Dispersal Morphs Research Articles

Identifying environmental factors affecting the production of pea aphid dispersal morphs in field populations

Abstract Many species of insects show phenotypically plastic dispersal traits in response to suboptimal environmental conditions. These polyphenetic traits can have large impacts on the population dynamics of the dispersing insects. Therefore, it is important to understand what environmental factors affect the development of dispersal traits and how these factors affect population dynamics via immigration/emigration. Pea aphids (Acyrthosiphon pisum) exhibit a polyphenetic wing dimorphism in which individuals are either winged (alate) or not (apterous). The production of alate offspring is determined by environmental cues experienced by the mother. Environmental factors associated with these cues have been intensively studied, with crowding, host plant quality, natural enemies, fungal infection, and viral infection being shown to affect the production of pea aphid alates under controlled laboratory conditions. Nonetheless, environmental factors affecting alate production have rarely been studied in field populations of pea aphids. Using data from a three‐year study of alate production in 5–9 alfalfa (lucerne) fields, we examined the effects of (i) pea aphid abundance, (ii) host plant maturity, (iii) temperature, (iv) predator and herbivore abundance, (v) parasitism, and (vi) entomopathic fungal incidence. In twice‐weekly samples taken over the growing season, alate production ranged from 0% to 83% of the nymphal population. Pea aphid abundance, temperature, and alfalfa maturity together explained 67% of the variation in alate production. The other factors we investigated explained little variation. These results suggest there is a limited number of key environmental factors that consistently predict changes in alate production in field populations, while many factors identified in lab studies may be unimportant. Our results highlight the value of investigating factors affecting the expression of plastic traits in insect species at a broad spatiotemporal scale and under natural conditions.

Physiological demands and nutrient intake modulate a trade-off between dispersal and reproduction based on age and sex of field crickets.

Animals adjust resource acquisition throughout life to meet changing physiological demands of growth, reproduction, activity and somatic maintenance. Wing-polymorphic crickets invest in either dispersal or reproduction during early adulthood, providing a system in which to determine how variation in physiological demands, determined by sex and life history strategy, impact nutritional targets, plus the consequences of nutritionally imbalanced diets across life stages. We hypothesized that high demands of biosynthesis (especially oogenesis in females) drive elevated resource acquisition requirements and confer vulnerability to imbalanced diets. Nutrient targets and allocation into key tissues associated with life history investments were determined for juvenile and adult male and female field crickets (Gryllus lineaticeps) when given a choice between two calorically equivalent but nutritionally imbalanced (protein- or carbohydrate-biased) artificial diets, or when restricted to one imbalanced diet. Flight muscle synthesis drove elevated general caloric requirements for juveniles investing in dispersal, but flight muscle quality was robust to imbalanced diets. Testes synthesis was not costly, and life history investments by males were insensitive to diet composition. In contrast, costs of ovarian synthesis drove elevated caloric and protein requirements for adult females. When constrained to a carbohydrate-biased diet, ovary synthesis was reduced in reproductive morph females, eliminating their advantage in early life fecundity over the dispersal morph. Our findings demonstrate that nutrient acquisition modulates dispersal-reproduction trade-offs in an age- and sex-specific manner. Declines in food quality will thus disproportionately affect specific cohorts, potentially driving demographic shifts and altering patterns of life history evolution.

Effects of the habitat-soil factor on transgenerational plasticity in a diaspore-polymorphic cold desert annual

We determined effects of soil from three habitats differing in physiochemical properties on transgenerational plasticity in diaspore production of the diaspore-polymorphic annual Ceratocarpus arenarius. Production of three disapore morphs and size of F0 plants in a sand desert (S) were evaluated. F1 plants from morphs at soil surface (a) and middle (c) and upper (f) parts of canopy of F0 plants were grown in soil from S, salt desert (SD) and desert steppe (DS) and evaluated for diaspore production and plant size. Number of dispersal unit morphs for F0 plants in S was (f > c)> > a. Plants from all three morphs varied in size, mass and mass allocation. For F1 plants (compared to F0), a:(c + f) was the same or increased in S (poor soil) and decreased in DS (good soil); c:(a + f) increased in S and DS; and f:(a + c) decreased in S and DS. In SD, ratios for F1 plants from a and c were the same as those in S, but ratios for F1 plants from f were the same as those in S and DS. Differences in soil physiochemistry caused flexibility in plant size, mass and mass allocation and proportion of diaspore morphs across generations.

