and adult stages of the peach-potato aphid Myzus persicae was lowered following long term acclimation at low temperatures.
First instars consistently showed greater cold hardiness than adult stages at each acclimation temperature, with the differential increasing as the temperature was lowered. When maintained at 5°C (the lowest acclimation regime) nymphs and adults had dLT₅₀8.3°C and 4.7°C respectively lower than those for non-acclimated individuals.
When 10°C acclimated adults were returned to 20°C, the acclimation effect was retained in full for 6 days but complete deacclimation occurred by day 10. In contrast the LT₅₀ of their progeny increased gradually from the first day of adult deacclimation towards the level of the unacclimated control over a period of 10 days.
A change in cold hardiness was observed in first instars according to their position in the birth sequence. The LT₅₀ of first-born nymphs (day 1 of reproduction) from 20°C parents was - 15.9°C rising to - 8.3°C by day 4 and remaining at this level until the end of the reproductive period.
The differential mortality between nymphs and adults observed in the laboratory was supported by the results of a field experiment. Adult aphids kept in clip-cages on a crop of oilseed rape showed greater mortality compared with those introduced as nymphs when the minimum temperature fell below -4°C for the first time in winter. At - 10°C mortality of aphids introduced as adults approached 100% whereas more than 50% of those introduced as nymphs were still alive at this temperature.     

Supercooling of Myzus persicae in relation to gut content

The supercooling distribution curves of Myzus persicae varied in response to feeding position, diet and age. Unfed instar I aphids supercooled to temperatures lower than –20 °C. Feeding aphids supercooled to higher temperatures, the earlier instars on subsidiary veins supercooling to lower temperatures than the late instars and adults on the main veins. Starvation decreased supercooling ability in some adult aphids. The supercooling ability of aphids feeding on sucrose solutions through a Parafilm membrane decreased in response to an increasing concentration, the position being reversed on 25% sucrose. Varying the amino acid concentration had no effect. The possible effects of this variable supercooling ability on the winter survival of the aphid are discussed.     

Relative consumption of three aphid species by the lacewing, Chrysoperla rufilabris, and effects on its development and survival

Relative consumption of three aphid species, Aphis gossypii Glover, Myzus persicae (Sulzer) and Lipaphis erysimi (Kaltenbach) (Homoptera: Aphididae), by larvae of the lacewing, Chrysoperla rufilabris (Burmeister) (Neuroptera: Chrysopidae), was determined in the laboratory, together with effects on lacewing development and survival. Percentages of survival of C. rufilabris from first instar to adult eclosion were significantly different among lacewing larvae fed different aphid species. When larvae were fed A. gossypii and M. persicae, all larvae developed to adulthood. All larvae died prematurely when they were fed L. erysimi. Developmental duration of C. rufilabris larvae was significantly shorter when larvae were fed A. gossypii (18.0 d) than when larvae were fed M. persicae (19.2 d). The number of fourth instar aphids consumed during development by C. rufilabris larvae differed significantly among individuals fed different aphid species. Chrysoperla rufilabris consumed an average of 168 M. persicae, followed by 141.6 A. gossypii, and only 26.6 L. erysimi. The percentage of these total number of aphids consumed by each larval stadium of C. rufilabris varied significantly among aphid species. The percentage of A. gossypii consumed by each larval stadium was similar to that for M. persicae, 12.1 and 11.4% by the first instar, 15.7 and 13.1% by the second instar, and 72.2 and 75.5% by the third instar, respectively; whereas in the case of L. erysimi, 23.3% of the total number of aphids were consumed by the first instar, 30.1% by the second instar, and 46.6% by the third instar.     

