Geographic variation in clutch size and a realized benefit of aggregative feeding

We investigated one causal explanation for geographic variation in clutch size and aggregative feeding of the pipevine swallowtail, Battus philenor. Populations in California lay larger clutches than those in Texas, and larger feeding aggregations grow at an accelerated rate on the California host plant. Using reciprocal transplant experiments with larvae from California and Texas populations, we found that the benefit of increased growth rate associated with feeding in larger groups occurred only on the California host plant and was observed for larvae from both populations. These results are consistent with the hypothesis that larger clutch size and aggregative feeding are adaptations to characteristics of the California host plant. Future studies on the evolution of clutch size and aggregative feeding of herbivorous insects should consider how these life-history traits affect host plant suitability.     

Antagonistic, stage-specific selection on defensive chemical sequestration in a toxic butterfly

Larvae of the pipevine swallowtail ( Battus philenor ) sequester toxic alkaloids called aristolochic acids from their Aristolochia host plants, rendering both larvae and adults chemically defended against most predators. Using a chemically controlled artificial diet, we observed substantial among-family variation in sequestration ability and larval developmental rate in a population occurring in central Texas. Early instar larvae from families that sequester greater amounts of aristolochic acid showed increased survivorship in a field experiment in which cohorts from each family were exposed to natural predators, whereas among-family variation in growth rate did not predict survivorship. Conversely, the aristolochic acid content of adult butterflies was negatively correlated with adult fat content, a fitness correlate. Sequestration ability positively affects the probability of larval survivorship, but at the cost of adult fat content. The costs and benefits of aristolochic acid sequestration vary during the course of the butterfly's development, and these antagonistic selection pressures may explain why variation in sequestration ability persists in wild populations.     

Host plant trichomes and the advantage of being big: progeny size variation of the pipevine swallowtail

1. Plants possess numerous traits that confer resistance against insect herbivores, and herbivores, in turn, can evolve traits to ameliorate the effectiveness of these traits. The pipevine swallowtail, Battus philenor, is an extreme specialist on plants in the genus Aristolochia. The only host plant available to the California population of B. philenor is A. californica. Aristolochia californica is distinct from most other B. philenor host plants in that it is pubescent. 2. The progeny of B. philenor are larger in California compared with populations examined in Texas. Size differences persist throughout larval development. 3. Regardless of maternal host plant, population differences in progeny size persist, and crosses between California (large progeny) and Texas (small progeny) B. philenor populations resulted in offspring producing intermediate sized progeny, indicating a heritable component to progeny size variation. 4. California neonate caterpillars more easily overcame the trichomes of A. californica compared with Texas neonates. When trichomes were removed from A. californica, time to feeding establishment was reduced for caterpillars from both populations. Texas caterpillars established feeding sites on A. californica with trichomes removed, in the same time required to establish feeding on their non‐pubescent host plant, A. erecta. 5. This study shows that plant trichomes might impose selection pressure on progeny size.     

No geographic variation in thermoregulatory colour plasticity and limited variation in heat‐avoidance behaviour in Battus philenor caterpillars

Phenotypic plasticity can help organisms cope with variation in their current environment, including temperature variation, but not all environments are equally variable. In the least variable or extreme environments, plasticity may no longer be used. In this case, the plasticity could be lost altogether, or it could persist with either the same or an altered reaction norm, depending on factors such as the plasticity's costs. In the pipevine swallowtail caterpillar (Battus philenor), I tested for changes in two forms of heat‐avoidance plasticity, colour change and refuge‐seeking behaviour, across the species’ range in the United states, including the cooler eastern parts of its range where colour change has not been observed and is unlikely to be needed. I found that both heat‐avoidance behaviour and colour change persisted in all surveyed populations. Indeed, the reaction norm for colour change remained nearly unaltered, whereas the threshold for refuge‐seeking only changed slightly across populations. These results suggest that the costs of these plastic traits are low enough for them to be maintained by whatever minimal gene flow the population receives. I show that plasticity can be maintained unaltered in populations where it is not used and discuss the potential consequences of this persistence for both the ecology and evolution of plasticity.     

