Pea Aphid Populations Research Articles

ФИТОСАНИТАРНОЕ СОСТОЯНИЕ АГРОФИТОЦЕНОЗОВ ГОРОХА ПОСЕВНОГО ПРИ РАЗЛИЧНЫХ СИСТЕМАХ ОСНОВНОЙ ОБРАБОТКИ ПОЧВЫ

Research was carried out to determine the influence of various systems of basic soil cultivation on the prevalence and development of pests in the agrophytocenosis of peas. The work was carried out in 2017-2019 in the Republic of Mordovia in the second rotation of the crop rotation: peas - winter wheat - soybeans - spring barley. The experimental scheme included the following options: autumn disking at 10…12 cm + moldboard plowing at 22…24 cm (disking + plowing); autumn disking with a disc harrow at 10…12 cm (disking); zero tillage (direct sowing). The use of disking and direct sowing increased the infestation of crops with young weeds, compared with plowing, by 12 and 24 weeds/m2. Plowing led to a decrease in the amount of thistle and horsetail. The prevalence of rust and gray rot in peas was largely influenced by weather conditions favorable to pathogens. In 2017 the prevalence of rust in the flowering phase was higher than in the drier years of 2018-2019 by 44...53%, gray mold - by 17...23%. The intensity of pathogen development increased as the crop matured; in the experiment, the lowest value of this indicator was against the background of disking. Plowing significantly reduced the density of the pea aphid population compared to disking (by 79%) and direct sowing (by 11%). A similar pattern was noted for gamma moth caterpillars. The results obtained can be used in the development of an integrated system for protecting peas from a complex of pests for the southern forest-steppe of the Non-Chernozem zone of the Russian Federation.

Development of economic thresholds for pea aphid (Hemiptera: Aphididae) management in lentil (Fabaceae) based on in-field insecticide efficacy trials.

Pea aphid (Acyrthosiphom pisum Harris, Hemiptera: Aphididae) presents a significant economic challenge to lentil (Lens culinaris Medik.) production in the major growing region of Saskatchewan, Canada. During 2019-2020, field experiments were conducted to optimize the management tools for pea aphid control on lentils. A randomized split-plot design was used with main plots consisting of different pea aphid pressures and subplots consisting of different insecticide treatments. The main plot design was aimed to assess the impact of A. pisum feeding on lentil yields during the late vegetative to early reproductive stages. Subplots of the study evaluated the efficacy of 3 insecticides in suppressing pea aphid populations on lentils. Lentil is susceptible to A. pisum feeding and requires management at low pest densities. The economic threshold for pea aphids on lentil crops varied depending on environmental conditions, ranging from 20 to 66 aphids per sweep, calculated using a discrete daily growth rate of 1.116. The estimated economic thresholds provided a 7-day lead time before aphid populations achieved the economic injury level (EIL). The EIL was defined as 78 ± 14 aphids per sweep net sample or 743 ± 137 cumulative aphid days from the first aphid present in the field. In addition, the results of the study found that, on average, foliar applications of insecticides containing the pyrethroid active ingredient lambda-cyhalothrin (IRAC group: 3A) reduced pea aphid populations by 83% compared with untreated control.

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.

Phenotypic specialization of the pea aphid in its southern limit of distribution

The success of biological invasions ultimately relies on phenotypic traits of the invasive species. Aphids, which include many important pests worldwide, may have been successful invading new environments partly because they can maximize reproductive output by becoming parthenogenetic and losing the sexual phase of their reproductive cycle. However, invasive populations of aphids invading wide ranges can face contrasting environmental conditions and requiring different phenotypic strategies. Besides transitions in their reproductive cycle, it is only partially known which phenotypic traits might be associated to the invasion success of aphid populations in extended novel ranges. Here, we used four genotypes of the pea aphid Acyrthosiphon pisum from two localities in Chile to test for phenotypic specialization that might explain their establishment and spread in habitats exhibiting contrasting environmental conditions. We show that lineages living at a higher latitude with low temperatures show, in addition to facultative sexual reproduction, smaller body sizes, lower metabolic rates and a higher tolerance to the cold than the obligate asexual lineages living in a mild weather, at the expense of fecundity. Conversely, at higher temperatures only asexual lineages were found, which exhibit larger body sizes, higher reproductive outputs and consequently enhanced demographic ability. As a result, in conjunction with the reproductive mode, lineage specialization in physiological and life-history traits could be taken into account as an important strategy for populations of pea aphid to effectively invade extended novel ranges comprising different climatic conditions.

