Immunological larval polyphenism in the map butterfly Araschnia levana reveals the photoperiodic modulation of immunity

The bivoltine European map butterfly (Araschnia levana) displays seasonal polyphenism characterized by the formation of two remarkably distinct dorsal wing phenotypes: The spring generation (A. levana levana) is predominantly orange with black spots and develops from diapause pupae, whereas the summer generation (A. levana prorsa) has black, white, and orange bands and develops from subitaneous pupae. The choice between spring or summer imagoes is regulated by the photoperiod during larval and prepupal development, but polyphenism in the larvae has not been investigated before. Recently, it has been found that the prepupae of A. levana display differences in immunity‐related gene expression, so we tested whether larvae destined to become spring (short‐day) or summer (long‐day) morphs also display differences in innate immunity. We measured larval survival following the injection of a bacterial entomopathogen (Pseudomonas entomophila), the antimicrobial activity in their hemolymph and the induced expression of selected genes encoding antimicrobial peptides (AMPs). Larvae of the short‐day generation died significantly later, exhibited higher antibacterial activity in the hemolymph, and displayed higher induced expression levels of AMPs than those of the long‐day generation. Our study expands the seasonal polyphenism of A. levana beyond the morphologically distinct spring and summer imagoes to include immunological larval polyphenism that reveals the photoperiodic modulation of immunity. This may reflect life‐history traits that manifest as trade‐offs between immunity and fecundity.     

Seasonal selection and resource dynamics in a seasonally polyphenic butterfly

Seasonal polyphenisms are widespread in nature, yet the selective pressures responsible for their evolution remain poorly understood. Previous work has largely focussed either on the developmental regulation of seasonal polyphenisms or putative ‘top‐down’ selective pressures such as predation that may have acted to drive phenotypic divergence. Much less is known about the influence of seasonal variation in resource availability or seasonal selection on optimal resource allocation. We studied seasonal variation in resource availability, uptake and allocation in Araschnia levana L., a butterfly species that exhibits a striking seasonal colour polyphenism consisting of predominantly orange ‘spring form’ adults and black‐and‐white ‘summer form’ adults. ‘Spring form’ individuals develop as larvae in the late summer, enter a pupal diapause in the fall and emerge in the spring, whereas ‘summer form’ individuals develop directly during the summer months. We find evidence for seasonal declines in host plant quality, and we identify similar reductions in resource uptake in late summer, ‘spring form’ larvae. Further, we report shifts in the body composition of diapausing ‘spring form’ pupae consistent with a physiological cost to overwintering. However, these differences do not translate into detectable differences in adult body composition. Instead, we find minor seasonal differences in adult body composition consistent with augmented flight capacity in ‘summer form’ adults. In comparison, we find much stronger signatures of sex‐specific selection on patterns of resource uptake and allocation. Our results indicate that resource dynamics in A. levana are shaped by seasonal fluctuations in host plant nutrition, climatic conditions and intraspecific interactions.     

FITNESS CONSEQUENCES OF SEASONAL POLYPHENISM IN WESTERN WHITE BUTTERFLIES

The western white butterfly, Pontia occidentalis, has distinctly different wing phenotypes during spring and summer generations as a result of phenotypic plasticity (seasonal polyphenism). We experimentally generated different seasonal phenotypes in the lab by altering photoperiodic conditions during rearing, and released the resulting butterflies in the field. Mark-recapture studies were then used to estimate the effects of the polyphenism on activity patterns and adult survival in both late-spring (one study) and summer (two studies) conditions. There were no significant effects of rearing treatment on temporal patterns of behavioral activity during either the late-spring or the summer field studies. Recapture probabilities were consistently higher for males than females in all three field studies; in the summer 1992 study, recapture probabilities were higher for long-day (LD) than for short-day (SD) treatment groups. During the late-spring 1992 study, there were no significant differences between LD and SD treatment groups for survival probabilities. In the two summer studies, there were significant effects of photoperiodic treatment on survival probabilities; in the summer 1992 study, LD individuals consistently had higher survival probabilities than SD individuals; in the summer 1991 study, there was a significant interaction between treatment and time period on survival probabilities, such that survival probabilities were higher for LD than for SD individuals in two of four time periods. The consistent differences in survival probabilities between treatment groups in the summer 1992 study can be accounted for by the differences in wing traits between the treatment groups. Micrometeorological data from the study site showed that midday ambient temperatures averaged ~3°C hotter during the 1992 than the 1991 summer study and that ambient conditions during the late-spring 1992 study were relatively warm and sunny for the season. These results document the varying relationships between phenotype and fitness in the temporally fluctuating environments experienced by this population.     

