Natural flightless morphs of the ladybird beetle Adalia bipunctata improve biological control of aphids on single plants

The challenge of using ladybird beetles for biological control of insect pests such as aphids is that the adult beetles tend to fly away from the host plants. Therefore, flightless ladybirds might improve biocontrol. There are several artificial ways to obtain flightless beetles, but it may be preferable to use natural variation in flight ability. We investigated, for the first time, biocontrol by inundative augmentation of natural flightless morphs of the ladybird beetle Adalia bipunctata. Microcosm experiments using single leaves with one of three species of aphid revealed no differences in consumption behavior between flightless and winged beetles. Monitoring for 48 h of single, caged pepper plants infested with aphids of Myzus persicae nicotianae or Aulacorthum solani showed that flightless beetles had a longer residence time on the plants than winged beetles. This only translated into significantly better biocontrol of M. persicae. Despite their difference in residence time, both beetle morphs reduced the population growth of A. solani. This is probably explained by the tendency of A. solani to drop from the plant upon disturbance, and we predict that flightless beetles may outperform winged ones in the long term. Overall, our results provide a proof of principle that natural flightless A. bipunctata can improve biocontrol of aphids by ladybird beetles. However, we recognize that the effect of biocontrol will vary with the species of aphid used and that further examination in long term and large scale experiments is required.     

Effect of size,extraguild prey and habitat complexity on intraguild interactions: a case study with the invasive ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis> and the hoverfly <Emphasis Type="Italic">Episyrphus balteatus</Emphasis>

Intraguild predation (IGP) is a widespread interaction between predatory arthropods, and is influenced by several factors. The harlequin ladybird, Harmonia axyridis (Coleoptera: Coccinellidae), has frequently been reported as an intraguild predator of other Coccinellidae, but little is known about its interactions with other aphidophagous predators, including syrphids. This study investigated the incidence of IGP between H. axyridis and Episyrphus balteatus (Diptera: Syrphidae), the most abundant hoverfly species in Europe and a commercially available aphid biocontrol agent. The influence of size, presence of extraguild prey and habitat complexity were investigated through laboratory experiments in Petri dishes and on potted broad bean plants. In both types of arenas, IGP between H. axyridis and E. balteatus was found to be asymmetric, with the coccinellid in the majority of cases being the intraguild predator. There was a significant effect of size on the frequency of IGP. The efficiency of H. axyridis as an intraguild predator increased with the developmental stage. Early instars of E. balteatus were the most vulnerable to IGP. Pupae of either species were not attacked. In the presence of extraguild prey, the frequency of IGP was substantially reduced. However, IGP still occurred, mainly in combinations of older larvae of H. axyridis with first or second instars of E. balteatus. The size of the arena affected the incidence of IGP in combinations with second instars of E. balteatus, but not in combinations with third instars. Field research is needed to elucidate the ecological relevance of IGP among these predators.     

Predatory responses of selected lines of developmental variants of ladybird,Propylea dissecta (Coleoptera: Coccinellidae) in relation to increasing prey and predator densities

Individuals of the same species, population and generation frequently exhibit sub-maximal and significant genetic and phenotypic variation in their rate of development, showing slow and fast developers. Fast developers commonly have higher foraging and predation rates than slow developers. The consequence of such differences and foraging for the efficacy of biocontrol species remains under-explored. Slow and fast developers from a population of the ladybird, Propylea dissecta were separated and selected experimentally for F15 generations, and the predatory response of fourth instar larvae of control and experimentally selected slow and fast developers was then assessed at differing levels of prey (pea aphid, Acyrthosiphon pisum) and conspecific predator abundance. All individuals, whether slow or fast developers, showed a Type-II functional response, decrease in proportion of prey consumed with increasing prey biomass and an increase in proportion of prey consumed with increasing predator density. The proportion of prey consumed was highest in experimental fast developers and lowest in experimental slow developers. Attack rate was highest and handling time longest in slow developers of control/experimental groups. Mutual interference was least while area of discovery was highest in experimental fast developers. Thus, selection of fast developers for F15 generations led to higher functional responses, slower attack rates and faster prey consumption. This lower mutual interference and high searching efficiency indicates that they can be experimentally selected and used for better control of the pea aphids. This study is the first attempt to evaluate predatory responses of selected lines of an aphidophagous ladybird.     

