Do bioassays adequately predict ecological effects of pollutants?

With some notable exceptions, such as the echinoderm and oyster larvae tests, the species traditionally used in bioassays are not sufficiently sensitive to detect subtle ecological effects of pollutants. It is suggested that by using ecological criteria, species can be identified from any pollution gradient that are sensitive to subtle effects of pollution. Examples are given using gradients of oil, sewage and titanium dioxide pollution, showing how ecologically sensitive species for use in laboratory bioassays can be selected objectively.Many marine molluscs show microgrowth bands, which can be used as in situ field bioassays. An example is given using the bivalve Cerastoderma edule.     

南麂列岛海洋自然保护区浮游植物的种类多样性及其生态分布

于2006年5月至2007年2月之间,对南麂列岛海域的浮游植物类群进行了4个季节的调查,分析了该海域浮游植物的种类组成、优势种类、群落结构以及水平分布等特征参数的季节变化。共鉴定浮游植物80种,隶属于4个门,硅藻种类最多,甲藻其次。浮游植物可划分为3个生态类群,以广温类群为主。春季和夏季分别以三角棘原甲藻和中肋骨条藻为绝对优势种,秋冬季的优势种类组成多样化。共鉴定57种赤潮生物,占浮游植物种类数的71.25%。调查期间,三角棘原甲藻和中肋骨条藻分别于春季和夏季形成赤潮。浮游植物的物种丰富度呈现春、夏、秋、冬递减的趋势。浮游植物细胞丰度的年平均值为1.03×106cells/L,春夏季显著高于秋冬季。春季和夏季时,浮游植物高值区集中在南麂岛西北近岸海域;秋季和冬季时,浮游植物高值区相对集中在南麂岛东南近岸海域。浮游植物群落的多样性指数(H')以秋季最高,冬季最低。春季的三角棘原甲藻赤潮期间,水体中N/P值显著升高;夏季的中肋骨条藻赤潮期间,水体中N/P值显著降低。     

Population‐dependent acclimatization capacity of thermal tolerance in larvae of the rocky‐shore barnacle Pollicipes elegans

As environmental temperatures increase and become more seasonally variable, the ability of individuals to plastically alter their physiological responses to temperature (=acclimatize) may affect the potential for species persistence. Among marine organisms, the larval stage is often the most physiologically sensitive; larvae are also often the main dispersal stage in the life history. However, studies that address the acclimatization of marine larvae are rare. We investigated whether larvae of the gooseneck barnacle Pollicipes elegans from two temperate populations, one from the Northern Hemisphere (Mexico) and one from the Southern Hemisphere (Peru), show patterns of seasonal acclimatization to temperature. We compared the effects of temperature on swimming activity, oxygen consumption, and mortality of larvae from the two populations in both warm and cold seasons. Larvae from Mexico had higher thermal tolerances when collected in the boreal summer compared to the boreal winter, while no similar indication of seasonal acclimatization was seen in larvae from Peru. The lack of acclimatization in larvae of P. elegans from Peru may be related to recent thermal history, low selection for acclimatization due to irregular patterns of seasonal temperature change during ENSO events, or to different phylogeographic histories of Northern‐ and Southern‐hemisphere populations.     

Effects of oil pollution on the development of sex cells in sea urchins

The sea urchinStrongylocentrotus nudus is highly sensitive to oil pollution. Experiments were performed in winter, spring and summer over periods of 15 to 45 days. Experimental urchins were kept in water with hydrocarbon concentrations of 10 to 30 mg l⁻¹, and control urchins in pure sea water. Thermal stimulation by Evdokimov's method was applied to obtain mature sexual products during winter and spring tests. Summer investigations were conducted at temperatures of 17 to 18 °C. The gonads were studied histologically and morphometrically, and the sexual cells obtained were analyzed at the embryological level. No histological and morphometrical differences were recorded between sexual cells of controls and experimentals. However, marked hydrocarbon effects were observed in the embryonic development of artificially fertilized cells from experimental urchins. Control embryos developed normally. Embryogenesis of artificially fertilized gametes from control females and experimental males, and vice versa, was found to be distinctly abnormal. Many abnormalities were identified at the first cleavage stage, as well as in blastula, gastrula and pluteus. Fertilization of experimental eggs with experimental sperm resulted in serious disturbances of embryos, followed by the development of non-viable larvae. On the whole, embryogenesis of sexual cells from experimental urchins was characterized by prominent delay, asynchronism and presence of abnormal non-viable larvae. Consequently, long-term effects of sublethal hydrocarbon concentrations resulted in the formation of defective sex cells and high larval mortality.     