Induction and termination of prepupal summer diapause in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

The seasonal life cycle of the zygaenid moth, Pseudopidorus fasciata is complicated by two different developmental arrests: a winter diapause as a fourth larval instar and a summer diapause as a prepupa in a cocoon. Both larval diapause induction and termination are under photoperiodic control. Short days induce larval diapause with a critical daylength of 13.5h and long days terminate diapause with a critical daylength of 14h. In the present study photoperiodic control of summer diapause was investigated in Pseudopidorus fasciata. Under long photoperiods ranging from LD 14:10 to LD 18:6, only part of the population entered summer diapause, the rest continued to develop. The lowest number of prepupae entered diapause at LD 14:10, followed by LD 16:8 and LD 17:7. The highest incidence of diapause occurred with photoperiods of LD 15:9 and LD 18:6. By transferring the diapausing prepupae induced by various long photoperiods (LD 14:10, LD 15:9, LD 16:8, LD 17:7, LD 18:6) to LD 13:11, 25 degrees C, the duration of diapause induced by LD 14:10 was significantly shorter than those induced by longer photoperiods. By keeping aestivating prepupae induced by LD 15:9, 28 degrees C or by natural conditions at short photoperiods (LD 11:13 and LD 13:11) and at a long photoperiod (LD 15:9), the duration of diapause at LD 15:9 was more than twice as long as than those at LD 11:13 and LD 13:11. Moreover, adult emergence was highly dispersed with a high mortality at LD 15:9 but was synchronized with low mortality at LD 11:13 and LD 13:11. When the naturally induced aestivating prepupae were kept under natural conditions, the early aestivating prepupae formed in May exhibited a long duration of diapause (mean 126 days), whereas the later-aestivating prepupae formed in July exhibited a short duration of diapause (mean 69 days). These results indicate that aestivating prepupae require short or shortening photoperiod to terminate their diapause successfully. By transferring naturally induced aestivating prepupae to 25, 28 and 30 degrees C, the duration of diapause at the high temperature of 30 degrees C was significantly longer than those at 25 and 28 degrees C, suggesting that high temperature during summer also plays an important role in the maintenance of summer diapause in Pseudopidorus fasciata. All results reveal that summer diapause can serve as a "bet hedging" against unpredictable risks due to fluctuating environments or as a feedback mechanism to synchronize the period of autumn emergence.     

Nutrition,sex and season contribute to variation in fat and glycerol levels in the long‐lived moth Caloptilia fraxinella

The ash leaf cone roller Caloptilia fraxinella Ely (Lepidoptera: Gracillariidae) is an invasive leaf‐mining moth pest of horticultural ash Fraxinus spp. in the Canadian Prairie Provinces. Caloptilia fraxinella overwinter as adults in reproductive diapause and mating occurs after overwintering in the spring. The effect of a carbohydrate food source on fat and glycerol reserves throughout the long adult life stage of this moth is investigated. Insects collected as pupae are given access to either water or sugar water upon adult eclosion. Moths held under the different feeding regimes are sampled before (summer and autumn) and after overwintering in the spring. Analysis of either glycerol or lipid content is conducted for male and female moths from each collection period. Both moth weight and glycerol concentration are affected by moth sex, food regime and season of collection. Although female moths are heavier than males, a higher glycerol concentration occurs in males. Moths fed sugar are heavier and have a higher glycerol concentration than water‐fed moths late in reproductive diapause and after overwintering. Moths collected in the spring after overwintering are lighter and have a lower glycerol content than moths collected before winter. There is a significant influence of feeding regime and season on moth body lipid content, with sugar‐fed moths having more fat than water‐fed moths; however, this difference is smaller in the summer than the autumn or spring. An initial understanding of the overwintering biology and diapause of this pest is provided in the present study.     