Diversity of endosymbiotic bacteria in the pea aphid (Acyrthosiphon pisum) on alfalfa in Chile

Facultative bacterial endosymbionts are prevalent in aphid pest species and key organisms for the rapid adaptation to changing environment conditions under agricultural management. Facultative endosymbiont interactions allow beneficial phenotypes of insect hosts, providing resistance against natural enemies, fungal infections as well as enhanced resistance to thermal conditions, among others. These interactions have been highlighted as important modulators in the evolution of aphid defenses, being a key aspect for the invasive potential of aphids as pests. In this study, we have examined the bacterial community diversity associated to the pea aphid Acyrthosiphon pisum on alfalfa (Medicago sativa L.) crops in Central Chile by determining the presence and abundance of the seven most common facultative endosymbionts reported for aphid species, including: Hamiltonella defensa, Regiella insecticola, Serratia symbiotica, Spiroplasma sp., Rickettsia sp., Rickettsiella viridis and Fukatsuia symbiotica (or pea aphid X-type symbiont’ PAXS), using a PCR-based approach across species-specific primers, in 16 alfalfa fields during the Chilean spring in the Maule Region. We reported six of the seven endosymbiont species studied, observing a high frequency of infected aphids being the endosymbiont H. defensa, the most predominant among the aphid populations in the different alfalfa fields studied. As H. defensa has been extensively studied in its defensive role to parasitoids, we discuss implications for biological control of this aphid pests. We also discuss the absence of a relevant defensive endosymbiont such as F. symbiotica in the Chilean pea aphid populations and potential implications for the success of the pea aphid biological control program.

Light at night disrupts trophic interactions and population growth of lady beetles and pea aphids.

Natural variation in light has historically correlated with seasonality, providing an honest cue to organisms with seasonal life history cycles. However, with the onset of widespread light at night (LAN), the reliability of light as a cue has decreased in polluted areas, making its timing or intensity potentially clash with temperature trends. These clashing cues may influence biological systems on multiple levels. Yet, a few studies have connected behavioral underpinnings and larger community-level processes, resulting in a knowledge gap bridging individual-, population-, and community-level responses to mismatched cues. We experimentally investigated impacts of cool temperature and LAN on a lady beetle-aphid-fava system to test how light and temperature influenced aphid population growth and their underlying behavioral drivers. We used Coccinella septempunctata and Coleomegilla maculata beetles to understand the interaction of the environment and predation on pea aphid (Acyrthosiphon pisum) population growth. Aphids and their predators reacted differently to variation in light and temperature, influencing the strength of aphid-driven and predator-driven dynamics in the different conditions. We observed evidence of aphid-driven dynamics in the cool, light conditions where aphids excel and exhibited strong anti-predator behavior. In contrast, we found stronger predator-driven dynamics in warm conditions where lady beetle predatory success was higher. Overall, we found that LAN has context-dependent effects on insect communities due to the varied responses each player has to its environment.

Population Dynamics and Yield Loss Assessment for Pea Aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), on Lentil in Morocco.

