Non‐consumptive effects of a natural enemy on a non‐prey herbivore population

1. Predator–prey interactions have traditionally focused on the consumptive effects that predators have on prey. However, predators can also reduce the abundance of prey through behaviourally‐mediated non‐consumptive effects. For example, pea aphids (Acyrthosiphon pisum Harris) drop from their host plants in response to the risk of attack, reducing population sizes as a consequence of lost feeding opportunities. 2. The objective of the present study was to determine whether the non‐consumptive effects of predators could extend to non‐prey herbivore populations as a result of non‐lethal incidental interactions between herbivores and foraging natural enemies. 3. Polyculture habitats consisting of green peach aphids (Myzus persicae Sulzer) feeding on collards and pea aphids feeding on fava beans were established in greenhouse cages. Aphidius colemani Viereck, a generalist parasitoid that attacks green peach aphids but not pea aphids, was released into half of the cages and the abundance of the non‐host pea aphid was assessed. 4. Parasitoids reduced the population growth of the non‐host pea aphid by increasing the frequency of defensive drops; but this effect was dependent on the presence of green peach aphids. 5. Parasitoids probably elicited the pea aphid dropping behaviour through physical contact with pea aphids while foraging for green peach aphids. It is unlikely that pea aphids were responding to volatile alarm chemicals emitted by green peach aphids in the presence of the parasitoid. 6. In conclusion, the escape response of the pea aphid provided the opportunity for a parasitoid to have non‐target effects on an herbivore with which it did not engage in a trophic interaction. The implication is that natural enemies with narrow diet breadths have the potential to influence the abundance of a broad range of prey and non‐prey species via non‐consumptive effects.     

Control of potato aphids by the addition of barley strips in potato fields: a successful example of vegetation management

The densities of barley and potato aphids, their natural enemies and hyperparasitoids were assessed in three experimental potato fields as a case study to investigate the effectiveness of the addition of barley strips in potato fields for conservation biological control. These fields were located in a low plant-diversity landscape, but common aphid species and their natural enemies were present. The barley strips in the potato fields were found to support different species of aphids of potato, but these different sets of aphids shared a common set of natural enemies. The amount of time between peak aphid densities and peaks of their natural enemies' populations was shorter in the potato fields than in the barley strips. The levels of winged aphids in a potato monoculture field were significantly higher than those in a field with barley strips. The wingless and winged aphid populations in the field without barley strips was almost three times higher than in the fields with the barley strips, as measured at the peak aphid density. This result is one of few examples of the application of the conservation effect of greenhouse banker plants on outdoor crops.     

The Role of Natural Enemy Foraging Guilds in Controlling Cereal Aphids in Michigan Wheat

Insect natural enemies (predators and parasitoids) provide important ecosystem services by suppressing populations of insect pests in many agricultural crops. However, the role of natural enemies against cereal aphids in Michigan winter wheat (Triticum aestivum L.) is largely unknown. The objectives of this research were to characterize the natural enemy community in wheat fields and evaluate the role of different natural enemy foraging guilds (foliar-foraging versus ground-dwelling predators) in regulating cereal aphid population growth. We investigated these objectives during the spring and summer of 2012 and 2013 in four winter wheat fields on the Michigan State University campus farm in East Lansing, Michigan. We monitored and measured the impact of natural enemies by experimentally excluding or allowing their access to wheat plants infested with Rhopalosiphum padi (L.) and Sitobion avenae (F.) (Hemiptera: Aphidae). Our results indicate that the natural enemy community in the wheat fields consisted mostly of foliar-foraging and ground-dwelling predators with relatively few parasitoids. In combination, these natural enemy groups were very effective at reducing cereal aphid populations. We also investigated the role of each natural enemy foraging guild (foliar-foraging versus ground-dwelling predators) independently. Overall, our results suggest that, in combination, natural enemies can almost completely halt early-season aphid population increase. Independently, ground-dwelling predators were more effective at suppressing cereal aphid populations than foliar-foraging predators under the conditions we studied. Our results differ from studies in Europe and the US Great Plains where foliar foraging predators and parasitoids are frequently more important cereal aphid natural enemies.     