Effects of nutrition and methoprene treatment upon reproductive diapause in Caloptilia fraxinella (Lepidoptera: Gracillariidae)

Abstract Female Caloptilia fraxinella exhibit a prolonged reproductive diapause immediately post adult emergence in mid‐summer until the next spring when mating, egg development and oviposition on fresh Fraxinus spp. leaflets occur. Factors that effect the termination of reproductive diapause are investigated in this species. Caloptilia fraxinella diapausing adults held in overwintering conditions (2 °C, LD 0 : 24 h) for 24 weeks terminate diapause after placement for 2 weeks in simulated summer conditions (24 °C, LD 16 : 8 h) only if they are provided with 10% sugar water. Exogenous application of the Juvenile Hormone (JH) analogue methoprene to moths in both early‐ (summer) and mid‐ (autumn) reproductive diapause demonstrates that JH affects diapause termination but a carbohydrate nutrition source also mediates mating and vitellogenesis. Mating between moth pairs early in diapause occurs only after treatment with methoprene and provision with sugar water. However, there is no impact of mating on the propensity of females to produce vitellogenic oöcytes. Moths collected in the autumn in mid‐diapause respond in a dose‐dependent fashion to methoprene treatment and the response is greater than that of moths early in diapause collected in the summer. Treatment with methoprene and access to sugar water results in vitellogenic oöcytes in 18.75% of females from mid‐diapause moth pairs treated with 0.01 μg methoprene per insect and in all females from pairs treated at the two highest doses of methoprene (0.1 and 1 μg per insect). Mating occurs only between moths in mid‐diapause treated with the two highest doses of methoprene and these doses induce 91% and 100% mating, respectively. Both control and methoprene‐treated males in mid‐diapause held under summer conditions mate successfully and pass a spermatophore to their methoprene‐treated female partner. These data demonstrate that female C. fraxinella undergo a prolonged reproductive diapause in which termination is dependent on JH and further mediated by a carbohydrate nutrition source. The production of vitellogenic oöcytes is independent of mating. These data also provide evidence that response of moths in diapause to exogenous applications of methoprene differs throughout the diapause period and between male and female C. fraxinella.     

Regulation of heat shock proteins in the apple maggot Rhagoletis pomonella during hot summer days and overwintering diapause

Abstract Developing larvae of the apple maggot Rhagoletis pomonella are frequently exposed to summertime apple temperatures that exceed 40 °C and, during their overwintering diapause, pupae are exposed to sub‐zero soil temperatures for prolonged periods. To investigate the potential involvement of heat shock proteins (Hsps) in response to these environmental extremes, the genes encoding Hsp70 and Hsp90 in R. pomonella are cloned and expression monitored during larval feeding within the apple and during overwintering pupal diapause. Larvae reared in the laboratory at constant temperatures of 25, 28 or 35 °C express Hsp90 but very little Hsp70. Larvae do not survive rearing at 40 °C. The temperature cycles to which larvae were exposed inside apples in the field, ranging 16–46.9 °C over a 24‐h period, elicit strong Hsp70 and Hsp90 expression, which begins at mid‐day and reaches a peak in late afternoon, coinciding with peak air and apple temperatures. Heat shock proteins are also expressed strongly by pupae during their overwintering diapause. Hsp70 is not expressed in nondiapausing pupae but is highly expressed throughout diapause. Hsp90 is constitutively expressed in both diapausing and nondiapausing pupae. Rhagoletis pomonella thus strongly expresses its Hsps during pupal diapause, presumably as a protection against low temperature injury, and during larval development to cope with natural temperature cycles prevailing in late summer.     

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.     

A test of genomic modularity among life‐history adaptations promoting speciation with gene flow

Speciation with gene flow may require adaptive divergence of multiple traits to generate strong ecologically based reproductive isolation. Extensive negative pleiotropy or physical linkage of genes in the wrong phase affecting these diverging traits may therefore hinder speciation, while genetic independence or “modularity” among phenotypic traits may reduce constraints and facilitate divergence. Here, we test whether the genetics underlying two components of diapause life history, initial diapause intensity and diapause termination timing, constrain differentiation between sympatric hawthorn and apple‐infesting host races of the fly Rhagoletis pomonella through analysis of 10,256 SNPs measured via genotyping‐by‐sequencing (GBS). Loci genetically associated with diapause termination timing were mainly observed for SNPs mapping to chromosomes 1–3 in the genome, most notably for SNPs displaying higher levels of linkage disequilibrium (LD), likely due to inversions. In contrast, selection on initial diapause intensity affected loci on all five major chromosomes of the genome, specifically those showing low levels of LD. This lack of overlap in genetically associated loci suggests that the two diapause phenotypes are largely modular. On chromosome 2, however, intermediate level LD loci and a subgroup of high LD loci displayed significant negative relationships between initial diapause intensity and diapause termination time. These gene regions on chromosome 2 therefore affected both traits, while most regions were largely independent. Moreover, loci associated with both measured traits also tended to exhibit highly divergent allele frequencies between the host races. Thus, the presence of nonoverlapping genetic modules likely facilitates simultaneous, adaptive divergence for the measured life‐history components.     