A common strategy of cold hardiness in ants of the genus <Emphasis Type="Italic">Myrmica</Emphasis> (Formicidae,Hymenoptera) in Northeast Asia

The biotopic distribution, nest structure, wintering conditions, and cold hardiness of four ecologically contrasting ant species (Myrmica angulinodis, M. kamtschatica, M. bicolor, and M. transsibirica) are considered. The cold hardiness of these species is typical of the genus: the supercooling points vary from −28 to −31°C; cold hardiness levels (LT50%) are higher by 5°C. At this level of cold hardiness, ants can be practically ubiquitous across the whole Hypoarctic (Berman et al., 2007). However, the above Myrmica species are strictly segregated (M. kamtschatica occurs in moss bogs, M. angulinodis and M. transsibirica, on dry and warm south slopes, and M. bicolor, in sandy-gravel floodplains), probably due to different requirements for weather conditions in summer and depth of ground thawing. At present, the excess cold hardiness common to the species in question (exceeding the nest temperature by 5–10°C in different years) is not adaptive and may be considered as preadaptive. It could have been acquired during ancient cold epochs or inherited by the genus as a concomitant result of adaptation not to low temperatures but, for instance, to aridity. These Myrmica species do not undergo selection for resistance to negative temperatures since their current level of cold hardiness is excessive, considering the possible wintering temperatures.     

光周期对中华通草蛉自然越冬成虫及实验种群幼虫耐寒能力的影响

【目的】滞育诱导期进行短光照处理可影响昆虫耐寒性。为明确光周期对中华通草蛉Chrysoperla sinica (Tjeder)耐寒性的影响, 针对中华通草蛉滞育解除过程及非滞育虫态的耐寒性进行了一系列研究。【方法】测定了中华通草蛉自然越冬成虫的过冷却点（supercooling point, SCP）以及长光周期（15L∶9D）和短光周期（9L∶15D）条件下自然越冬成虫在滞育解除过程中在-12℃下的死亡率, 并测定了室内长、 短两种光周期下实验种群2龄和3龄幼虫的过冷却点（SCP）、 结冰点（freezing point, FP）以及-7℃下的死亡率。【结果】中华通草蛉12月份的自然越冬成虫SCP集中在-10~-14℃之间。SCP低于-12℃的个体占43.70%, 且-12℃处理1 d死亡率为62.00%。-12℃处理1 d条件下的长、 短光周期处理自然越冬成虫, 除处理0 d外, 长光周期处理死亡率均高于短光周期处理的, 且在处理15 （P=0.012）, 20 （P=0.01）和25 d （P=0.001）差异显著。中华通草蛉试验种群相同龄期幼虫在短光周期下的SCP和FP均高于长光周期下, 但差异不显著(P>0.05); 但在-7℃下, 2龄幼虫短光周期下的低温死亡率为67.00%±4.04%, 显著低于长光周期下的低温死亡率（78.00%±1.33%）（P=0.011）, 3龄幼虫短光周期条件下低温死亡率为24.33%±1.33%, 显著低于长光周期下的低温死亡率（53.00%±3.46%）（P=0.002）。【结论】中华通草蛉为结冰敏感型, 诱导滞育的短光照处理可提高其幼虫期及滞育解除过程中成虫的耐寒能力。     

Parasitism byEphedrus cerasicola [Hym.: Aphidiidae] developing in different stages ofMyzus persicae [Hom.: Aphididae]

The parasitoidEphedrus cerasicola Stary oviposited in the 4 nymphal instars and in newly moulted apterous adults ofMyzus persicae (Sulzer). Development and reproduction of unparasitized and parasitized aphids at 21°C were compared. Unparasitized aphids developed to adults in 6.5 days and started to reproduce after 7 days. Longevity varied between 7 and 42 days. Net reproductive rate (R₀) was 40.7. In contrast to older nymphs, aphids parasitized in the 1 st instar almost never reached the adult stage before mummification. Aphids parasitized in 2nd, 3rd and 4th instar and as newly moulted adults produced respectively 0.07 %, 2 %, 23 % and 32 % of offspring produced by unparasitized aphids. Corresponding reproductive periods were 1, 1.4, 3 and 4 days. Host age at parasitization had a slight effect on the parasitoid's developmental rate and had no effect on egg or pupal survival, or on the sex ratio of the emerging parasitoids.     