Biological control of aphids in the presence of thrips and their enemies

Generalist predators are often used in biological control programs, although they can be detrimental for pest control through interference with other natural enemies. Here, we assess the effects of generalist natural enemies on the control of two major pest species in sweet pepper: the green peach aphid Myzus persicae (Sulzer) and the western flower thrips Frankliniella occidentalis (Pergande). In greenhouses, two commonly used specialist natural enemies of aphids, the parasitoid Aphidius colemani Viereck and the predatory midge Aphidoletes aphidimyza (Rondani), were released together with either Neoseiulus cucumeris Oudemans, a predator of thrips and a hyperpredator of A. aphidimyza, or Orius majusculus (Reuter), a predator of thrips and aphids and intraguild predator of both specialist natural enemies. The combined use of O. majusculus, predatory midges and parasitoids clearly enhanced the suppression of aphids and consequently decreased the number of honeydew-contaminated fruits. Although intraguild predation by O. majusculus on predatory midges and parasitoids will have affected control of aphids negatively, this was apparently offset by the consumption of aphids by O. majusculus. In contrast, the hyperpredator N. cucumeris does not prey upon aphids, but seemed to release aphids from control by consuming eggs of the midge. Both N. cucumeris and O. majusculus did not affect rates of aphid parasitism by A. colemani. Thrips were also controlled effectively by O. majusculus. A laboratory experiment showed that adult predatory bugs feed on thrips as well as aphids and have no clear preference. Thus, the presence of thrips probably promoted the establishment of the predatory bugs and thereby the control of aphids. Our study shows that intraguild predation, which is potentially negative for biological control, may be more than compensated by positive effects of generalist predators, such as the control of multiple pests, and the establishment of natural enemies prior to pest invasions. Future work on biological control should focus on the impact of species interactions in communities of herbivorous arthropods and their enemies.     

Density-dependent regulation in populations of potato-colonizing aphids

Scarcity of long-term (over 30 years) data series represents a major challenge for an accurate estimation of the role of density-dependent processes in population regulation. We analyzed population densities of the wingless parthenogenic morphs of buckthorn aphid (BA), Aphis nasturtii Kaltenbach, potato aphid (PA), Macrosiphum euphorbiae (Thomas), and green peach aphid (GPA), Myzus persicae (Sulzer) from 1949 to 2003 for signs of density-dependent regulation. We found strong evidence of density-dependent regulation, with detection of density dependence being fairly consistent among the different statistical techniques. Direct density dependence was detected for the populations of all three species. There was also evidence of delayed density dependence for PA. The periodicity of population fluctuations for BA and GPA was 6.1 years and 3.9 years, respectively. The periodicity for PA was not explicit, being highly variable throughout the time series. Effects of density-independent weather factors were relatively minor compared to density-dependent regulation. The BA populations experienced a significant reduction in both density and annual oscillations starting in 1995, while GPA populations experienced a similar reduction in 1991. No such change was apparent for PA. The most likely explanation for the observed phenomenon is a change in the composition of the lady beetle community following the establishment of two alien coccinellid species, and/or changes in insecticide use by commercial growers in the area of the study.     

Non-additive effects of multiple natural enemies on aphid populations

The question of whether multiple natural enemies often interact to produce lower host mortality than single enemies acting alone has not yet been resolved. We compared the effects of four different combinations of natural enemies-parasitoids, predators, parasitoids plus predators, and no enemies-on caged aphid populations on marsh elder, Iva frutescens, in west-central Florida. Using starting densities of natural enemies commonly found in the field, we showed that parasitoid wasps reduced aphid population densities more than predatory ladybird beetles. The addition of predators to cages containing parasites reduced the ability of parasitoids to decrease aphid population densities. Because the experiments ran only over the course of one generation, such a reduction in the effectiveness of parasites is likely caused by interference of predators with parasitoid behavior. Parasitism in the cages containing both parasitoids and predators was reduced when compared to percent parasitism in parasitoid-only cages, but this could also be due to predation. Our experiments showed that ladybird beetles prey on parasitized aphids. Thus over the long-term, the effectiveness of parasites is impaired by the interference of predators on ovipositing parasitoids and by the predation of parasitized aphids. The effects of natural enemies in this system are clearly non-additive.     