Effects of elevated temperature and carbon dioxide on the growth and survival of larvae and juveniles of three species of northwest Atlantic bivalves

Rising CO(2) concentrations and water temperatures this century are likely to have transformative effects on many coastal marine organisms. Here, we compared the responses of two life history stages (larval, juvenile) of three species of calcifying bivalves (Mercenaria mercenaria, Crassostrea virginica, and Argopecten irradians) to temperatures (24 and 28°C) and CO(2) concentrations (～250, 390, and 750 ppm) representative of past, present, and future summer conditions in temperate estuaries. Results demonstrated that increases in temperature and CO(2) each significantly depressed survival, development, growth, and lipid synthesis of M. mercenaria and A. irradians larvae and that the effects were additive. Juvenile M. mercenaria and A. irradians were negatively impacted by higher temperatures while C. virginica juveniles were not. C. virginica and A. irradians juveniles were negatively affected by higher CO(2) concentrations, while M. mercenaria was not. Larvae were substantially more vulnerable to elevated CO(2) than juvenile stages. These findings suggest that current and future increases in temperature and CO(2) are likely to have negative consequences for coastal bivalve populations.     

Larval growth rates of the blowfly, Calliphora vicina, over a range of temperatures

Blowfly larvae (Diptera: Calliphoridae) fulfil an important ecological function in the decomposition of animal remains. They are also used extensively in forensic entomology, predominantly to establish a minimum time since death, or a minimum post-mortem interval, using the larval length as a 'biological clock'. This study examined the larval growth rate of a forensically important fly species, Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae) at temperatures of between 4 degrees C and 30 degrees C, under controlled laboratory conditions. The laboratory flies had been trapped initially in London, U.K. The minimum developmental temperature was estimated to be 1 degrees C and 4700 accumulated degree hours (ADH) were required for development from egg hatch to the point of pupariation. Lines fitted to the laboratory larval growth data were found to adequately explain the growth of larvae in the field. The nature of variation in growth rates from geographically isolated populations is discussed.     

Thermotropic behavior of major phospholipids from marine invertebrates: changes with warm-acclimation and seasonal acclimatization

The crystal-liquid crystal-isotropic melt phase transitions of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) from muscle tissue of five species (actinia Metridium senile fimbriatum, mussel Crenomytilus grayanus, sea-urchin Strongylocentrotus intermedius, starfish Distolasterias nipon and the ascidian Halocynthia aurantium) of marine invertebrates, collected in winter at 0 degrees C and then acclimated to 18.5 degrees C for 5 days, were studied by differential scanning calorimetry and polarising microscopy. To elevate temperature from 0 to 18.5 degrees C, we used the rate of 4.5 degrees C/h. Although phase transitions of both phospholipids from animals collected in summer occurred already at temperatures below -1.7 oC (minimal temperature of seawater in winter), compensatory mechanisms resulted in a decrease by 29-43 oC in the phase transition temperature of PE in winter. Thermotropic behavior of PCs changed in various trends. However, the total heat of their phase transitions always decreased in winter compared with summer. For all species, except the mussel, the time of warm-acclimation was insufficient to adjust the thermotropic behavior of either phospholipid. Nevertheless, the unsaturation index decreased to achieve summer values, due primarily to decreased proportions of eicosapentaenate and docosahexaenate. The accumulation of arachidonate, during warm-acclimation, might be connected to the signalling properties of n-6 eicosanoids. Absence of effective homeoviscous mechanisms suggests that most of the studied marine invertebrates have very limited capacity to survive an acute temperature elevation, e.g. at the appearance of thermal currents.     