Phenotypic plasticity in diaspore production of a amphi-basicarpic cold desert annual that produces polymorphic diaspores

Phenotypic plasticity has been studied in diaspore-dimorphic species, but no such study has been done on a diaspore-polymorphic species. Our aim was to determine the effects of abiotic and biotic factors on phenotypic plasticity of the diaspore-polymorphic cold desert annual Ceratocarpus arenarius. Plants produced from dispersal units near the soil surface (a, basicarps) and at the middle (c) and upper (f) parts of the plant canopy were subjected to different levels of soil moisture, nutrient supply and intramorph and intermorph densities. Different levels of these biotic and abiotic factors resulted in significant variation in total plant mass, diaspore mass, mass allocation to stem and reproductive organs and total number and proportion of morphs a, c and f on an individual. The effect of stress on number and mass of a dispersal unit morph varied by treatment, with dispersal unit f having the highest CV and dispersal unit a the lowest. The success of this diaspore polymorphic species in its rainfall-unpredictable environment likely is enhanced by plasticity in production of the different types of diaspores.

Insight into the durability of plant resistance to aphids from a demo-genetic study of Aphis gossypii in melon crops.

Resistance breakdown has been observed following the deployment of plant cultivars resistant to pests. Assessing the durability of a resistance requires long‐term experiments at least at a regional scale. We collected such data for melon resistance conferred by the Vat gene cluster to melon aphids. We examined landscape‐level populations of Aphis gossypii collected in 2004–2015, from melon‐producing regions with and without the deployment of Vat resistance and with different climates. We conducted demo‐genetic analyses of the aphid populations on Vat and non‐Vat plants during the cropping seasons. The Vat resistance decreased the density of aphid populations in all areas and changed the genetic structure and composition of these populations. Two bottlenecks were identified in the dynamics of adapted clones, due to the low levels of production of dispersal morphs and winter extinction. Our results suggest that (i) Vat resistance will not be durable in the Lesser Antilles, where no bottleneck affected the dynamics of adapted clones, (ii) Vat resistance will be durable in south‐west France, where both bottlenecks affected the dynamics of adapted clones and (iii) Vat resistance will be less durable in south‐east France, where only one of the two bottlenecks was observed.

Post-release fates of seeds in dehiscent and indehiscent siliques of the diaspore heteromorphic species Diptychocarpus strictus (Brassicaceae)

Abstract Fruit/seed heteromorphic plants have been studied most extensively for differences in dispersal and germination of the morphs. However, fates of the heteromorphic diaspores beginning with seed release and ending with germination/seed death have been documented for relatively few species. Our primary aim was to follow the fates of the two dispersal unit morphs of Diptychocarpus strictus over a several-year period or until they germinated or died. Our hypothesis was that the winged, mucilaginous seeds (dispersal unit) from the dehiscent upper siliques form a short-lived seed bank and the nearly-wingless, nonmucilaginous seeds inside the indehiscent lower siliques (dispersal unit) a long-lived seed bank. Thus, for the two morphs we monitored phenology of seed release in 2010, germination phenology from 2010 to 2014 and germination and survival during burial in soil from 2010 to 2012. All lower siliques had been released from mother plants by late July, whereas seed release of the winged seeds of upper siliques was not completed until late October, after the main stem had become detached (near the base). Most seeds (18%) from upper siliques that did so (c. 20%) germinated in spring 2011, whereas no seeds in lower siliques germinated until autumn 2011 (0.2%). All seeds from upper siliques had germinated or lost viability by the end of the autumn 2012 germination season, whereas 11% of seeds inside lower siliques germinated in autumn 2014 and a high proportion of the nongerminated seeds was still viable, thus supporting our hypothesis. Buried seeds of both morphs exhibited a 6-mo dormancy cycle in which the nondormant phase coincided with the two germination seasons. Dehiscence of the upper siliques and indehiscence of the lower siliques are required for D. strictus to have a high risk-low risk strategy (H/H-L/L) for diaspore dispersal and dormancy.