The lethal plant defense paradox remains: inducible host-plant aristolochic acids and the growth and defense of the pipevine swallowtail

Toxic plants with sequestering specialists are presented with a problem because plant derived toxins protect herbivores against natural enemies. It has been suggested that early induction of toxins and later relaxation of these defenses may help the plant resolve this problem because neonate caterpillars incur the physiological cost of dealing with toxins in early life, but are denied toxins when they are able to sequester them efficiently. In California, the pipevine swallowtail, Battus philenor L. (Lepidoptera: Papilionidae), feed exclusively on Aristolochia californica Torrey (Aristolochiaceae), an endemic vine that contains toxic alkaloids called aristolochic acids that caterpillars sequester to provide chemical defense in immature and adult stages. In a field experiment, the concentration of aristolochic acids doubled in the plant following leaf damage and returned to constitutive levels after six days. Neonate pipevine swallowtail caterpillars showed no aversion to high levels of aristolochic acid in a preference test. Caterpillars reared on leaves with supplemented aristolochic acid showed no physiological cost or increased mortality compared to caterpillars reared on un-supplemented leaves. Searching efficiency and capture rate of lacewing larvae (Chrysoperla), a common predator of first instar caterpillars, was compromised significantly after feeding on caterpillars reared on leaves with supplemented concentrations of aristolochic acid compared to caterpillars feeding on control plants. Additionally, mortality of lacewings increased when they were provided with a diet of B. philenor caterpillars reared on supplemented leaves compared to caterpillars reared on control leaves. Thus, the induction of aristolochic acids in the plant following leaf damage does not resolve the problem confronted by the plant and may confer benefits to this sequestering specialist.     

Genetic differences and phenotypic plasticity as causes of variation in oviposition preference in Battus philenor

Summary Bradshaw (1965) proposed that phenotypic plasticity would be more common than adaptive genetic variability in species for which environmental fluctuations occur over periods roughly equal to that species' generation time. In an effort to examine this notion, sources of seasonal variation in two components of oviposition behavior in an east Texas population of pipevine swallowtail butterflies (Battus philenor) were investigated under natural and seminatural conditions. Variability in a visually-based prealighting component involving orientation to leaf shape was primarily due to phenotypic plasticity in the form of adult learning; no seasonally-based genotypic differences in leaf-shape discrimination behavior were observed. By contrast, a chemotactile post-alighting component involving elicitation of oviposition after landing on the host plant was not phenotypically plastic, i.e., not susceptible to learning. In addition, only slight and nonsignificant seasonally-based differences in post-alighting responses to different host species were observed.     

Contemporary patterns in a historical context: phylogeographic history of the pipevine swallowtail,Battus philenor (Papilionidae)

Abstract We examined mitochondrial DNA (mtDNA) variation in pipevine swallowtail butterflies ( Battus philenor ) from throughout its extant range to provide a historical, phylogeographical context for ecological studies of the disjunct population in California. We evaluate current hypotheses regarding host plant use, behavior, and mimetic relationships of B. philenor populations and generate alternative hypotheses. Compared to populations throughout the rest of the species' range, California populations are ecologically distinct in that they lack mimics, lay significantly larger clutches of eggs, and exclusively use a unique, endemic larval host plant. Analysis of molecular variance, tests of population differentiation, and nested clade analysis of mtDNA variation indicate that, despite low levels of population genetic structure across the species' range, there is evidence of recent range expansion from presumed Pleistocene refuge(s) in southeastern North America. Colonization of California appears to have been a recent event. This phylogeographic investigation also suggests that the evolution of life-history adaptations to a novel larval host has occurred rapidly in California and the lack of mimics in California may be attributable to the recency of colonization.     

Differences in heat tolerance plasticity between supratidal and intertidal snails indicate complex responses to microhabitat temperature variation