Supercooling ability and cold hardiness of the pollen beetle Meligethes aeneus

Supercooling point (SCP) and cold‐hardiness of the pollen beetle Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae) were investigated. Mature eggs from the oviduct were supercooled on average to ?28.0 °C and from oilseed rape buds to ?24.4 °C; first instars were supercooled to ?21.0 °C and second instars to ?16.8 °C. Despite their high supercooling ability, none of the eggs survived 24 h exposure to ?2.5 °C. The supercooling ability of adults varied significantly among feeding and non‐feeding beetles: high SCPs prevailed during the whole warm period, being about ?12 °C; low values of SCP of ?20 °C dominated in non‐feeding beetles. In spring and autumn, beetles displayed the same acclimation efficiency: after 1 week of exposure at 2.0 °C with no access to food their SCPs were depressed equally by about 3 °C. Meligethes aeneus beetles have a different response to low temperatures depending on the season. The lowest tolerance was found in reproductively active beetles after emergence from overwintering sites; the time needed to kill 50% of individuals (Ltime₅₀) was 56.2 h at ?7 °C and the lower lethal temperature needed to kill 50% (Ltemp₅₀) after 24 h exposure was ?8.6 °C. Cold hardiness increased from midsummer to midwinter; Ltime₅₀ was 80 h in August, 182.8 h in September, and 418.1 h in January. Lethal temperature after 24 h exposure was ?9.1 °C in August and ?9.8 °C in September. In February, after diapause, the beetles started to loose their cold tolerance, and Ltemp₅₀ was slightly increased to ?9.5 °C. Hibernating beetles tolerated long exposure at ?7 °C well, but mortality was high after short exposure if the temperature dropped below ?9 °C for 24 h. Despite the season, the beetles died at temperatures well above their mean SCP; consequently, SCP is not a suitable index for cold hardiness of M. aeneus.     

Age‐dependent changes in tolerance to cold and accumulation of cryoprotectants in overwintering and non‐overwintering larvae of European corn borer Ostrinia nubilalis

Abstract The age‐dependent cold hardiness profile of Ostrinia nubilalis is compared between nondiapausing and diapausing larvae, as well as with field‐collected larvae. The results suggest that both cold tolerance and accumulation of cryoprotectants depends upon the age of O. nubilalis larva. Late fifth‐instar nondiapausing larvae are more cold tolerant than younger fifth‐instars because they show enhanced ability to withstand sub‐zero temperatures. No appreciable difference is observed between the experimental groups of diapausing larvae as far as their supercooling ability and tolerance at sub‐zero temperatures above the supercooling point. In general, both field‐collected and diapausing larvae are more cold tolerant than nondiapausing larvae, indicating a direct link between diapause and cold hardiness. The age of diapausing larvae affects the ability to accumulate glycerol. Glycerol levels of 45‐day‐old diapausing larvae are significantly higher (2.7‐fold) compared with 90‐day‐old diapausing larvae. Moreover, diapausing larvae display a five‐ to 13‐fold higher glycerol content compared with nondiapausing larvae. There is a trend for an age‐dependent cold hardiness profile in O. nubilalis and further tests that could demonstrate a causal relationship between age and cold tolerance are needed.     

Factors affecting cold hardiness in the small striped flea beetle, Phyllotreta undulata

The striped flea beetle, Phyllotreta undulata Kutschera (Coleoptera: Chrysomelidae), is a pest of cruciferous crops. It overwinters as an adult. During winter in northern European countries, such as Estonia, it is subject to sometimes severe temperatures that may fluctuate daily, over the season, and between seasons. The objective of this study was to investigate factors that affect its cold hardiness. In a series of five experiments, the effects of food plant, starvation, and acclimatization on the beetles’ ability to supercool and survive exposure to sub‐zero temperatures was investigated. The supercooling points (SCP) of overwintered beetles field‐collected from white mustard and Indian mustard differed from those caught from white cabbage and oilseed rape, but these differences disappeared after a 4‐day period of starvation at room temperature, indicating that gut content probably influences the potential to supercool. The duration and temperature of acclimation affected SCP in overwintered beetles. The decrease in SCP was more rapid at 22 °C than at 0 °C, probably because of faster dehydration and gut evacuation at the higher temperature. Acclimation at 0 °C for a week increased the ability of overwintered beetles to survive sub‐zero temperatures, lowering both SCP and lower lethal temperature (LLT₅₀). Some pre‐freeze mortality occurred; SCP and LLT₅₀ were correlated but the latter was a constant 3 °C higher than the former. The SCP of field‐collected pre‐winter beetles decreased gradually during the autumn. It also decreased when field‐collected pre‐winter beetles were acclimated at 0 °C in the laboratory, attaining its lowest level after 18 days. Phyllotreta undulata is well‐adapted to unstable and sometimes severe winter conditions; its high potential to supercool enhances its cold hardiness and ability to survive short periods at sub‐zero temperatures although it cannot survive freezing of its body fluids.     