Antibiosis and antixenosis of six commonly produced potato cultivars to the green peach aphid, Myzus persicae Sulzer (Hemiptera: Aphididae)

The antibiotic and antixenotic resistance of six commonly produced potato cultivars in Iran including Aozonia, Agria, Cosima, Cosmos, Kondor and Savalan to the green peach aphid, Myzus persicae Sulzer, were investigated under laboratory conditions at 20 ± 2oC, 65 ± 5% RH and 16:8h (L:D) in 2009. Antibiosis experiments showed significant differences in the developmental time, nymphal survivorship, fecundity, adult longevity of the green peach aphid among the potato cultivars. Intrinsic rate of natural increase (r m) for apterous aphids varied significantly with the potato cultivars on which the aphids were reared. This value ranged from 0.225 to 0.293 females/female/day, which was lowest on Cosmos and highest on Aozonia. Additionally, the estimated net reproductive rate (R0) and finite rate of increase (λ) for apterous aphids were the lowest on Cosmos. For the antixenosis experiment, no significant difference was found in aphid's preference to the potato cultivars. However, Aozonia was preferred more than the other five cultivars by the apterous aphids. Therefore, our results demonstrated that among the investigated cultivars the Cosmos cultivar is moderately resistant to the green peach aphid.     

作物多样性对大豆蚜的控蚜效应

【目的】研究大豆蚜发生为害及大豆与多种作物间邻作种植对大豆蚜的控制作用,为大豆蚜的可持续综合治理提供理论依据。【方法】采用系统调查的方法,研究大豆蚜和天敌田间种群动态;通过田间罩笼、人工接蚜和释放天敌的方法,研究捕食性天敌对大豆蚜种群的控制作用;在佳木斯地区进行大豆与早熟马铃薯间作,牡丹江地区进行黄瓜-大豆-玉米、甜葫芦-大豆-玉米、烟草-大豆-香瓜、甜菜-大豆-玉米等多作物带状穿插种植模式,以单作大豆田为对照,对不同种植模式的大豆田大豆蚜与天敌进行调查,研究作物多样性对大豆蚜的控制作用。【结果】2009年6月中下旬大豆蚜开始侵入大豆田,3~5周后田间有蚜株率达到100%,大豆蚜种群发生高峰期在7月下旬至8月上旬,9月上旬在田间逐渐消失。草蛉、瓢虫和寄生蜂等为蚜虫天敌优势种;按大豆蚜与天敌数量之比700︰1,释放异色瓢虫和叶色草蛉成虫7 d后,蚜虫种群减退率分别为54.78%和78.79%;大豆与早熟马铃薯间作,在大豆蚜种群迅速增长期早熟马铃薯收获(7月20日)后第5天,豆田蚜虫天敌总数是收获前的2.6倍,与同期单作大豆田相比,间作田大豆蚜种群数量降低了51.3%。大豆与甜葫芦、香瓜、烟草和玉米等作物进行多样性间作种植,在大豆蚜田间发生高峰期,单作豆田益害比为1︰65.2,多样性种植区的大豆田益害比为1︰26~1︰42,与单作大豆田相比,间作田大豆蚜种群数量降低40.7%~83.5%。【结论】2009年大豆蚜的种群高峰期为8月3日,田间的天敌优势种类为草蛉、瓢虫和寄生蜂。早熟马铃薯与大豆间作,在大豆蚜种群迅速增长期间收获早熟马铃薯,大量蚜虫天敌转移至间作的大豆田,从而形成对大豆蚜的控制。大豆与其它经济作物间邻作,大豆田天敌昆虫与蚜虫的益害比明显提高,表明利用农田作物多样性能充分发挥自然天敌的生物控害作用。     