High-frequency variation of tree-ring width of some native and alien tree species in Latvia during the period 1965–2009

The plasticity of climate-growth relationships of trees is one of the main factors determining the climate-induced changes in forest productivity and composition. In this study, high-frequency variation of tree-ring width (TRW) of four native and three alien tree species and two hybrids of Populus L. growing in Latvia (hemiboreal zone) was compared using a principal component analysis based on TRW indices for the period 1965–2009. The effect of climatic factors was assessed using a bootstrapped correlation analysis. Influence of common climatic factors related to the length of the vegetation season, winter temperature, and water regime in summer was traced in the TRW of the studied species and hybrids. The combination and effect of the identified factors differed by species (and hybrids), to a certain extent explaining the diversity of TRW patterns. Nevertheless, some similarities among the species were also observed, suggesting the plasticity of growth response. Scots pine was generally sensitive to winter temperatures, but Norway spruce was mainly sensitive to summer water regime, while black alder was sensitive to winter temperatures and precipitation in spring. In contrast, silver birch showed the lowest sensitivity to the tested climatic factors (demonstrating sensitivity to winter precipitation in a few sites), suggesting tolerance to weather fluctuations. The TRW of the alien species was primarily sensitive to climatic factors related to water regime in the summer of the year preceding the formation of tree-ring, implying differences in mechanisms regulating wood increment. Nevertheless, temperature in the dormant period was significant for European larch in a few sites, suggesting sensitivity to cold damage. The variation of TRW of Populus hybrids diverged from others, as their growth was negatively correlated with the temperature in autumn, spring, and summer and positively correlated with water balance. Although the annual water balance in Latvia is positive, the effect of water deficit on tree growth was apparent.     

Experimental evaluation of planktonic respiration response to warming in the European Arctic Sector

The Arctic Ocean is the region on Earth supporting the steepest warming rate and is also particularly vulnerable due to the vanishing ice cover. Intense warming in the Arctic has strong implications for biological activity and the functioning of an Arctic Ocean deprived of ice cover in summer. We evaluated the impact of increasing temperature on respiration rates of surface marine planktonic communities in the European Arctic sector, a property constraining the future role of the Arctic Ocean in the CO₂ balance of the atmosphere. We performed experiments under four different temperature elevation regimes (in situ, +2, +4 and +6°C above the temperature of the sampled water) during cruises conducted in the Fram Strait region and off Svalbard during late fall–early winter, spring and summer. During late fall–early winter, where only three different temperatures were used, no response to warming was observed, whereas respiration rates increased in response to warming in spring and summer, although with variable strength.     

Effects of season and temperature acclimation on electrocardiogram and heart rate of toads

Electrocardiogram (ECG) was recorded from unrestrained toads of two species, one resistant (Bufo viridis) and the other sensitive (B. regularis) to low temperatures. Although the temperature-sensitive species could not survive at temperatures below 7-8 degrees C, heart rate is linearly and similarly related to ambient temperature in the two species. In B. viridis, the cold resistant species, heart rate in winter was 25% lower than in summer, and the dependency of heart rate on temperature was reduced by 50% in toads acclimated to 10 degrees C in winter. It was not possible to disclose any effect of acclimation on heart rate in summer, due to the large variation in the recorded values. The QRS amplitude in the ECG was considerably reduced at low temperature only in B. viridis. It is concluded that differences in cardiac activity cannot account for the distinct difference in thermal relations of the two species, and that it should reside at other regulatory levels.     

Water temperature determines strength of top-down control in a stream food web

1. We examined effects of water temperature on the community structure of a three trophic level food chain (predatory fish, herbivorous caddisfly larvae and periphyton) in boreal streams. We used laboratory experiments to examine (i) the effects of water temperature on feeding activities of fish and caddisfly larvae and on periphyton productivity, to evaluate the thermal effects on each trophic level (species‐level experiment), and (ii) the effects of water temperature on predation pressure of fish on abundance of the lower trophic levels, to evaluate how temperature affects top‐down control by fish (community‐level experiment). 2. In the species‐level experiment, feeding activity of fish was high at 12 °C, which coincides with the mean summer temperature in forested streams of Hokkaido, Japan, but was depressed at 3 °C, which coincides with the mean winter temperature, and also above 18 °C, which coincides with the near maximum summer temperatures. Periphyton productivity increased over the range of water temperatures. 3. In the community‐level experiments, a top‐down effect of fish on the abundance of caddisfly larvae and periphyton was clear at 12 °C. This effect was not observed at 3 and 21 °C because of low predation pressure of fish at these temperatures. 4. These experiments revealed that trophic cascading effects may vary with temperature even in the presence of abundant predators. Physiological depression of predators because of thermal stress can alter top‐down control and lead to changes in community structure. 5. We suggest that thermal habitat alteration can change food web structure via combinations of direct and indirect trophic interactions.     