Photoperiodism and control of summer diapause in the Mediterranean tiger moth Cymbalophora pudica

Photoperiodic responses to both constant and changing photoperiods were studied in the Mediterranean tiger moth Cymbalophora pudica. Embryos, larval instars and prepupae were all sensitive to photoperiod, and the responses of larvae and prepupae to changing photoperiods were similar. At 23+/-2 degrees C, constant 24-h photoperiods with short photophases (11, 12h) induced a long diapause (mean 88days) whereas long photophases (14, 16h) induced a short diapause (mean 52days). A change to a longer or shorter photophase during larval development or during diapause caused a significant extension (up to a maximum of 138days) or shortening (down to a minimum of 10days) of diapause, respectively, but only when at least one of the photophases was longer than 14h. Thus, shortening and prolongation of photophase had an opposite effect than constant short and long photophases, respectively. Changes within the range of photophases of 10-14h did not cause a significant change in diapause duration.Experimental results enabled us to outline the mechanisms regulating voltinism and the duration of summer diapause. For the monovoltine cycle, cold autumn/winter temperatures slow down larval development, and prepupal aestivation starts relatively late (March, April). Prepupae are then kept in diapause by the increasing daylength (>14h after late April). Pupation is synchronized by decreasing daylength after summer solstice, and imagoes emerge in September/October. For the bivoltine cycle, when the autumn/winter temperatures are relatively warm, a certain portion of the population (depending on the individual rate of growth) may be diverted to a bivoltine life-cycle. In such a case, larval development is fast and short enough to allow an early start of diapause (prior to or during February). The duration of such early diapause is not influenced by changes in daylength (<14h); pupation occurs very early (April/May), and spring generation imagoes fly and oviposit in May/June. Summer larvae and prepupae live under decreasing daylength (>14h), which shortens their diapause to 20-30days. Imagoes of the autumnal generation thus occur in September/October, together with the univoltine portion of the population.     

Interactive effects of photoperiod and temperature on diapause induction and termination in the yellow-spotted longicorn beetle, Psacothea hilaris

Abstract. The interactive effects of temperature (20 °C or 25 °C) and photoperiod (LD 12 : 12 h or LD 15 : 9 h) on diapause induction and termination are investigated in the west‐Japan type yellow‐spotted longicorn beetle, Psacothea hilaris (Pascoe) (Coleoptera: Cerambycidae). Larval diapause of P. hilaris is induced under three diapause‐inducing conditions (20 °C–SD, 20 °C–LD and 25 °C–SD), and the diapause larvae are transferred to one of four conditions (20 °C–SD, 20 °C–LD, 25 °C–SD or 25 °C–LD) for observation of pupation, which indicates termination of diapause. The intensity of diapause induced under the three conditions increases in the order 20 °C–SD < 25 °C–SD < 20 °C–LD, when assessed by the time course of pupation after the transfer. On the other hand, the effectiveness of the temperature–photoperiod combinations to terminate diapause is in the order 25 °C–SD (ineffective) < < 20 °C–LD < 25 °C–LD < 20 °C–SD. Among the temperatures (5, 10, 15 and 20 °C) examined, 15 °C is the most effective in terminating diapause under the short day; diapause in most larvae appears to have been completed in 15 days.     