Predator biodiversity strengthens herbivore suppression

Species diversity at lower trophic levels generally improves ecosystem functioning. However, the impact of greater predator diversity on herbivore regulation is uncertain because predator species both compete with and prey on each other. In a large-scale field experiment we examined the relationship between predator species diversity and the suppression of two herbivores, green peach and cabbage aphids, on collard plants. We show that, for both aphid species, the strength of herbivore suppression increased with higher predator biodiversity. Greater resource exploitation by predators in diverse communities generally led to improved predator survivorship and reproduction. Herbivore population size was negatively correlated with plant biomass, providing evidence that greater aphid suppression leads to improved plant growth. Our study suggests a harmonious relationship between predator conservation and herbivore control, and a relatively weak role for predator interference, within this community.     

The role of the perennial stinging nettle, Urtica dioica, as a reservoir of beneficial natural enemies

A wide range of natural enemies, including predators, parasites and entomophagous fungi were observed to feed on the stinging nettle aphid, Microlophium carnosum, populations of which increased rapidly in late April and early May. Patches of stinging nettles thus served as an important alternate feeding site for some beneficial natural enemies before pest aphids appeared on cultivated plants. Anthocoridae, Miridae and Coccinellidae were the most abundant specific predators sampled on nettles; only the Coccinellidae appeared to disperse over a defined period to other habitats. The hymenopterous parasites Aphidius ervi and Ephedrus lacertosus parasitized up to 10% of M. carnosum populations in June: two species of the fungus Entomophthora occurred spasmodically. Cutting patches of nettles in May or June had the most striking effect on the species and numbers of Coccinellidae. Cutting in mid-June might increase the numbers and impact of natural enemies on nearby pest infestations.     

Assessment of cotton aphids, Aphis gossypii, and their natural enemies on aphid-resistant and aphid-susceptible wheat varieties in a wheat–cotton relay intercropping system

The cotton aphid, Aphis gossypii Glover (Homoptera: Aphididae), is an important cotton pest in northern China, especially in the seedling stage of cotton. After large scale commercial use of transgenic Bt cotton, cotton aphids became one of the most important cotton pests. A 2‐year study was conducted to evaluate the role of four winter wheat varieties that were resistant or susceptible to wheat aphid, Sitobion avenae Fabricius (Homoptera: Aphididae), in conserving arthropod natural enemies and suppressing cotton aphids in a wheat–cotton relay intercropping system in northern China. The results indicated that wheat–cotton intercropping preserved and augmented natural enemies more than a monoculture of cotton. The density of natural enemies in cotton was significantly different among relay‐intercropping fields with different wheat varieties. The highest density of natural enemies and low cotton aphid populations were found in the treatment of cotton in relay intercropped with the wheat variety Lovrin10, which is susceptible to wheat aphid. The lowest density of predators and parasitoids associated with high cotton aphid populations were found with the wheat variety KOK1679, which is resistant to wheat aphid. The results showed that wheat varieties that are susceptible or moderately resistant to wheat aphid might reduce cotton aphids more effectively than an aphid‐resistant variety in the intercropping system by enhancing predators to suppress cotton aphids during the cotton seedling stage.     

Natural enemies act faster than endophytic fungi in population control of cereal aphids