Ideal free distribution of fixed dispersal phenotypes in a wing dimorphic beetle in heterogeneous landscapes

According to the ideal free distribution (IFD) theory, individuals that are able to perceive the quality of different patches in a landscape and disperse freely are expected to redistribute themselves proportionally to the carrying capacities of heterogeneous patches. Here, we argue that, when dispersal is unconditional and genetically fixed, a coalition of sedentary and dispersing phenotypes can attain an IFD under spatiotemporally uncorrelated variation in fitness. This not only leads to a stable polymorphism of both dispersal phenotypes, but also implies that the number of dispersing individuals should on average be equal among patches and determined by the carrying capacity of the smallest local populations in the landscape. Differences in carrying capacity among patches are thus only reflected by changes in the number of sedentary individuals. Individual-based simulations show that this mechanism can be generalized over a wide range of spatiotemporal conditions and dispersal strategies. Moreover, these expectations are in strong agreement with empirical data on the density of both dispersal phenotypes of the wing dimorphic ground beetle Pterostichus vernalis within and among 10 different landscapes. Hence, for the first time, these results demonstrate that this mechanism serves as a plausible alternative to the competition-colonization model to explain the spatial distribution of fixed dispersal phenotypes in heterogeneous landscapes. Understanding of the frequency distributions of individuals expressing discrete dispersal morphs moreover improves our predictive and management capabilities for a broad range of species, for which we currently typically rely on using mean dispersal rates.

Flexible dispersal dimorphism in zooplankton resting eggs: an example of repeated phenotypic coin flipping?

Theory predicts that spatio-temporal variation in habitat suitability will promote selection for dispersal. In addition to movement in space, dispersal gains an extra dimension in freshwater zooplankton because resting eggs can disperse in time as well, via dormancy. Potential trade-offs between both strategies, however, remain largely unexplored. Using a temporary pool fairy shrimp population as a model, we tested for consistent differences in buoyancy among resting eggs during consecutive inundations in a standardized laboratory experiment and explored a potential trade-off between dispersal (floaters versus sinkers) and dormancy (high versus low hatching fractions). Although discrete dispersal morphs were present, this trait was not fixed. Irrespective of their dispersal phenotype during previous inundations, floating eggs hatched more frequently than sinking eggs. Egg morphology did not affect buoyancy and, between inundations, approximately half of the eggs changed their dispersal phenotype. Although this mechanism has affinities with conservative bet hedging and adaptive coin flipping, it is unique because the dispersal phenotype can switch at the onset of each inundation. Despite possible selection against dispersal at the population level, such a strategy ensures variation in dispersal ability at any moment and could promote population persistence in a metapopulation context. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 749–756.

Multiple Cues for Winged Morph Production in an Aphid Metacommunity

Environmental factors can lead individuals down different developmental pathways giving rise to distinct phenotypes (phenotypic plasticity). The production of winged or unwinged morphs in aphids is an example of two alternative developmental pathways. Dispersal is paramount in aphids that often have a metapopulation structure, where local subpopulations frequently go extinct, such as the specialized aphids on tansy (Tanacetum vulgare). We conducted various experiments to further understand the cues involved in the production of winged dispersal morphs by the two dominant species of the tansy aphid metacommunity, Metopeurum fuscoviride and Macrosiphoniella tanacetaria. We found that the ant-tended M. fuscoviride produced winged individuals predominantly at the beginning of the season while the untended M. tanacetaria produced winged individuals throughout the season. Winged mothers of both species produced winged offspring, although in both species winged offspring were mainly produced by unwinged females. Crowding and the presence of predators, effects already known to influence wing production in other aphid species, increased the percentage of winged offspring in M. tanacetaria, but not in M. fuscoviride. We find there are also other factors (i.e. temporal effects) inducing the production of winged offspring for natural aphid populations. Our results show that the responses of each aphid species are due to multiple wing induction cues.