The life cycle of Paraquimperia tenerrima: a parasite of the European eel Anguilla anguilla

Previous studies on the life history of the nematode eel specialist Paraquimperia tenerrima (Nematoda: Quimperiidae) have failed to determine whether an intermediate host is required in the life cycle. In the laboratory, eggs failed to hatch below 10 degrees C, hatching occurring only at temperatures between 11 and 30 degrees C. Survival of the free-living second stage larvae (L2) was also temperature dependent, with maximal survival between 10 and 20 degrees C. Total survival of the free-living stages (eggs and L2) is unlikely to exceed a month at normal summer water temperatures, confirming that parasite could not survive the 6 month gap between shedding of eggs in spring and infection of eels in early winter outside of a host. Eels could not be infected directly with L2, nor could a range of common freshwater invertebrate species. Third stage larvae (L3) resembling P. tenerrima were found frequently and abundantly in the swimbladder of minnows Phoxinus phoxinus from several localities throughout the year and were able to survive in this host in the laboratory for at least 6 months. Third stage larvae identical to these larvae were recovered from minnows experimentally fed L2 of P. tenerrima, and eels infected experimentally with naturally and experimentally infected minnows were found to harbour fourth stage larvae (L4) and juvenile P. tenerrima in their intestines. Finally, the whole life cycle from eggs to adult was completed in the laboratory, confirming that minnows are an obligate intermediate host for P. tenerrima.     

兴化湾浮游动物群落季节变化和水平分布

兴化湾为福建北部最大的海湾,于2006年对该海湾浮游动物群落进行了四季9个站位的调查。共检出浮游动物及幼虫124种,其中春季42种,夏季89种,秋季71种,冬季20种;分属近岸暖温、近岸暖水和广布外海3个生态类群;优势种15种,春季以水母和桡足类占优势,夏季以水母占优势,秋季以水母、桡足类和箭虫占优势,冬季则以桡足类占优势。不同季节兴化湾浮游动物生物量湿重和丰度水平分布特征变化明显,并与温度和盐度呈显著相关。聚类分析显示兴化湾浮游动物群落夏季类群和秋季类群相似度较高;各季节水平分布基本可分为湾口区和湾内区两大类群。与20世纪80年代相比,尽管本次调查浮游动物群落没有表现出显著差异,但随着电厂等大规模工程的投产,兴化湾海域生态系统健康面临着极大威胁,其环境压力需引起持续关注。     

Temperature responses in larvae of Macoma balthica from a northerly and southerly population of the European distribution range

Planktonic marine invertebrate embryos and larvae experience high mortality rates. Processes during these early vulnerable stages of development are an important determinant of the dynamics of marine invertebrate populations. In order to evaluate possible specific local adaptations of the bivalve Macoma balthica (L.), larvae from parents living in Norway (Balsfjord) and France (Gironde Estuary) were reared in the laboratory at 10, 15 and 20 °C. The rate of growth and the time it took to develop a foot were measured. Larvae grew faster and developed quicker at higher temperatures. This was true for both origins tested. Within temperature treatments, the French larvae always developed and grew fastest. Size at metamorphosis (defined as the appearance of the foot) was 250 μm (SD=12.7) in five out of the six cases; the only exception was Norwegian larvae kept at the highest temperature that metamorphosed at a smaller size (229 μm, SD=6.4). Size at metamorphosis thus appears to be largely independent of temperature. In both populations, instantaneous survival rates declined with temperature with no effect of origin. Instantaneous survival declined faster with temperature than development rates increased, resulting in lower net survival of larvae to metamorphosis at the higher temperatures. Although the French larvae had a shorter development time at the same temperature than the Norwegian larvae, the total survival of larvae from the two origins was not significantly different. The larvae of M. balthica of both populations prove to be tolerant to considerably higher rearing temperatures than they will ever experience in their natural habitat.     