The effects of temperature on the availability and acquisition of potato leafroll luteovirus by Myzus persicae

The concentration of potato leafroll luteovirus (PLRV) did not differ in potato plants with secondary infections grown at 15°C or 27°C. Detached leaves of plants grown at 15°C or 27°C were used as sources of PLRV for peach-potato aphids (Myzus persicae Sulz.) both at 15°C and 27°C. At comparable temperature during virus acquisition, aphids which fed on leaves of plants kept previously at 15°C contained more viral antigen detected by ELISA than aphids which fed on leaves of plants grown at 27°C. The aphids which acquired PLRV at 27°C contained evidently more viral antigen than those which acquired PLRV at 15°C. The greatest amount of PLRV was found in the aphids which acquired the virus at 27°C from the leaves of plants kept at 15°C. The ability of M. persicae to transmit PLRV to Physalis ftoridana Rydb. generally decreased with decrease in the amount of PLRV in vectors.     

Cold shock injury and ecological costs of rapid cold hardening in the grain aphid Sitobion avenae (Hemiptera: Aphididae)

The ability of first instar nymphs and newly moulted pre-reproductive adults of the grain aphid S. avenae to rapidly cold harden was investigated. When nymphs reared at 20 degrees C were transferred directly to -8 degrees C for 3 h, there was 18% survival. This exposure was selected as the discriminating temperature. Maximum increases in survival were achieved by acclimating nymphs for 2 h at 0 degrees C and adults for 3 h at 0 degrees C, resulting in survival of 83% and 68%, respectively. Cooling nymphs from 10 to 0 degrees C at different rates (1, 0.1 and 0.05 degrees C min(-1)) also increased cold hardiness, with the slowest rate of 0.05 degrees C min(-1) conferring the highest survival following exposure to the discriminating temperature. Adult aphids also expressed a rapid cold hardening response but to a lesser extent, with survival increasing from 16% to 68% following 3 h at 0 degrees C. There were no 'ecological costs' associated with rapid cold hardening in terms of development, longevity or fecundity. The data support the hypothesis that rapid cold hardening can be induced during the cooling phase of natural diurnal temperature cycles, allowing insects to track daily changes in environmental temperatures.     

Photoperiodic and Thermal Regulation of Development and Cold Hardiness in Larvae of the Clover Leaf Weevil, Hypera punctata

Effects of photoperiod and temperature on the development and cold hardiness were investigated in larvae of Hypera punctata. At a relatively low temperature (15°C), the larvae fed less and developed more slowly under a 12L:12D (SD) photoperiod than under a 16L:8D photoperiod (LD). SD larvae had lower gut weight against the whole body weight and lower supercooling point (SCP) than the LD counterparts for the same instar and same body weight. This was because the larval SCP is markedly affected by the quantity of the gut content. Laboratory experiments indicated that the low temperature mortality of this larvae occurred mainly due to freezing irrespective of the photoperiod and temperature, suggesting that the lower lethal temperature (LLT) depends on the supercooling ability of larvae. The SD larvae tended to have a lower SCP and hence a lower LLT than the LD counterparts at 15 or 10°C, unlike at 20°C. Thus, the slower larval development under SD conditions at relatively low temperatures may prevent larvae from reaching the later instar, which have a higher SCP and thus less cold tolerance, during the coldest season. The suppressed feeding activity under SD conditions would lower the SCP, thereby reducing the possibility of lethal tissue freezing. Such a photoperiodic and thermal regulation of the larval development and the supercooling ability appear to represent adaptive mechanisms for winter survival in this beetle.