1. Fast-growing populations of phytophagous insects can be limited by the presence of natural enemies and by alkaloids that are produced by symbiotic associations of many temperate grass species with endophytic fungi. It is unclear if and how acquired plant defences derived from endophytic fungi interact with natural enemies to affect phytophagous insect populations. 2. To assess the relative importance of endophytic fungi compared to that of natural enemies on the population dynamics of phytophagous insects, we carried out a fully factorial field experiment, in which the presence of natural enemies and the presence of endophytic fungi were manipulated simultaneously. Target colonies of aphids were monitored for 8 weeks starting from their natural appearance in the field to the end of the aphid season. 3. We show that on Lolium perenne increased natural enemy densities reduced the individual numbers of two common cereal aphids, Rhopalosiphum padi and Metopolophium festucae. 4. The presence of the endophytic fungi Neotyphodium lolii reduced the number of M. festucae but did not affect the number of R. padi. The reduction in R. padi numbers by predators and parasitoids was not influenced by the presence of endophytes. For adult M. festucae, however, the negative effects of natural enemies were significant only in the absence of endophytes. 5. Over the duration of the experiment, the effect of natural enemies on aphid colony growth was much stronger than the effect of the endophytic fungi N. lolii, presumably because predator and parasitoid action on aphid colonies is much faster than any effects of endophytes. 6. Our results demonstrate that with simultaneous action of acquired endosymbionts and natural enemies, both factors can control aphid colony growth but they generally act independently of each other.     

Evaluation of artificial diets for rearing Aphis glycines (Hemiptera: Aphididae)

Artificial aphid diets have been previously developed for the pea aphid, Acyrthosiphon pisum (Harris), and the green peach aphid, Myzus persicae (Sulzer). The ability to rear aphids on an artificial diet allows for selectively adding or subtracting compounds from an aphid's food source to determine the effect on fecundity and longevity. Five diets previously developed for the green peach aphid and the pea aphid were tested for their suitability for rearing soybean aphid, Aphis glycines Matsumura. The best diet, originally developed for the green peach aphid and based on the amino acid profile of young potato plants, allowed 12 generations of soybean aphids to develop. For all diets tested, aphid fecundity, and longevity were greatly reduced in comparison with aphids reared on soybean, Glycine max (L.) Merr., plants or on detached soybean leaves. In addition, mean developmental time was significantly longer for aphids reared on artificial diets.     

Interspecific and intraspecific variation in the performance of three pest aphid species on five grain legume hosts

The suitability of five grain legume species (narrow-leafed lupin, chickpea, faba bean, field pea, lentil) as hosts for three aphid species (green peach aphid, cowpea aphid, bluegreen aphid) was evaluated by measuring the mean relative growth rate (MRGR) and survivorship of nymphs over a 5 day period. For each aphid species, intraspecific (interclonal) variation was also determined by independently measuring the performance of 30 clones collected from a variety of hosts and from different parts of the Western Australia (WA) wheatbelt. The suitability of the grain legumes varied among aphid species. Chickpea was not a suitable host for any of the aphids tested. Averaged over all clones, lentil and faba bean were the most suitable hosts for cowpea aphid, and narrow-leafed lupin was the most suitable host for green peach aphid. Field pea was a suitable host for all three species, but only at a suboptimal level. Cowpea aphid showed the greatest amount of intraspecific variation, with significant variation in MRGR among clones on all hosts except chickpea and significant variation in survivorship on chickpea and lupin. For green peach aphid, there was significant variation in MRGR among clones on field pea and lupin, but in survivorship on lupin only. Bluegreen aphid clones showed significant variation only for MRGR on faba bean and lupin. There were positive correlations in performance of green peach aphid clones on faba bean and lentil, and of cowpea aphid clones on faba bean and lentil. Bluegreen aphid clones showed a negative correlation in performance on field pea and faba bean. These results show the importance of screening cultivars against a wide variety of aphid clones when assessing aphid susceptibility in breeding programmes. The implications of these results on the adaptability of parthenogenetic aphids are discussed.     