Simple SummaryIn Morocco, lentil is being grown under rainfed conditions and plays an important role in cereal-based cropping systems. Pea aphid (Acyrthosiphon pisum Harris) is one of the most important constraints limiting the yield of lentil in several countries; however, the extent of yield loss it causes in Morocco is unknown. Pea aphid weakens the plant directly by sucking its sap, and can also spread viruses from infected plants to healthy ones. Currently, there are no effective tools available for A. pisum control, with most farmers being reliant on chemical insecticides. The aim of this study is to investigate the population fluctuation of pea aphid over different seasons and their effects on yield loss in Morocco. Correlation analysis was performed to find out the extent of influence of weather parameters on the population dynamics of an aphid population over different seasons. The results demonstrated that the avoidable losses due to aphid infestation were in the range of 4.56–12.51%. The pea aphid populations increased rapidly between March and April when climate factors and the plants became more suitable for aphid development. The maximum temperature, relative humidity, and wind speed influenced pea aphid infestation on different lentil varieties.Pea aphid (Acyrthosiphon pisum Harris) is the major insect pest of lentil in Morocco. We investigated pea aphid mean numbers and yield losses on three lentil varieties at one location during three successive cropping seasons during 2015–2018. The effects of several weather factors on pea aphid population dynamics were investigated. Population density increased in early spring followed by several peaks during March–April and then steeply declined during the late spring. Aphid populations peaked at different times during the three years of the study. In 2016, higher populations occurred during the second and third weeks of April for Abda and Zaria varieties with averages of 27 and 28 aphids/20 twigs, respectively. In 2017, higher populations occurred on the 12th and 13th standard meteorological weeks (SMWs) for Zaria with averages of 24.7 and 27.03 aphids/20 twigs, respectively. In 2018, the population peaked for all varieties at three different times, 11th, 13th, and 17th SMW, with the highest for Zaria being 26.00, 47.41, and 32.33 aphids/20 twigs. Pea aphid population dynamics changed with weather conditions. The number of aphids significantly and positively correlated with maximum temperature, but significantly negatively correlated with relative humidity and wind speed. The minimum temperature and rainfall had non-significant correlations. Pea aphid infestation resulted in losses of total seed weight for all lentil varieties, with the highest avoidable losses for Bakria being 12.51% followed by Zaria with 7.72% and Abda with 4.56%. These losses may justify the development of integrated management options for control of this pest.

Diversity and geographic variation of endosymbiotic bacteria in natural populations of the pea aphid (Acyrthosiphon pisum) in China

• We firstly characterized symbiont communities and diversity of the pea aphid in China. • The obligate symbiont, Buchnera , was present in all aphid samples. • Four facultative symbionts ( Serratia , Hamiltonella , Regiella , and Rickettsia ) showed a significant geographic variation. • Bacterial communities of the Chinese pea aphid populations were mainly influenced by environmental factors. Bacterial symbionts in aphids are known to benefit the insect host and associated with aphid’s ecological adaptation. The pea aphid ( Acyrthosiphon pisum ), an important legume pest worldwide, carries at least eight endosymbionts, providing a model system to study insect–bacteria interactions. However, species diversity and geographic variations of endosymbionts are unknown in Chinese populations; therefore, we characterized symbiont communities and diversity of 27 pea aphid samples from 13 geographic populations of China. Via amplicon high-throughput sequencing and diagnostic PCR, we found that bacterial communities of Chinese populations were dominated by Proteobacteria and Firmicutes. Among eight known endosymbionts, five ( Buchnera , Serratia , Hamiltonella , Regiella , and Rickettsia ) were detected by both methods, with a specific geographical distribution. The obligate symbiont, Buchnera , was present in all aphid samples, while the four facultative symbionts showed a significant geographic variation. Each population was randomly infected with distinct endosymbionts, ranging from three to five species. Serratia and Rickettsia showed relatively higher abundance in central regions of China, Regiella was predominant in eastern and western China, whereas Hamiltonella showed an extremely low abundance and was absent in four populations. Samples grouped by altitudes showed a significant diversity difference, whereas there was no significant difference between red and green body colors. Bacterial community structures of the Chinese pea aphid populations were mainly influenced by environmental factors, other than body colors. These data can guide the development of potential biocontrol techniques against this aphid.

Does getting defensive get you anywhere?-Seasonal balancing selection, temperature, and parasitoids shape real-world, protective endosymbiont dynamics in the pea aphid.

Facultative, heritable endosymbionts are found at intermediate prevalence within most insect species, playing frequent roles in their hosts' defence against environmental pressures. Focusing on Hamiltonella defensa, a common bacterial endosymbiont of aphids, we tested the hypothesis that such pressures impose seasonal balancing selection, shaping a widespread infection polymorphism. In our studied pea aphid (Acyrthosiphon pisum) population, Hamiltonella frequencies ranged from 23.2% to 68.1% across a six-month longitudinal survey. Rapid spikes and declines were often consistent across fields, and we estimated that selection coefficients for Hamiltonella-infected aphids changed sign within this field season. Prior laboratory research suggested antiparasitoid defence as the major Hamiltonella benefit, and costs under parasitoid absence. While a prior field study suggested these forces can sometimes act as counter-weights in a regime of seasonal balancing selection, our present survey showed no significant relationship between parasitoid wasps and Hamiltonella prevalence. Field cage experiments provided some explanation: parasitoids drove modest ~10% boosts to Hamiltonella frequencies that would be hard to detect under less controlled conditions. They also showed that Hamiltonella was not always costly under parasitoid exclusion, contradicting another prediction. Instead, our longitudinal survey - and two overwintering studies - showed temperature to be the strongest predictor of Hamiltonella prevalence. Matching some prior lab discoveries, this suggested that thermally sensitive costs and benefits, unrelated to parasitism, can shape Hamiltonella dynamics. These results add to a growing body of evidence for rapid, seasonal adaptation in multivoltine organisms, suggesting that such adaptation can be mediated through the diverse impacts of heritable bacterial endosymbionts.