Effects of photoperiod and temperature on testicular function in amphibians

Most amphibians present an annual testicular cycle characterized by a quiescent period (late autumn-winter) and a spermatogenic period (spring and summer). At the end of the period of spermatogenesis undifferentiated interstitial cells transform into steroid-secreting Leydig cells which regress in spring at the beginning of the new spermatogenetic cycle. The testicular cycle is controlled by the pituitary gonadotropin levels which are high in autumn and winter, low in spring and increase temporarily in the middle of summer. Photoperiod and temperature seem to be the most important external factors involved in the regulation of this cycle in many amphibian species since the colder the geographic area, the longer the quiescent period and the shorter the spermatogenic period. This suggests the occurrence of a potentially continuous cycle in these species, in contrast with that which occurs in other species having an endogenous rhythm of testicular function which is much less sensitive to environmental factors. Although the specific response to temperature can vary widely between species, the most frequent observation in amphibians with a potentially continuous cycle is that exposure to mild temperatures (15-20 degrees C, according to the spring temperatures of the different geographic areas) stimulates spermatogenesis even during the period of testicular quiescence. If this mild temperature is combined with a long photoperiod, complete spermatogenesis is attained. Experiments performed during the period of germ-cell proliferation (development from spermatogonia to round spermatids) indicated that low temperatures (below 11 degrees C) as well as short photoperiods (less than 8 h of light) hinder germ-cell proliferation. Moderately high temperatures (about 30 degrees C) do not impair this proliferation. In the newt Triturus marmoratus, it has been shown that an excessively long photoperiod (over 16 h) has the same effect as a short photoperiod. In this species eyes are not required for the testicular photoperiodic response. Photoperiod appears to have no effect on spermiogenesis (differentiation of round spermatids into spermatozoa), because once round spermatids are formed, spermiogenesis will occur even in total darkness. Mild temperatures seem to be necessary for spermiogenesis as well as for androgen biosynthesis because neither process will take place at extreme temperatures. Results on the effect of photoperiod in steroidogenesis differ between species.     

Viability loss of neem (Azadirachta indica) seeds associated with membrane phase behaviour.

Storage of neem (Azadirachta indica) seeds is difficult because of their sensitivity to chilling stress at moisture contents (MC) > or =10% or imbibitional stress below 10% MC. The hypothesis was tested that an elevated gel-to-liquid crystalline phase transition temperature (Tm) of membranes is responsible for this storage behaviour. To this end a spin probe technique, Fourier transform infrared microspectroscopy, and electron microscopy were used. The in situ Tm of hydrated membranes was between 10 degrees C and 15 degrees C, coinciding with the critical minimum temperature for germination. During storage, viability of fresh embryos was lost within two weeks at 5 degrees C, but remained high at 25 degrees C. The loss of viability coincided with an increased leakage of K+ from the embryos upon imbibition and with an increased proportion of cells with injured plasma membranes. Freeze-fracture replicas of plasma membranes from chilled, hydrated axes showed lateral phase separation and signs of the inverted hexagonal phase. Dehydrated embryos were sensitive to soaking in water, particularly at low temperatures, but fresh embryos were not. After soaking dry embryos at 5 degrees C (4 h) plus 1 d of further incubation at 25 degrees C, the axis cells were structurally disorganized and did not become turgid. In contrast, cells had a healthy appearance and were turgid after soaking at 35 degrees C. Imbibitional stress was associated with the loss of plasma membrane integrity in a limited number of cells, which expanded during further incubation of the embryos at 25 degrees C. It is suggested that the injuries brought about by storage or imbibition at sub-optimal temperatures in tropical seeds whose membranes have a high intrinsic Tm (10-15 degrees C), are caused by gel phase formation.     

Acquisition of freezing tolerance in early autumn and seasonal changes in gall water content influence inoculative freezing of gall fly larvae,Eurosta solidaginis (Diptera,Tephritidae)

We examined seasonal changes in freeze tolerance and the susceptibility of larvae of the gall fly, Eurosta solidaginis to inoculative freezing within the goldenrod gall (Solidago sp.). In late September, when the water content of the galls was high (approximately 55%), more than half of the larvae froze within their galls when held at -2.5 degrees C for 24 h, and nearly all larvae froze at -4 or -6 degrees C. At this time, most larvae survived freezing at > or = -4 degrees C. By October plants had senesced, and their water content had decreased to 33%. Correspondingly, the number of larvae that froze by inoculation at -4 and -6 degrees C also decreased, however the proportion of larvae that survived freezing increased markedly. Gall water content reached its lowest value (10%) in November, when few larvae froze during exposure to subzero temperatures > or = -6 degrees C. In winter, rain and melting snow transiently increased gall water content to values as high as 64% causing many larvae to freeze when exposed to temperatures as high as -4 degrees C. However, in the absence of precipitation, gall tissues dried and, as before, larvae were not likely to freeze by inoculation. Consequently, in nature larvae freeze earlier in the autumn and/or at higher temperatures than would be predicted based on the temperature of crystallization (T(c)) of isolated larvae. However, even in early September when environmental temperatures are relatively high, larvae exhibited limited levels of freezing tolerance sufficient to protect them if they did freeze.     