Virus mediated trophic interactions between aphids and their natural enemies

Microbial endosymbionts alter the phenotype of their host which may have cascading effects at both population and community levels. However, we currently lack information on whether the effects of viruses on both host phenotypic traits and host population demography can modify interactions with upper trophic levels. To fill this gap, we investigated whether a prevalent densovirus infecting the aphid Myzus persicae (i.e. MpDNV) can modify trophic interactions between host aphids and their natural enemies (i.e. predators and parasitoids) by influencing aphid phenotypic traits (i.e. body mass and defensive behaviours), population demography (i.e. density and age-structure) and susceptibility towards both predation and parasitism. We found that the virus decreased aphid body mass but did not influence their behavioural defences. At the population level, the virus had a minor effect on aphid adult mortality whereas it strongly reduced the density of nymphs and influenced the stage structure of aphid populations. In addition, the virus enhanced the susceptibility of aphids to parasitism regardless of the parasitoid species. Predation rate on adult aphids was not influenced by the virus but ladybeetle predators strongly decreased the number of aphid nymphs, especially for uninfected ones compared to infected ones. As a result, the virus decreased predator effect on aphid populations. By reducing both aphid quality and availability, increasing their susceptibility to parasitism, and modulating predator effect on aphid populations, we highlighted that viral endosymbionts can be prevalent drivers of their host ecology as they modify their phenotypes and interspecific interactions. These virus-mediated ecological effects may have consequences on enemies foraging strategies as well as trophic webs dynamics and structure.     

Bottom–up regulates top–down: the effects of hybridization of grass endophytes on an aphid herbivore and its generalist predator

The ecological consequences of hybridization of microbial symbionts are largely unknown. We tested the hypothesis that hybridization of microbial symbionts of plants can negatively affect performance of herbivores and their natural enemies. In addition, we studied the effects of hybridization of these symbionts on feeding preference of herbivores and their natural enemies. We used Arizona fescue as the host‐plant, Neotyphodium endophytes as symbionts, the bird cherry–oat aphid as the herbivore and the pink spotted ladybird beetle as the predator in controlled experiments. Neither endophyte infection (infected or not infected) nor hybrid status (hybrid and non‐hybrid infection) affected aphid reproduction, proportion of winged forms in the aphid populations, aphid host‐plant preference or body mass of the ladybirds. However, development of ladybird larvae was delayed when fed with aphids grown on hybrid (H+) endophyte infected grasses compared to larvae fed with aphids from non‐hybrid (NH+) infected grasses, non‐hybrid, endophyte‐removed grasses (NH?) and hybrid, endophyte‐removed (H?) grasses. Furthermore, adult beetles were more likely to choose all other types of grasses harboring aphids rather than H+ infected grasses. In addition, development of ladybirds was delayed when fed with aphids from naturally uninfected (E?) grasses compared to ladybirds that were fed with aphids from NH+ and NH? grasses. Our results suggest that hybridization of microbial symbionts may negatively affect generalist predators such as the pink spotted ladybird and protect herbivores like the bird cherry–oat aphids from predation even though the direct effects on herbivores are not evident.     

The influence of Lasius neoniger (Hymenoptera: Formicidae) on population growth and biomass of Aphis glycines (Hemiptera: Aphididae) in soybeans

In the United States, the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), are often tended by the aphid-tending ant, Lasius neoniger Emery (Hymenoptera: Formicidae). In this study, we examined the effects of tending by ants on the density and biomass of soybean aphids on soybeans in Kentucky. We performed cage studies that limited access by ants and/or natural enemies. We used a split-plot design with natural enemy access as the main plot and ant attendance as the sub plot. We found that natural enemy access negatively affected aphid population density in the presence of tending ants, seen as a three- to four-fold increase in aphid density when natural enemies were excluded. In addition, we found that ant tending positively affected aphid biomass, both when natural enemies were given access to aphids or when natural enemies were excluded, seen by a two-fold increase in aphid biomass when ants tended aphids, both in the presence or absence of natural enemies. Biomass accumulation is seen as an important measurement for assessing aphid performance, and we argue that aphid-tending by ants can have an influence on natural field populations of soybean aphids. Agronomic practices that affect ant abundance in soybeans may influence the performance and hence pest outbreaks for this economically important pest.     