Delineating the effects of a plant trait on interactions among associated insects

Plant traits can affect ecological interactions between plants, herbivores, and predators. Our study tests whether reduced leaf wax in peas alters the interaction between the pea aphid ( Acyrthosiphon pisum), a foliar-foraging predator (a lady beetle, Hippodamia convergens) and a ground-foraging predator (a ground beetle, Poecilus scitulus). We performed a 2×2×2 factorial experiment in which wax level, presence of H. convergens, and presence of P. scitulus were manipulated. Experimental arenas consisted of a cage surrounding three pea plants. One plant in each cage was stocked with 15 pea aphids. In greenhouse and field cage experiments, we assessed the effect of each factor and their interactions on aphid density. As in previous studies, H. convergens foraged for aphids more effectively on reduced wax peas than on normal peas. Other interactions among H. convergens, P. scitulus , and A. pisum were the same on both types of peas. We consider how aphid movement, plant growth, and a high frequency of predation by P. scitulus on H. convergens influenced pea aphid density.     

Response of walnut aphid populations to increasing foliar nitrogen content

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The escape response of pea aphids to foliar-foraging predators: factors affecting dropping behaviour

1. The effects of predator species, aphid density, aphid age, diel period, and habitat complexity on the dropping behaviour of the pea aphid Acyrthosiphon pisum were assessed in a series of laboratory and field-cage experiments.
2. The presence of foliar-foraging predators significantly increased the proportion of aphids that dropped from alfalfa plants. In the absence of predators, less than 7% of the aphids dropped. Dropping more than doubled (14%) when one of three hemipteran predators , N. americoferus, G. punctipes or O. insidiosus , was present. Nearly 60% of the aphids dropped when the ladybird beetle, Coccinella septempunctata , was present.
3. Adult aphids showed a significantly higher propensity to drop than immature aphids, regardless of the presence or absence of predators. Aphid density had no effect on dropping behaviour.
4. Neither diel period nor habitat complexity had an effect on aphid dropping behaviour. Aphids were significantly more likely to drop in the presence of predators during either the day or night and from either early or late regrowth alfalfa.
5. A review of the factors affecting dropping behaviour, including those elucidated in this study, indicates that the propensity to drop from a plant is influenced by three factors: the risk of predation on the plant, the quality of the resource to be abandoned, and the risk of mortality in the new microhabitat.     

Predator and parasitoid attacking ant-attended aphids: effects of predator presence and attending ant species on emerging parasitoid numbers

Interaction between a predator and a parasitoid attacking ant-attended aphids was examined in a system on photinia plants, consisting of the aphid Aphis spiraecola, the two ants Lasius japonicus and Pristomyrmex pungens, the predatory ladybird beetle Scymnus posticalis, and the parasitoid wasp Lysiphlebus japonicus. The ladybird larvae are densely covered with waxy secretion and are never attacked by attending ants. The parasitoid females are often attacked by ants, but successfully oviposit by avoiding ants. The two ants differ in aggressiveness towards aphid enemies. Impacts of the predator larvae and attending ant species on the number of parasitoid adults emerging from mummies per aphid colony were assessed by manipulating the presence of the predator in introduced aphid colonies attended by either ant. The experiment showed a significant negative impact of the predator on emerging parasitoid numbers. This is due to consumption of healthy aphids by the predator and its predation on parasitized aphids containing the parasitoid larvae (intraguild predation). Additionally, attending ant species significantly affected emerging parasitoid numbers, with more parasitoids in P. pungens-attended colonies. This results from the lower extent of interference with parasitoid oviposition by the less aggressive P. pungens. Furthermore, the predator reduced emerging parasitoid numbers more when P. pungens attended aphids. This may be ascribed to larger numbers of the predator and the resulting higher levels of predation on unparasitized and parasitized aphids in P. pungens-attended colonies. In conclusion, a negative effect of the predator on the parasitoid occurs in ant-attended aphid colonies, and the intensity of the interaction is affected by ant species.     