Tropical intertidal gastropods that experience extreme and highly variable daily temperatures have evolved significant and complex heat tolerance plasticity, comprising components that respond to different timescales of temperature variation. An earlier study showed different plasticity attributes in snails from differently-heated coastlines, suggesting lifelong irreversible responses that matched habitat thermal regimes. To determine whether heat tolerance plasticity varied at a finer, within-shore spatial scale, we compared the responses of supratidal (predominantly shade-dwelling) and intertidal (frequently solar-exposed) populations of the tropical thermophilic gastropod, Echinolittorina malaccana. Snails modified lethal temperature (LT₅₀) under warm or cool laboratory acclimation, with the overall variation in LT₅₀ being greater in the supratidal (56.0–58.0 °C) than in the intertidal population (57.1–58.1 °C). Similar maximum LT₅₀s expressed by the populations after warm acclimation suggest a capacity limitation under these temperature conditons. The different minimum LT₅₀s after cool acclimation corresponded with microhabitat temperature and field acclimatization of the snails. Different responses to the same laboratory acclimation treatment imply long-term (and possibly lifelong) thermal acclimatization, which could benefit sedentary organisms that are randomly recruited as larvae from a common thermally-stable aquatic environment to thermally-unpredictable intertidal microhabitats. These findings provide another example of thermal tolerance plasticity operating at microhabitat scales, suggesting the importance of considering microhabitat thermal responses when assessing broad-scale environmental change.     

Aggregative feeding of pipevine swallowtail larvae enhances hostplant suitability

Many hypotheses have been proposed to explain the adaptive significance of aggregative feeding in the Lepidoptera. One hypothesis that has received little attention is how induced plant responses may be influenced by aggregative feeding, as compared to feeding by solitary larvae. This study investigated the role of aggregative feeding of the pipevine swallowtail, Battus philenor, in California with special emphasis on the induced responses to herbivory of its hostplant. Here, I show that first-instar larvae develop faster when feeding in a large aggregation compared to solitary or small groups of larvae. Furthermore, I show that this effect is mediated by a larval-density-dependent response in the plant and is independent of prior larval experience and direct interaction among larvae. These results indicate that large groups of larvae can effectively enhance hostplant suitability. A separate experiment showed that larvae feeding on previously damaged leaves had a reduced growth rate. Thus, following initial damage a plant first goes through a period of increased suitability, followed by induced resistance against subsequent herbivory. Aggregative feeding in this system may be an adaptive strategy for larvae to manipulate hostplant suitability, adding a new dimension to the role of aggregative feeding for the Lepidoptera.     

Effects of temperature and elevation on habitat use by a rare mountain butterfly: implications for species responses to climate change

Abstract 1. The present study used the mountain specialist butterfly Parnassius apollo as a model system to investigate how climate change may alter habitat requirements for species at their warm range margins. 2. Larval habitat use was recorded in six P. apollo populations over a 700 m elevation gradient in the Sierra de Guadarrama (central Spain). Larvae used four potential host species (Sedum spp.) growing in open areas amongst shrubs. 3. Parnassius apollo host‐plant and habitat use changed as elevation increased: the primary host shifted from Sedum amplexicaule to Sedum brevifolium, and larvae selected more open microhabitats (increased bare ground and dead vegetation, reduced vegetation height and shrub cover), suggesting that hotter microhabitats are used in cooler environments. 4. Larval microhabitat selection was significantly related to ambient temperature. At temperatures lower than 27 °C, larvae occupied open microhabitats that were warmer than ambient temperature, versus more shaded microhabitats that were cooler than ambient conditions when temperature was higher than 27 °C. 5. Elevational changes in phenology influenced the temperatures experienced by larvae, and could affect local host‐plant favourability. 6. Habitat heterogeneity appears to play an important role in P. apollo larval thermoregulation, and may become increasingly important in buffering populations of this and other insect species against climatic variation.     

Between-clutch interactions affect a benefit of group feeding for pipevine swallowtail larvae

Abstract.  1. In California, early instar larvae of the pipevine swallowtail ( Battus philenor ) develop at an accelerated rate when feeding in large groups compared with small groups due to a plant-mediated response to feeding group size. Larvae benefit from accelerated growth because the time larvae remain in early stages, where mortality is highest, is reduced. Occasionally, multiple clutches are laid on the same plant stem. Clutch size modification by females ovipositing on plant with previously laid clutches and the effect of kinship and group size on larval behaviour was examined. The direct and indirect interactions between clutches were investigated to determine if group size and time between clutch establishment affects the performance of early instar larvae.
2. Larger groups consume the young foliage more quickly and develop at an accelerated rate compared with smaller groups. Older foliage available to later clutches is an inferior food resource compared with younger foliage.
3. There was no evidence that females adjust clutch size in response to the presence of conspecific clutches.
4. Second groups of larvae readily joined previously established feeding groups. There were no observed behavioural differences between sibling and mixed-family groups.
5. The effect of a second group on the growth of the initial group was dependent on the size of both groups and the time interval between the arrival of the two groups.
6. Accelerated growth associated with larger feeding aggregations was absent when these groups were introduced to plants with previously established groups.
7. It is beneficial for ovipositing females to avoid plants with previously laid clutches because direct and indirect interactions with established clutches compromises larval performance.     