Polyphagous predatory rove beetles (Coleoptera: Staphylinidae) induce winged morphs in the pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae)

A number of aphid species have been shown to produce winged dispersal morphs in the presence of natural enemies. Pre- vious studies tested specialized aphid predators such as ladybirds or lacewing larvae. We confronted colonies of pea aphids with the polyphagous rove beetles, Drusilla canaliculata and Tachyporus hypnorum. For both predators we found that the percentage of winged morphs increased in predator-attacked pea aphid colonies compared to a control. The behaviour of the two rove beetles spe- cies was noticeably different. D. canaliculata mostly foraged on the ground and rarely on the plant, while T. hypnorum was almost exclusively observed on the plants, causing a higher number of aphids to drop to the ground, which resulted in a stronger increase in winged morph production. Our results clearly show that not only monophagous aphid predators but also more polyphagous insect predators, which include aphids in their diet, can induce aphids to produce winged morphs.

Pheromones: Fish Fear Factor

Fish, like many other animals, panic when another individual is injured. Now, the chemical nature of a substance that mediates this reaction has been uncovered.

Polyphenism in Insects

(Current Biology 21, R738–R749; September 27, 2011) The authors wish to make an important clarification regarding the above review article on insect polyphenism. In the text we used a passage from Ernst Mayr's classic 1963 book [1Mayr E. Animal Species and Evolution. Belknap Press of Harvard University Press, Cambridge, MA1963Crossref Google Scholar] to help provide a historical context, noting that the study of phenotypic plasticity had fallen out of respectability and that with this statement Mayr helped return the subject to respectability. It has since come to our attention that our use of this passage could also be interpreted as a claim that Mayr coined the term “polyphenism.” This was certainly not our intention. As a point of fact, it was Charles Michener [2Michener C.D. Social polymorphism in Hymenoptera.Symp. R. Entomol. Soc. Lond. 1961; 1: 43-56Google Scholar], who first suggested the term “polyphenic” 2 years before Mayr's publication and should be credited with its origin. We greatly thank our colleague Meir Paul Pener for his keen attention to this matter. Polyphenism in InsectsSimpson et al.Current BiologySeptember 27, 2011In BriefPolyphenism is the phenomenon where two or more distinct phenotypes are produced by the same genotype. Examples of polyphenism provide some of the most compelling systems for the study of epigenetics. Polyphenisms are a major reason for the success of the insects, allowing them to partition life history stages (with larvae dedicated to feeding and growth, and adults dedicated to reproduction and dispersal), to adopt different phenotypes that best suit predictable environmental changes (seasonal morphs), to cope with temporally heterogeneous environments (dispersal morphs), and to partition labour within social groups (the castes of eusocial insects). Full-Text PDF Open Archive

Polyphenism in Insects

Polyphenism is the phenomenon where two or more distinct phenotypes are produced by the same genotype. Examples of polyphenism provide some of the most compelling systems for the study of epigenetics. Polyphenisms are a major reason for the success of the insects, allowing them to partition life history stages (with larvae dedicated to feeding and growth, and adults dedicated to reproduction and dispersal), to adopt different phenotypes that best suit predictable environmental changes (seasonal morphs), to cope with temporally heterogeneous environments (dispersal morphs), and to partition labour within social groups (the castes of eusocial insects). We survey the status of research onsome of the best known examples of insect polyphenism, in each case considering the environmental cues that trigger shifts in phenotype, the neurochemical and hormonal pathways that mediate the transformation, the molecular genetic and epigenetic mechanisms involved in initiating and maintaining the polyphenism, and the adaptive and life-history significance of the phenomenon. We conclude by highlighting some of the common featuresof these examples and consider future avenues for research on polyphenism.