Intraguild predation by aphidophagous predators on parasitised aphids: the use of multiple video cameras

A video technique that allows simultaneous behavioural observations of several experimental replicates under field and laboratory conditions is described. The technique was used to analyse predation risk of parasitised aphids in a sugar beet field. The images of 16 black and white video cameras were recorded by a video multiplexer in combination with a time-lapse video recorder. Each camera was weather protected and equipped with a single infrared diode to allow observations during night times. Single leaves carrying aphid mummies only or mummies and unparasitised aphids were monitored. All colonies were exposed to predation and parasitation by the community of natural enemies in the field. Colonies with mummies and unparasitised aphids were visited significantly more often by predators than those without additional aphids. Predators also stayed significantly longer in patches with unparasitised aphids. Although an equal proportion of aphid mummies were destroyed in both treatments, the video analysis showed differences in predator species spectrum between treatments. In patches with aphids, coccinellid and hemipteran predators preyed on mummies, while in patches with only mummies, chrysopids accounted for most of the damage. The decrease in parasitoid survival could be attributed to the increasing number of predator visits in aphid patches and to a lesser extent to the decreasing number of unparasitised aphids (alternative prey). Parasitoid survival in colonies without alternative prey was correlated with the number of predator visits and the time predators spent on a leaf.Continuous video observations gave additional behavioural information for the interpretation of field data. Other prospective research fields for the use of the multi video camera technique are outlined and general advantages and disadvantages are discussed.     

The role of natural enemy guilds in Aphis glycines suppression

Generalist natural enemy guilds are increasingly recognized as important sources of mortality for invasive agricultural pests. However, the net contribution of different species to pest suppression is conditioned by their biology and interspecific interactions. The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is widely attacked by generalist predators, but the relative impacts of different natural enemy guilds remains poorly understood. Moreover, low levels of A. glycines parasitism suggest that resident parasitoids may be limited through intraguild predation. During 2004 and 2005, we conducted field experiments to test the impact of different guilds of natural enemies on A. glycines. We contrasted aphid abundance on field cages with ambient levels of small predators (primarily Orius insidiosus) and parasitoids (primarily Braconidae), sham cages and open controls exposed to large predators (primarily coccinellids), and cages excluding all natural enemies. We observed strong aphid suppression (86- to 36-fold reduction) in treatments exposed to coccinellids, but only minor reduction due to small predators and parasitoids, with aphids reaching rapidly economic injury levels when coccinellids were excluded. Three species of resident parasitoids were found attacking A. glycines at very low levels (<1% parasitism), with no evidence that intraguild predation by coccinellids attenuated parasitoid impacts. At the plant level, coccinellid impacts resulted in a trophic cascade that restored soybean biomass and yield, whereas small natural enemies provided only minor protection against yield loss. Our results indicate that within the assemblage of A. glycines natural enemies in Michigan, coccinellids are critical to maintain aphids below economic injury levels.     

Population dynamics of cereal aphids: influence of a shared predator and weather

1 Aphid populations may show strong year-to-year fluctuations, but questions remain regarding the dominance of factors that cause this variation, especially the role of natural enemies. To better understand the dynamics of aphid species that occur as pests in cereals, we investigated the relative influence of top-down control by a predator and weather (temperature and precipitation) on population fluctuations of three cereal aphid species.
2 From 1987 to 2005, populations of Metopolophium dirhodum , Sitobion avenae and Rhopalosiphum padi in insecticide-free stands of winter wheat were monitored in the Praha-Ruzyné region of the Czech Republic. Densities of an aphidophagous predator, the ladybeetle Coccinella septempunctata , were recorded from an overwintering site in the landscape. Weather was quantified using historical records.
3 A significant bottom-up effect of densities of aphids on those of C.   septempunctata was found, but evidence of direct top-down regulation of aphids by C.   septempunctata was only significant in the case of R.   padi . There was no significant periodicity in the dynamics of the aphid or C.   septempunctata , suggesting that there was no clear predator-prey cycle. Combinations of C.   septempunctata and weather variables could be used to explain M.   dirhodum and R.   padi per capita rate of change. There were also indications that weather directly affected peak density of M.   dirhodum .
4 We conclude that regional estimates of C.   septempunctata densities are not sufficient to determine whether aphid population dynamics are driven by predator–prey interactions. Feasibility of time series analysis as an investigative tool in aphid population dynamics studies is discussed.