The conceptual and practical implications of interpreting diet breadth mechanistically in generalist predatory insects

Seasonal polyphenism in animal colour patterns indicates that temporal variation in selection pressures maintains phenotypic plasticity. Spring generation of the polyphenic European map butterfly Araschnia levana has an orange–black fritillary‐like pattern whilst individuals of the summer generation are black with white bands across the wings. What selects for the colour difference is unknown. Because predation is a major selection pressure for insect coloration, we first tested whether map butterfly coloration could have a warning function (i.e. whether the butterflies are unpalatable to birds). In a following field experiment with butterfly dummies we tested whether the spring form is better protected than the summer form from predators in the spring, and vice versa in the summer. The butterflies were palatable to birds (blue tits Cyanistes caeruleus) and in the field the spring and summer form dummies were attacked equally irrespective of season. Therefore, we found no evidence that the map butterfly is warning‐coloured or that seasonal polyphenism is an adaptation to avian predation. Because insect coloration has multiple functions and map butterfly coloration is linked to morphology, life history and development it is likely that the interplay of several selection pressures explains the evolution of colour polyphenism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Ecophysiological attributes of adult overwintering in insects: insights from a field study of the nut weevil,Curculio nucum

Abstract Diapausing insect species have evolved a great diversity of life cycles, although overwintering occurs at a single development stage within most species. Understanding why diapause has evolved towards a given life stage requires investigation of both the ecological and physiological attributes. Notably, it is suggested that adult overwintering is more energy‐demanding than larval overwintering but it brings fitness gains by allowing adults to be synchronized with their seasonal requisites through an early spring emergence. This hypothesis is tested in field conditions in the nut weevil Curculio nucum, whose life cycle comprises an obligate 2‐year, nonfeeding underground phase, including a larval, followed by an adult, overwintering. In this species, adult wintering leads to an early spring emergence; at first glance, however, this does not enhance synchronization between weevils and their host because adults emerge more than 1 month before starting to breed. It is suggested that adult overwintering ultimately evolved in response to the phenology of the host, by allowing females to oviposit in nuts before their full sclerotinization. Adult overwintering appears to be costly because adults postpone reproduction for 1 year, incur a significant weight loss and require feeding before egg laying. Surprisingly, lipids are unaffected during diapause, lipogenesis even being likely in the summer metamorphosis. These results suggest that the lipids involved in egg production may entirely come from the larval stages, whereas the other nutrients are acquired through adult feeding but this remains to be tested.     

Overwintering success in adults of the Japanese common grass yellow Eurema mandarina

The evolution of reproductive diapause is controversial in males as compared to females that must overwinter to leave offspring, because late‐autumn males can obtain offspring by pre‐overwintering copulation. The Japanese common grass yellow Eurema mandarina is suitable to examine the evolution of male reproductive diapause, because direct comparisons are possible between males that do and do not exhibit reproductive diapause. Approximately one‐half of males are insensitive to diapause‐inducing conditions, and emerge as non‐diapause summer‐form. Most autumn‐form females mate with summer‐form males in late autumn. Females that have overwintered re‐mate with autumn‐form males before the onset of oviposition. Because last‐male‐precedence is general in sperm competition in Lepidoptera, it is unclear why half of males emerge as summer‐form in late autumn. A potential adaptive benefit for emerging as summer‐form is increased sperm overwintering success, if autumn‐form females have a higher overwintering success than autumn‐form males. In the present study, overwintering success was estimated for both sexes of autumn‐form adults by rearing under seminatural conditions and a mark–release–recapture technique. Both approaches estimated an overwintering success of approximately 5% for both sexes. The absence of difference in overwintering success between the sexes suggests that pre‐overwintering copulation does not increase sperm overwintering success. However, a considerably low overwintering success may explain, at least partly, the presence of summer‐form males in late autumn. The degree of overwintering success might be more important than the sexual differences of overwintering success in the evolution of male reproductive diapause.     

Effect of mating on survival at low temperature in females of the Japanese common grass yellow,Eurema mandarina

1. In some insects that overwinter as adults, mating occurs both before and after overwintering. Two hypotheses have been proposed to explain the adaptive significance of pre‐overwintering copulation of females. One is the bet‐hedging hypothesis, which explains pre‐overwintering copulation as a preparation for less chance of mating in the following spring. The other is the nuptial gift hypothesis, which states that secretions derived from males increase overwintering success of females. 2. In Eurema mandarina, both diapause autumn‐ and non‐diapause summer‐form male adults emerge with autumn‐form female adults in the last generation in a year. Most autumn‐form females mate with summer‐form males before winter, and re‐mate with autumn‐form males in the following spring. Because autumn‐form females have sufficient chances for mating after overwintering, the nuptial gift hypothesis has been regarded as the more probable hypothesis. 3. To test the nuptial gift hypothesis, the survival period was compared under short‐day conditions at 10 °C between mated and unmated females that had been reared on sucrose solution at 25 °C for 15–21 days. The mated females had significantly greater longevity than the unmated females, supporting the nuptial gift hypothesis. Body size also affected the survival period. 4. The results suggest that the nuptial gift is an important factor for the evolution of pre‐overwintering copulation in species in which females mate both before and after overwintering.     