Wing plasticity and associated gene expression varies across the pea aphid biotype complex

Developmental phenotypic plasticity is a widespread phenomenon that allows organisms to produce different adult phenotypes in response to different environments. Investigating the molecular mechanisms underlying plasticity has the potential to reveal the precise changes that lead to the evolution of plasticity as a phenotype. Here, we study wing plasticity in multiple host-plant adapted populations of pea aphids as a model for understanding adaptation to different environments within a single species. We describe the wing plasticity response of different "biotypes" to a crowded environment and find differences within as well as among biotypes. We then use transcriptome profiling to compare a highly plastic pea aphid genotype to one that shows no plasticity and find that the latter exhibits no gene expression differences between environments. We conclude that the loss of plasticity has been accompanied by a loss of differential gene expression and therefore that genetic assimilation has occurred. Our gene expression results generalize previous studies that have shown a correlation between plasticity in morphology and gene expression.

Non-consumptive effects stabilize herbivore control over multiple generations.

Understanding the factors that influence predator-prey dynamics requires an investigation of oscillations in predator and prey population sizes over time. However, empirical studies are often performed over one or fewer predator generations. This is particularly true for studies addressing the non-consumptive effects of predators on prey. In a previous study that lasted less than one predator generation, we demonstrated that two species of parasitoid wasps additively suppressed aphid populations through a combination of consumptive and non-consumptive effects. However, the non-consumptive effects of one wasp reduced the reproductive success of the other, suggesting that a longer-term experiment may have revealed antagonism between the wasps. The goal of our current study is to evaluate multi-generation consumptive and non-consumptive interactions between pea aphids (Acyrthosiphon pisum) and the wasps Aphidius ervi and Aphidius colemani. Aphidius ervi is a common natural enemy of pea aphids. Aphidius colemani is a non-consumptive enemy that does not consume pea aphids, but negatively affects pea aphid performance through behavioral disturbance. Large field cages were installed to monitor aphid abundance in response to the presence and absence of both species of wasp over four weeks (two parasitoid generations). We found that the non-consumptive enemy A. colemani initially controlled the pea aphid population, but control in the absence of parasitism was not sustainable over the long term. Aphidius ervi suppressed pea aphids through a combination of consumptive and non-consumptive effects. This suppression was more effective than that of A. colemani, but aphid abundance fluctuated over time. Suppression by A. ervi and A. colemani together was complementary, leading to the most effective and stable control of pea aphids. Therefore, promoting a diverse natural enemy community that contributes to pest control through consumptive and non-consumptive interactions may enhance the stability of herbivore population suppression over time.

Comparison of native and non-native predator consumption rates and prey avoidance behavior in North America and Europe.

Novel predator–prey interactions can contribute to the invasion success of non‐native predators. For example, native prey can fail to recognize and avoid non‐native predators due to a lack of co‐evolutionary history and cue dissimilarity with native predators. This might result in a competitive advantage for non‐native predators. Numerous lady beetle species were globally redistributed as biological control agents against aphids, resulting in novel predator–prey interactions. Here, we investigated the strength of avoidance behavior of the pea aphid (Acyrthosiphon pisum) toward chemical cues of native lady beetles and non‐native Asian Harmonia axyridis and European Coccinella septempunctata and Hippodamia variegata in North America, hypothesizing that cues of non‐native lady beetles induce weaker avoidance behavior than cues of co‐evolved native lady beetles. Additionally, we compared aphid consumption of lady beetles, examining potential predation advantages of non‐native lady beetles. Finally, we compared cue avoidance behavior between North American and European pea aphid populations and aphid consumption of native and non‐native lady beetles in North America and Europe. In North America, pea aphids avoided chemical cues of all ladybeetle species tested, regardless of their origin. In contrast to pea aphids in North America, European pea aphids did not avoid cues of the non‐native H. axyridis. The non‐native H. axyridis and C. septempunctata were among the largest and most voracious lady beetle species tested, on both continents. Consequently, in North America non‐native lady beetle species might have a competitive advantage on shared food resources due to their relatively large body size, compared to several native American lady beetle species. In Europe, however, non‐native H. axyridis might benefit from missing aphid cue avoidance as well as a large body size. The co‐evolutionary time gap between the European and North American invasion of H. axyridis likely explains the intercontinental differences in cue avoidance behavior and might indicate evolution in aphids toward non‐native predators.