Induction of parturition in snow skinks: can low temperatures inhibit the actions of AVT?

The influence of environmental factors on the timing of parturition has not been investigated in viviparous squamates. We investigated the interaction between temperature and parturition in two viviparous skink species, the southern snow skink (Niveoscincus microlepidotus) and the metallic skink (N. metallicus). In these species, the timing of parturition is separated from the completion of embryonic development; the delay is attributed to their cool and variable habitats. We examined whether the neurohypophyseal hormone arginine vasotocin (AVT) stimulated parturition in southern snow skinks with late stage embryos in autumn (approximately 6-7 months prior to parturition) and in metallic skinks with late stage embryos in summer (approximately 2-3 weeks prior to parturition). The experiments were conducted at a range of environmentally relevant temperatures (6 degrees C, 15 degrees C, 22 degrees C, and 28 degrees C). AVT induced parturition in both species at all temperatures; time until birth, however, occurred more quickly at warmer temperatures (22 degrees and 28 degrees C), whereas cooler temperatures delayed parturition. We hypothesize that if cool temperatures are preventing parturition, then temperature must act at some level within the brain to prevent or slow the secretion of AVT. Future experiments will need to determine how temperature influences AVT production. Further research is also required to determine how the timing of parturition is influenced by interactions between temperature, photoperiod, and seasonal hormone patterns.     

Seasonal changes of fatty acid composition and thermotropic behavior of polar lipids from marine macrophytes

Major glyco- and phospholipids as well as betaine lipid 1,2-diacylglycero-O-4'-(N,N,N-tri-methyl)-homoserine (DGTS) were isolated from five species of marine macrophytes harvested in the Sea of Japan in summer and winter at seawater temperatures of 20-23 and 3 degrees C, respectively. GC and DSC analysis of lipids revealed a common increase of ratio between n-3 and n-6 polyunsaturated fatty acids (PUFAs) of polar lipids from summer to winter despite their chemotaxonomically different fatty acid (FA) composition. Especially, high level of different n-3 PUFAs was observed in galactolipids in winter. However, the rise in FA unsaturation did not result in the lowering of peak maximum temperature of phase transition of photosynthetic lipids (glycolipids and phosphatidylglycerol (PG)) in contrast to non-photosynthetic ones [phosphatidylcholine (PC) and phosphatidylethanolamine (PE)]. Different thermotropic behavior of these lipid groups was accompanied by higher content of n-6 PUFAs from the sum of n-6 and n-3 PUFAs in PC and PE compared with glycolipids and PG in both seasons. Seasonal changes of DSC transitions and FA composition of DGTS studied for the first time were similar to PC and PE. Thermograms of all polar lipids were characterized by complex profiles and located in a wide temperature range between -130 and 80 degrees C, while the most evident phase separation occurred in PGs in both seasons. Polarizing microscopy combined with DSC has shown that the liquid crystal - isotropic melt transitions of polar lipids from marine macrophytes began from 10 to 30 degrees C mostly, which can cause the thermal sensitivity of plants to superoptimal temperatures in their environment.     

The effect of weather on the life cycle of the speckled wood butterfly Pararge aegeria

ABSTRACT.

• 1 Pararge aegeria (L.) is a very unusual butterfly of Britain, having a long period of adult activity, from April to October, without discrete flight periods. In central Britain it overwinters in two stages: pupae and third instar larvae, both being the progeny of late summer adults. Other larval stages die at the onset of cold winter weather. The overwintering stages give rise to the first adult generation in spring, split into two parts.
• 2 Different temperature regimes affect development rates in larvae and pupae differently. Late larval development is more rapid than that of pupae at low temperatures, thus in cool spring weather the overlap of the two parts of the first generation is greater than in warm spring weather.
• 3 Adults emerge continuously throughout the summer because larval development rates are variable. When summer is warm there is a partial third generation but when cool only two.
• 4 The timing of the end of the flight period is consistent with the hypothesis that both temperature and photoperiod are important in determining whether individuals enter diapause or develop directly. In warm summers larvae develop beyond a sensitive stage before critical daylength is reached and develop directly, but in cool summers individuals enter diapause because they are at the sensitive stage when critical daylength is reached.
• 5 It is suggested that variable development rates can facilitate parasite escape in autumn and increase the probability of adult success when weather is unpredictable, and this strategy is maintained because these benefits are greater than the cost of winter mortality of larvae.