Alternative life cycles controlled by temperature and photoperiod in the oligophagous bug, Dybowskyia reticulata

Abstract. Effects of temperature and photoperiod on the induction and re-induction of adult diapause were examined in Dybowskyia reticulata (Dallas) (Heteroptera: Pentatomidae). Adults collected from the field after overwintering in early summer continued oviposition under long-day conditions of LD 16:8 h at 20 or 25°C, while they re-entered diapause under short-day conditions of LD 12:12 h at 25, 27.5 or 30°C. By contrast, adults reared in the laboratory from eggs at 20 or 25°C entered diapause under both long-day and short-day conditions, whereas those reared at 27.5 and 30°C entered diapause only under short-day conditions. Under quasi-natural conditions in 1993, when summer temperature was low, most adults of the first generation entered diapause in late July. However, in the warmer summer of 1996, oviposition was recorded in many females that ecdysed into adults from July to early August. Even though the seeds of the host plants occur in a restricted period from early summer to early autumn, in warmer years D. reticulata may produce a second generation. The response to temperature with a threshold between 25 and 27.5°C in D. reticulata brings about a switch between the univoltine and bivoltine life cycles.     

Partial bivoltinism in a gregarious endoparasitoid: larval diapause as influenced by season and sharing a host

The yearly timing of the life cycle of a parasitoid is a key element of its life‐history strategy. I examine here factors influencing the expression of partial bivoltinism in Tetrastichus julis Walker (Hymenoptera: Eulophidae), a specialist parasitoid introduced to North America to attack its univoltine host, the cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae). The varying tendency was assessed of individuals of this gregarious larval parasitoid to either emerge as adults in the same summer they mature, or to enter diapause to emerge the following year. Parasitized hosts were obtained by rearing cereal leaf beetles collected as mature larvae from grain fields in northern Utah (western USA) throughout the growing seasons in 2013 and 2014. Cocoons spun by these beetles were held to determine patterns over the spring and summer in the tendency of the parasitoid to forgo larval diapause. A high percentage (nearly 90% in 2013) of parasitoid individuals were found to forgo diapause and emerge in the same summer from earliest maturing hosts. This percentage rapidly declined to 20% or less of individuals forgoing diapause and emerging from cocoons as the summer advanced. The percentage of parasitoid individuals forgoing diapause increased significantly at a given time of season (early or late) as the number of conspecifics with which an individual shared a host larva increased. These results may reflect a trade‐off for individual parasitoids in which greater success in finding – and ovipositing in – host larvae the following spring vs. in summer, is countered by reduced survivorship in diapausing over the winter vs. emerging in the same summer in which the parasitoid matures. Expression of partial bivoltinism of T. julis, as affected strongly by both season and within‐host density, results in high rates of parasitism of cereal leaf beetles both early and late in the season.     

Longer life span is associated with elevated immune activity in a seasonally polyphenic butterfly

Seasonal polyphenism constitutes a specific type of phenotypic plasticity in which short‐lived organisms produce different phenotypes in different times of the year. Seasonal generations of such species frequently differ in their overall lifespan and in the values of traits closely related to fitness. Seasonal polyphenisms provide thus excellent, albeit underused model systems for studying trade‐offs between life‐history traits. Here, we compare immunological parameters between the two generations of the European map butterfly (Araschnia levana), a well‐known example of a seasonally polyphenic species. To reveal possible costs of immune defence, we also examine the concurrent differences in several life‐history traits. Both in laboratory experiments and in the field, last instar larvae heading towards the diapause (overwintering) had higher levels of both phenoloxidase (PO) activity and lytic activity than directly developing individuals. These results suggest that individuals from the diapausing generation with much longer juvenile (pupal) period invest more in their immune system than those from the short‐living directly developing generation. The revealed negative correlation between pupal mass and PO activity may be one of the reasons why, in this species, the diapausing generation has a smaller body size than the directly developing generation. Immunological parameters may thus well mediate trade‐offs between body size‐related traits.