Increased nitrogen fertilization inhibits the biocontrol activity promoted by the intercropping partner plant.

The examination of the compatibility between agricultural practices and biocontrol activities is crucial for establishing an efficient, eco-friendly, and sustainable pest management program. In this study, we examined the population dynamics of two specialist aphids, the English grain aphid (Sitobion avenae) on potted wheat and the pea aphid (Acyrthosiphon pisum) on potted alfalfa, as well as the biocontrol activity of a generalist predator, the harlequin ladybird beetle (Harmonia axyridis). We investigated their responses to the presence of the intercropping partner plant species (alfalfa and wheat, respectively) through plant volatiles or visual cues at three nitrogen fertilizer levels in a greenhouse. In the absence of the predator, the English grain aphid population growth rate increased significantly with increasing nitrogen levels, whereas the pea aphid population increased significantly more slowly in response to high nitrogen levels. The English grain aphid and pea aphid population dynamics were unaffected by the presence of the intercropping partner. However, the presence of the intercropping partner enhanced the control of both aphid populations by the harlequin ladybird beetle. Increasing nitrogen fertilizer levels decreased the predation rates, which were otherwise increased by the intercropping partner. The beneficial effects of the intercropping partner were eventually non-existent at the highest nitrogen level tested. These results imply that the interaction between the presence of intercropping partner and the nitrogen fertilizer application affects the biocontrol activity of the natural enemies of insect pests. Thus, the compatibility between agricultural intensification and biocontrol strategies in integrated pest management programs need to be investigated.

Latitudinal trend in the reproductive mode of the pea aphid Acyrthosiphon pisum invading a wide climatic range.

The maintenance of sexuality is a puzzling phenomenon in evolutionary biology. Many universal hypotheses have been proposed to explain the prevalence of sex despite its costs, but it has been hypothesized that sex could be also retained by lineage‐specific mechanisms that would confer some short‐term advantage. Aphids are good models to study the maintenance of sex because they exhibit coexistence of both sexual and asexual populations within the same species and because they invade a large variety of ecosystems. Sex in aphids is thought to be maintained because only sexually produced eggs can persist in cold climates, but whether sex is obligate or facultative depending on climatic conditions remains to be elucidated. In this study, we have inferred the reproductive mode of introduced populations of the pea aphid Acyrthosiphon pisum in Chile along a climatic gradient using phenotypic assays and genetic‐based criteria to test the ecological short‐term advantage of sex in cold environments. Our results showed a latitudinal trend in the reproductive mode of Chilean pea aphid population from obligate parthenogenesis in the north to an intermediate life cycle producing both parthenogenetic and sexual progeny in the southernmost locality, where harsh winters are usual. These findings are congruent with the hypothesis of the ecological short‐term advantage of sex in aphids.

Oxidative stress links response to lead and Acyrthosiphon pisum in Pisum sativum L.