Divergence in larval responses to food plants between temperate and tropical populations of the black swallowtail butterfly

ABSTRACT.

• 1 Larvae of the black swallowtail butterfly, Papilio polyxenes Fabr., from New York and Costa Rica were reared under the same environmental conditions on Daucus carota L. and Spananthe paniculata Jacq. (Umbelliferae), which are the host plants most commonly used by the two populations, respectively. Feeding rates, growth rates and feeding efficiencies of the fifth instar larvae were measured by standard techniques.
• 2 Larvae from each population grew faster on the host-plant species from their region of origin than did larvae from the other population, though differences on S.paniculata were not significant. On D.carota the faster growth by New York larvae was attributable to greater efficiency. On S.paniculata, by contrast, the somewhat faster growth by Costa Rican larvae seemed to have resulted chiefly from faster feeding. The two populations have thus come to differ in metabolic and, probably, behavioural responses to host plants.
• 3 S.paniculata was a superior host plant for larvae of both populations, even though the total energy, nitrogen and water content of its foliage was similar to that of D.carota.
• 4 These results are consistent with the hypothesis that insects in the two populations have diverged in adaptation to their host plants following curtailment of gene flow between them as the species colonized Central America from the north.

     

The environmental and genetic control of seasonal polyphenism in larval color and its adaptive significance in a swallowtail butterfly

Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth-instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full-sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn.     

Pit-Building Decisions of Larval Ant Lions: Effects of Larval Age, Temperature, Food, and Population Source

Foraging decisions are an integral component of growth and maintenance and may reflect both environmental and genetic effects. We used a common garden experiment to evaluate the effects of food, temperature, and population source on pit-building decisions of the larval ant lion Myrmeleon immaculatus. In a laboratory common garden experiment, first-instar larvae from two southern (Georgia, South Carolina) and two northern (Connecticut, Rhode Island) populations were reared for 14 months in incubators under high- and low-food and high- and low-temperature regimes. For all populations, there was no effect of larval age on pit-building behavior. All larvae built and maintained pits more frequently at high temperatures than at low temperatures, and larvae in the low-food treatments built and maintained pits more frequently than larvae in the high-food treatments. Larvae from the southern populations built and maintained pits more frequently than larvae from northern populations. These results suggest that regional differences in foraging behavior may contribute to latitudinal gradients in life history strategies seen in this insect.     

INTERPOPULATION DIFFERENCES IN HOST PREFERENCE AND THE EVOLUTION OF LEARNING IN THE BUTTERFLY,BATTUS PHILENOR

The host-discrimination behavior of the adult female pipevine swallowtail butterfly (Battus philenor) was investigated for two populations, an east Texas population that uses two host species with different leaf shapes and a Virginia montane population that uses one host species with a single leaf shape. While Texas and Virginia females exhibited similar chemotactile responses after landing on various host species, butterflies from each population landed more frequently on certain host species used by that particular population. Despite this difference in searching behavior, Texas and Virginia populations were equally capable of learning to search for the leaf shape of a particular host species in artificial enclosure arrays. In addition, learning of leaf-shape preference was confounded equally when each population was introduced into arrays containing all the host species found in both populations. The lack of differentiation in learning of leaf-shape preference between populations that differ in host breadth and host preference argues against the specific hypothesis that learning of leaf-shape preference represents a local adaptation that permits foraging butterflies in the east Texas population to respond to seasonal changes in the relative quality or abundance of host species that differ in leaf shape. Three hypotheses are proposed to explain the apparent absence of interpopulation genetic variation in learning traits. One hypothesis supposes that learning of leaf-shape discrimination is mediated by the same physiological mechanisms that permit females to learn to discriminate among suitable and unsuitable conspecific plants. Selection for restriction of learning of leaf-shape preference in the Virginia montane population may therefore be constrained by selection for learning of other types of discrimination behavior.     