The role of microgynes in the reproductive strategy of the neotropical ant Ectatomma ruidum

Miniaturized queens, microgynes, are regarded as an alternative reproductive strategy sparsely present through the ant world. The described roles of miniaturized queens include alternative short-distance dispersal morphs, an adaptation to polygyny and inquiline parasites. Some of these inquiline parasite microgynes have been described as a separate species from their host. In the poneromorph group, miniaturized queens are only reported in two Mexican populations of two Ectatomminae: Ectatomma tuberculatum, in which small queens represent an inquiline species (Ectatomma parasiticum) and Ectatomma ruidum. E. ruidum presents apparently facultative polygyny with microgynes. We used mitochondrial DNA markers and newly developed microsatellite loci to investigate the status as well as the role of microgynes in E. ruidum. We confirmed that microgynes and macrogynes are from the same species. This species is almost exclusively monogynous and monoandrous, supernumerary dealate queens of both types being actually daughters of the mother queen. An apparently polygynous nest was more often headed by a macrogyne than a microgyne. We didn't find any inbreeding or isolation by distance in the studied population, indicating that new gynes are inseminated by unrelated males and can establish a new nest far from their natal nest. However, re-adoption of daughter queens seems to be the rule and rate of microgyny appears to be linked to nest density and environmental factors.

Seasonal Group Behavior of AdultJadera haematoloma(Hemiptera: Rhopalidae) in Central Florida Following Host Seed Exhaustion

Jadera haematoloma Herrich-Sch?ffer (Hemiptera: Rhopalidae) is a specialist seed predator on Sapindales (Sapindaceae) and is common throughout North and Central America (Carroll & Loye 1987). In North-Central Florida J. haematoloma occurs on the recently-acquired host, the Goldenrain tree (Koelreuteria sp. Lax mann). Koelreuteria trees produce tens of thou sands of seeds in the canopy on floral rods after flowering in early fall. By mid-fall, seeds fall to the ground and are fed upon by J. haematoloma until the seed base is exhausted in early sum mer (Carroll & Loye 1987; Carroll 1988). Though dispersal morphs are common in other host populations of J. haematoloma, they are not favored in Koelreuteria populations due to the synchronous exploitation of Koelreuteria sp. seeds among host plants. Instead, non-repro ductive diapause in adults follows seed exhaus tion (Carroll et al. 2003). During this period from May to Sep 2009,1 observed adults forming dense, physically-touching groups in the canopy following the exhaustion of seeds. My goal was to determine the demographic composition of these groups and identify where they occur in time as well as their position within and among host trees. I observed individuals foraging on seeds on the ground around host trees in large aggrega tions through late May and early Jun in the northwest Gainesville area (Alachua County, FL) in the manner described by previous investi gators (Carroll & Loye 1987). Following seed ex haustion and a noticeable decline in the density of individuals on the ground, I completed an ex tensive survey of the canopy and surrounding ground area of 12 large Koelreuteria trees using binoculars and a spotting scope to view higher canopies. Trees were classified according to their seed productivity, branching, height, and trunk diameter to determine if these influenced group number or insect abundance. Seed productivity was estimated by counting the number of bare floral rods in the canopy as a proxy for the num ber of seed-yielding flowers produced the previ ous fall. Samples of seeds were collected from the ground at each tree to verify seed exhaustion (af ter Carroll et al. 2003). I defined a group of bugs as 10 or more stationary individuals spaced less than 2 body lengths apart from one another (Fig. 1) in hopes of avoiding confusion with the ground foraging aggregations around host plants pre viously described in Jadera literature (Carroll & Loye 1987). To determine whether groups were randomly distributed around host trees, I re corded the branch position, compass position rel ative to trunk, height, radius from the trunk, and number of participating insects for each ob served group; a picture was taken to verify counts and each group was assigned a number. One quarter of the number of groups was ran domly selected based on a random numbers ta ble and then destructively sampled using a bag net at each tree. Sex ratios within groups were estimated for each tree from the collected group(s). Statistical analyses were conducted with JMP (SAS institute 2008). In total, 46 groups were observed. Fewer than 20 individuals were observed on the trunk or ground of any given tree during sampling. Trees contained between 0 and 17 groups within the canopy (mean = 4.6). The proportion of terminal branches with florescence rods (a proxy for seed productivity) reliably predicted the total number of individuals found on a tree (ANOVA, df= 45, F = 37.29, = 0.0005) and the number of groups within a tree (ANOVA, df = 8, F = 7.14, = 0.0319). Three trees without insects were ex cluded from the following analyses of group loca tion and participation. Group positions were bi ased towards more terminal branches as de scribed by Strahler and Botanical branch order ing systems (Strahler 1957; Wilson 1966, Fig 2). Number of branch meet-ups from most terminal branches (Strahler) was biased towards terminal branches, but position was not biased in the num ber of branch meet-ups from the trunk (Botani cal). This is contrary to the expectation that groups are more likely to occur in positions on the tree which are more abundant, which would bias the number of groups both towards most terminal branches, and branches furthest from the trunk, as more positions are available at the exterior of the crown. These data show that bugs are distrib uted in terminal sites regardless of where termi nal sites occur within the crown. The distribution