This study demonstrates the impact of lead at hormetic (0.075 mM Pb(NO3)2) and sublethal (0.5 mM Pb(NO3)2) doses on the intensity of oxidative stress in pea seedlings (Pisum sativum L. cv. ‘Cysterski’). Our first objective was to determine how exposure of pea seedlings to Pb alters the plant defence responses to pea aphid (Acyrthosiphon pisum Harris), and whether these responses could indirectly affect A. pisum. The second objective was to investigate the effects of various Pb concentrations in the medium on demographic parameters of pea aphid population and the process of its feeding on edible pea. We found that the dose of Pb sublethal for pea seedlings strongly reduced net reproductive rate and limited the number of A. pisum individuals reaching the phloem. An important defence line of pea seedlings growing on Pb-supplemented medium and next during combinatory effect of the two stressors Pb and A. pisum was a high generation of superoxide anion (O2−). This was accompanied by a considerable reduction in superoxide dismutase (SOD) activity, and a decrease in the level of Mn2+ ions. A the same time, weak activity of Mn-SOD was detected in the roots of the seedlings exposed to the sublethal dose of Pb and during Pb and aphid interaction. Apart from the marked increase in O2−, an increase in semiquinone radicals occurred, especially in the roots of the seedlings treated with the sublethal dose of Pb and both infested and non-infested with aphids. Also, hydrogen peroxide (H2O2) generation markedly intensified in aphid-infested leaves. It reached the highest level 24 h post infestation (hpi), mainly in the cell wall of leaf epidermis. This may be related to the function of H2O2 as a signalling molecule that triggers defence mechanisms. The activity of peroxidase (POX), an important enzyme involved in scavenging H2O2, was also high at 24 hpi and at subsequent time points. Moreover, the contents of thiobarbituric acid reactive substances (TBARS), products of lipid peroxidation, rose but to a small degree thanks to an efficient antioxidant system. Total antioxidant capacity (TAC) dependent on the pool of fast antioxidants, both in infested and non-infested and leaves was higher than in the control. In conclusion, the reaction of pea seedlings to low and sublethal doses of Pb and then A. pisum infestation differed substantially and depended on a direct contact of the stress factor with the organ (Pb with roots and A. pisum with leaves). The probing behavior of A. pisum also depended on Pb concentration in the plant tissues.

Seasonal dynamics of: the pea aphid, Acyrthosiphon pisum (Harris), its natural enemies the seven spotted lady beetle Coccinella septempunctata Linnaeus and variegated lady beetle Hippodamia variegata Goeze, and their parasitoid Dinocampus coccinellae (Schrank)

Abstract Two important lady beetle species commonly found in alfalfa fields in Iran are the variegated lady beetle Hippodamia variegata Goeze and the seven spotted lady beetle Coccinella septempunctata (Linnaeus, 1758) (Coleoptera: Coccinellidae). These two species attack many aphid species including the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae). In this study, the seasonal population changes of A. pisum, H. variegata, C. septempunctata and the parasitoid, Dinocampus coccinellae (Schrank) (Hymenoptera: Braconidae) were studied in alfalfa fields in the 2012 and 2013 seasonal periods. The highest ladybird densities were noted on July 5, 2012 and on September 6, 2013 (17.2±2.8 and 13.4±1.6) individuals per 20 sweeps, respectively. Parasitism rates by D. coccinellae ranged from approximately 3 to 6% in two subsequent years, respectively. Parasitism was higher early in the growing season. Most parasitised ladybirds were females. There was no significant relationship between the temperature and relative humidity with pea aphid populations, although the aphid populations declined during the hot summer period. In contrast, the relationship between temperature and the H. variegata population was significant and positive in both years of the study. It has been shown that these lady beetle species have a major role in reducing the pea aphid populations in alfalfa fields. Due to the relatively low percentage of field parasitism by D. coccinellae, this parasite might not reduce the biocontrol efficiency of lady beetle species.

Drawing the line.

The pea aphid Acyrthosiphon pisum is an agriculturally important pest of leguminous plants including peas and broad beans. The widespread use of chemical pesticides impacts heavily on the environment, and increases pesticide-resistant pea aphid populations, so alternative strategies are being actively sought. Pseudomonas syringae bacteria are known to infect and kill the pea aphid, and offer a possible control strategy. In this study, the authors measured the effects of injecting P. syringae on the survival of pea aphid populations at 24 and 48 hours. The pea aphid population was killed more rapidly (98 % after 24 hours) with a higher concentration of injected bacteria than in the control or with lower concentrations, indicating that a P. syringae-based control strategy may be a useful alternative to conventional pesticides.

Genetic characterisation of new host‐specialised biotypes and novel associations with bacterial symbionts in the pea aphid complex