A novel trade-off of insect diapause affecting a sequestered chemical defense

Diapause allows insects to temporally avoid conditions that are unfavorable for development and reproduction. However, diapause may incur a cost in the form of reduced metabolic energy reserves, reduced potential fecundity, and missed reproductive opportunities. This study investigated a hitherto ignored consequence of diapause: trade-offs involving sequestered chemical defense. We examined the aristolochic acid defenses of diapausing and non-diapausing pipevine swallowtail butterflies, Battus philenor. Pipevine swallowtail larvae acquire these chemical defenses from their host plants. Butterflies that emerge following pupal diapause have significantly less fat, a female fitness correlate, compared to those that do not diapause. However, butterflies emerging from diapaused pupae are more chemically defended compared to those that have not undergone diapause. Furthermore, non-diapausing butterflies are confronted with older, lower quality host plants on which to oviposit. Thus, a trade-off exists where butterflies may have greater energy reserves at the cost of less chemical defense and sub-optimal food resources for their larvae, or have substantially less energetic reserves with the benefit of greater chemical defense and plentiful larval food resources.     

Trade-offs between microhabitat selection and physiological plasticity in the Antarctic springtail, <Emphasis Type="Italic">Cryptopygus antarcticus</Emphasis> (Willem)

In the maritime Antarctic, terrestrial arthropods have recourse to two strategies to mitigate low summer temperatures: (1) physiological plasticity and (2) avoidance via microhabitat insulation. This study investigated the interaction between these strategies in the springtail, Cryptopygus antarcticus, established in situ within contrasting microcosms (buffered vs. exposed) and within two sets of habitat simulations (wet vs. dry) over diurnal scales through the Antarctic summer. Significant differences were found in the cold hardiness of springtails sampled simultaneously from each microcosm. Exposed animals showed greater plasticity in the “true” austral summer, but as field temperatures declined preceding the onset of winter, buffered animals showed greater resilience. Overall, water was found to inhibit the buffering effect of moss and there was a significantly greater discrimination between buffered and exposed microcosms in the dry treatment. Analysis of microhabitat temperatures indicate that it is thermal variability not lower temperature that is responsible for the greater plasticity of exposed animals.     

Influence of altitude, habitat and microhabitat on thermal adaptation of cicadas from Southwest Texas (Hemiptera: Cicadidae)

The thermal responses of 12 cicada species inhabiting Big Bend National Park, Texas, USA are investigated to determine the influence of altitude, habitat and microhabitat. The park provides an opportunity to analyze the thermal responses in animals from a variety of habitats and altitudinal gradients within a limited geographic range. The data suggest that thermal responses of cicadas are adaptations to their specific habitats. No thermal responses are significantly correlated with body size. The maximum voluntary tolerance temperature (an upper behavioral thermoregulatory point) and heat torpor temperatures show significant correlations with altitude. Variability in thermal responses can also be related to the characteristics of the microhabitat selected or the behavioral pattern of a species.     

Effect of low temperatures on mortality and oviposition in conjunction with climate mapping to predict spread of the root weevil Diaprepes abbreviatus and introduced natural enemies

The tropical root weevil, Diaprepes abbreviatus (L.), has been a pest of citrus and ornamental plants since its introduction into Lake County, FL, in 1964. Since then, it has colonized the Florida peninsula to the south of its point of introduction but has not expanded its range to the north. A lower threshold for oviposition by D. abbreviatus was estimated as 14.9 degrees C. Eggs were highly susceptible to cold, with 95% mortality (LTime95) occurring in 4.2 d at 12 degrees C. Relative susceptibility of life stages to cold was eggs > pupae > larvae > adults. Archived weather data from Florida were examined to guide a mapping exercise using the lower developmental threshold for larvae (12 degrees C) and the lower threshold for oviposition (15 degrees C) as critical temperatures for mapping the distribution of D. abbreviatus and the potential for establishment of egg parasitoids. Probability maps using the last 10 yr of weather data examined the frequency of at least 10, 15, 20, 25, or 30 d per winter when soil temperature was 相似文献   

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.