Aphid Wing Induction and Ecological Costs of Alarm Pheromone Emission under Field Conditions

The pea aphid, Acyrthosiphon pisum Harris, (Homoptera: Aphididae) releases the volatile sesquiterpene (E)-β-farnesene (EBF) when attacked by a predator, triggering escape responses in the aphid colony. Recently, it was shown that this alarm pheromone also mediates the production of the winged dispersal morph under laboratory conditions. The present work tested the wing-inducing effect of EBF under field conditions. Aphid colonies were exposed to two treatments (control and EBF) and tested in two different environmental conditions (field and laboratory). As in previous experiments aphids produced higher proportion of winged morphs among their offspring when exposed to EBF in the laboratory but even under field conditions the proportion of winged offspring was higher after EBF application (6.84±0.98%) compared to the hexane control (1.54±0.25%). In the field, the proportion of adult aphids found on the plant at the end of the experiment was lower in the EBF treatment (58.1±5.5%) than in the control (66.9±4.6%), in contrast to the climate chamber test where the numbers of adult aphids found on the plant at the end of the experiment were, in both treatments, similar to the numbers put on the plant initially. Our results show that the role of EBF in aphid wing induction is also apparent under field conditions and they may indicate a potential cost of EBF emission. They also emphasize the importance of investigating the ecological role of induced defences under field conditions.

Mutualist‐induced transgenerational polyphenisms in cotton aphid populations

Summary 1 In defensive mutualistic associations, reduced risk of predation should permit defended organisms to produce phenotypes with higher offspring production than non-mutualistic, unprotected conspecifics which require costly defensive traits. 2 Here, we show that cotton aphids, Aphis gossypii, which produce any combination of dwarf apterae (low intrinsic rate of increase), light green apterae (medium intrinsic rate of increase), dark green apterae (high intrinsic rate of increase) and alatae (winged dispersal morphs), alter offspring phenotypes when tended by predatory ants. 3 Aphids tended by the Argentine ant, Linepithema humile, have similar numbers of dwarf, dark green and alate offspring, but greater numbers of light green offspring, compared to untended colonies. 4 Because light green morphs have a higher intrinsic rate of increase than dwarf morphs but a decreased risk of parasitism compared to dark green morphs, increased production of the light green phenotype may optimize offspring production in order to maximize clone fitness. 5 Since many organisms have high levels of plasticity and mutualistic interactions are ubiquitous, mutualist-induced polyphenisms may be pervasive.

Evolution of dispersal and life history strategies--Tetrahymena ciliates.