Abstract The pea aphid, Acyrthosiphon pisum Harris, constitutes a complex of sympatric populations that are specialised to distinct species of Fabaceae. So far, 12 such populations have been characterised genetically as genetic clusters associated with one or few legumes species. These clusters form a continuum of genetic divergence linking host‐associated races, which show moderate hybridization, to nascent species. They are also known to be associated with different species of endosymbiotic bacteria, which have environmentally dependent effects on aphid fitness. Here, we report on the genetic characterisation of new host‐associated populations in this species complex. We sampled pea aphids in eastern France on Genista tinctoria, G. sagittalis, Onobrychis viciifolia, and Hippocrepis comosa. Bayesian clustering methods based on genotypes obtained at 25 microsatellite loci showed that each sampled plant species hosted a specific pea aphid population. Comparison with previously characterised biotypes showed that the population associated with Hippocrepis comosa was no different from that on Securigera varia. Migrant aphids from other host plants and hybrids were found at various frequencies on the newly sampled plant species. They were particularly frequent on Onobrychis viciifolia, occasional between the two Genista‐associated populations, and rare on Hippocrepis comosa. PCR‐based screening of bacterial species revealed new associations between aphid biotypes and facultative endosymbionts, chiefly the combination of Serratia symbiotica and Hamiltonella defensa within most individuals collected on Genista. The newly identified biotypes, which add up to a total of 15 within the pea aphid complex, offer new material to study the mechanisms and genetic bases of host‐specialisation and ecological speciation in this model aphid.

Bacterial communities associated with host-adapted populations of pea aphids revealed by deep sequencing of 16S ribosomal DNA.

Associations between microbes and animals are ubiquitous and hosts may benefit from harbouring microbial communities through improved resource exploitation or resistance to environmental stress. The pea aphid, Acyrthosiphon pisum, is the host of heritable bacterial symbionts, including the obligate endosymbiont Buchnera aphidicola and several facultative symbionts. While obligate symbionts supply aphids with key nutrients, facultative symbionts influence their hosts in many ways such as protection against natural enemies, heat tolerance, color change and reproduction alteration. The pea aphid also encompasses multiple plant-specialized biotypes, each adapted to one or a few legume species. Facultative symbiont communities differ strongly between biotypes, although bacterial involvement in plant specialization is uncertain. Here, we analyse the diversity of bacterial communities associated with nine biotypes of the pea aphid complex using amplicon pyrosequencing of 16S rRNA genes. Combined clustering and phylogenetic analyses of 16S sequences allowed identifying 21 bacterial OTUs (Operational Taxonomic Unit). More than 98% of the sequencing reads were assigned to known pea aphid symbionts. The presence of Wolbachia was confirmed in A. pisum while Erwinia and Pantoea, two gut associates, were detected in multiple samples. The diversity of bacterial communities harboured by pea aphid biotypes was very low, ranging from 3 to 11 OTUs across samples. Bacterial communities differed more between than within biotypes but this difference did not correlate with the genetic divergence between biotypes. Altogether, these results confirm that the aphid microbiota is dominated by a few heritable symbionts and that plant specialization is an important structuring factor of bacterial communities associated with the pea aphid complex. However, since we examined the microbiota of aphid samples kept a few generations in controlled conditions, it may be that bacterial diversity was underestimated due to the possible loss of environmental or transient taxa.

Patterns, causes and consequences of defensive microbiome dynamics across multiple scales.

The microbiome can significantly impact host phenotypes and serve as an additional source of heritable genetic variation. While patterns across eukaryotes are consistent with a role for symbiotic microbes in host macroevolution, few studies have examined symbiont-driven host evolution or the ecological implications of a dynamic microbiome across temporal, spatial or ecological scales. The pea aphid, Acyrthosiphon pisum, and its eight heritable bacterial endosymbionts have served as a model for studies on symbiosis and its potential contributions to host ecology and evolution. But we know little about the natural dynamics or ecological impacts of the heritable microbiome of this cosmopolitan insect pest. Here we report seasonal shifts in the frequencies of heritable defensive bacteria from natural pea aphid populations across two host races and geographic regions. Microbiome dynamics were consistent with symbiont responses to host-level selection and findings from one population suggested symbiont-driven adaptation to seasonally changing parasitoid pressures. Conversely, symbiont levels were negatively correlated with enemy-driven mortality when measured across host races, suggesting important ecological impacts of host race microbiome divergence. Rapid drops in symbiont frequencies following seasonal peaks suggest microbiome instability in several populations, with potentially large costs of 'superinfection' under certain environmental conditions. In summary, the realization of several laboratory-derived, a priori expectations suggests important natural impacts of defensive symbionts in host-enemy eco-evolutionary feedbacks. Yet negative findings and unanticipated correlations suggest complexities within this system may limit or obscure symbiont-driven contemporary evolution, a finding of broad significance given the widespread nature of defensive microbes across plants and animals.