BackgroundConsiderable attention has focused on how selection on dispersal and other core life-history strategies (reproductive effort, survival ability, colonization capacity) may lead to so-called dispersal syndromes. Studies on genetic variation in these syndromes within species could importantly increase our understanding of their evolution, by revealing whether traits co-vary across genetic lineages in the manner predicted by theoretical models, and by stimulating further hypotheses for experimental testing. Yet such studies remain scarce. Here we studied the ciliated protist Tetrahymena thermophila, a particularly interesting organism due to cells being able to transform into morphs differing dramatically in swim-speed. We investigated dispersal, morphological responses, reproductive performance, and survival in ten different clonal strains. Then, we examined whether life history traits co-varied in the manner classically predicted for ruderal species, examined the investment of different strains into short- and putative long-distance dispersal, while considering also the likely impact of semi-sociality (cell aggregation, secretion of 'growth factors') on dispersal strategies.ResultsVery significant among-strain differences were found with regard to dispersal rate, morphological commitment and plasticity, and almost all core life-history traits (e.g. survival, growth performance and strategy), with most of these traits being significantly intercorrelated. Some strains showed high short-distance dispersal rates, high colonization capacity, bigger cell size, elevated growth performance, and good survival abilities. These well performing strains, however, produced fewer fast-swimming dispersal morphs when subjected to environmental degradation than did philopatric strains performing poorly under normal conditions.ConclusionStrong evidence was found for a genetic covariation between dispersal strategies and core life history traits in T. thermophila, with a fair fit of observed trait associations with classic colonizer models. However, the well performing strains with high colonization success and short-distance dispersal likely suffered under a long-distance dispersal disadvantage, due to producing fewer fast-swimming dispersal morphs than did philopatric strains. The smaller cell size at carrying capacity of the latter strains and their poor capacity to colonize as individual cells suggest that they may be adapted to greater levels of dependency on clone-mates (stronger sociality). In summary, differential exposure to selection on competitive and cooperative abilities, in conjunction with selective factors targeting specifically dispersal distance, likely contributed importantly to shaping T. thermophila dispersal and life history evolution.

Alarm pheromone mediates production of winged dispersal morphs in aphids

AbstractThe aphid alarm pheromone (E)‐β‐farnesene (EBF) is the major example of defence communication in the insect world. Released when aphids are attacked by predators such as ladybirds or lacewing larvae, aphid alarm pheromone causes behavioural reactions such as walking or dropping off the host plant. In this paper, we show that the exposure to alarm pheromone also induces aphids to give birth to winged dispersal morphs that leave their host plants. We first demonstrate that the alarm pheromone is the only volatile compound emitted from aphid colonies under predator attack and that emission is proportional to predator activity. We then show that artificial alarm pheromone induces groups of aphids but not single individuals to produce a higher proportion of winged morphs among their offspring. Furthermore, aphids react more strongly to the frequency of pheromone release than the amount of pheromone delivered. We suggest that EBF leads to a ‘pseudo crowding’ effect whereby alarm pheromone perception causes increased walking behaviour in aphids resulting in an increase in the number of physical contacts between individuals, similar to what happens when aphids are crowded. As many plants also produce EBF, our finding suggests that aphids could be manipulated by plants into leaving their hosts, but they also show that the context‐dependence of EBF‐induced wing formation may hinder such an exploitation of intraspecific signalling by plants.

The interplay between density- and trait-mediated effects in predator-prey interactions: a case study in aphid wing polymorphism.

Natural enemies not only influence prey density but they can also cause the modification of traits in their victims. While such non-lethal effects can be very important for the dynamic and structure of prey populations, little is known about their interaction with the density-mediated effects of natural enemies. We investigated the relationship between predation rate, prey density and trait modification in two aphid-aphid predator interactions. Pea aphids (Acyrthosiphon pisum, Harris) have been shown to produce winged dispersal morphs in response to the presence of ladybirds or parasitoid natural enemies. This trait modification influences the ability of aphids to disperse and to colonise new habitats, and hence has a bearing on the population dynamics of the prey. In two experiments we examined wing induction in pea aphids as a function of the rate of predation when hoverfly larvae (Episyrphus balteatus) and lacewing larvae (Chrysoperla carnea) were allowed to forage in pea aphid colonies. Both hoverfly and lacewing larvae caused a significant increase in the percentage of winged morphs among offspring compared to control treatments, emphasising that wing induction in the presence of natural enemies is a general response in pea aphids. The percentage of winged offspring was, however, dependent on the rate of predation, with a small effect of predation on aphid wing induction at very high and very low predation rates, and a strong response of aphids at medium predation rates. Aphid wing induction was influenced by the interplay between predation rate and the resultant prey density. Our results suggests that density-mediated and trait-mediated effects of natural enemies are closely connected to each other and jointly determine the effect of natural enemies on prey population dynamics.