Erratum

Abstract: Fertilization levels for ornamental crops may influence pest population dynamics, crop quality, and pest management strategy. We examined the effect of fertilization on population growth and within‐plant distribution of melon or cotton aphid, Aphis gossypii Glover, on potted chrysanthemum, Dendranthema grandiflora (Tzvelev). In terms of pest management implications, we also investigated the effect of fertilization on the number of insecticide applications needed to control A. gossypii on potted chrysanthemum. Population growth rate of A. gossypii increased with fertilization levels from 0 to 38 ppm N and reached a plateau from 38 to 488 ppm N. Increased fertilization beyond 38 ppm N, 10% of the commercial standard, did not result in higher aphid number. Aphids responded to nutrient availability of plants by distributing themselves in areas with higher level of nitrogen. More aphids were found in the apical and middle strata of the plants than the basal stratum, which had the lowest nitrogen content. Leaf nitrogen content increased with increased fertilization level and was consistently higher in the apical and middle strata than the basal stratum. Increased fertilization from 0 to 375 ppm N did not result in higher number of insecticide applications. All three insecticides (bifenthrin, kinoprene or pymetrozine) were effective in keeping the aphid infestation below a pre‐determined level, five aphids per plant, but pymetrozine required the least number of applications. For chrysanthemum, a fast‐growing crop and heavy utilizer of nitrogen, increased fertilization shortened the time to flowering, which would allow growers to harvest their crop sooner and reduce the time for aphid population growth. Reduction in time to harvest could result in significant reduction of insecticide usage by reducing the time for aphid population growth. As a result, high fertilization together with minimal runoff may be a useful tactic to an integrated pest management (IPM) programme for managing A. gossypii on potted chrysanthemums.     

Influences of fertilization on Aphis gossypii and insecticide usage

Abstract: Fertilization levels for ornamental crops may influence pest population dynamics, crop quality, and pest management strategy. We examined the effect of fertilization on population growth and within‐plant distribution of melon or cotton aphid, Aphis gossypii Glover, on potted chrysanthemum, Dendranthema grandiflora (Tzvelev). In terms of pest management implications, we also investigated the effect of fertilization on the number of insecticide applications needed to control A. gossypii on potted chrysanthemum. Population growth rate of A. gossypii increased with fertilization levels from 0 to 38 ppm N and reached a plateau from 38 to 488 ppm N. Increased fertilization beyond 38 ppm N, 10% of the commercial standard, did not result in higher aphid number. Aphids responded to nutrient availability of plants by distributing themselves in areas with higher level of nitrogen. More aphids were found in the apical and middle strata of the plants than the basal stratum, which had the lowest nitrogen content. Leaf nitrogen content increased with increased fertilization level and was consistently higher in the apical and middle strata than the basal stratum. Increased fertilization from 0 to 375 ppm N did not result in higher number of insecticide applications. All three insecticides (bifenthrin, kinoprene or pymetrozine) were effective in keeping the aphid infestation below a pre‐determined level, five aphids per plant, but pymetrozine required the least number of applications. For chrysanthemum, a fast‐growing crop and heavy utilizer of nitrogen, increased fertilization shortened the time to flowering, which would allow growers to harvest their crop sooner and reduce the time for aphid population growth. Reduction in time to harvest could result in significant reduction of insecticide usage by reducing the time for aphid population growth. As a result, high fertilization together with minimal runoff may be a useful tactic to an integrated pest management (IPM) programme for managing A. gossypii on potted chrysanthemums.     

Biodiversity and biocontrol: emergent impacts of a multi-enemy assemblage on pest suppression and crop yield in an agroecosystem

The suppression of agricultural pests has often been proposed as an important service of natural enemy diversity, but few experiments have tested this assertion. In this study we present empirical evidence that increasing the richness of a particular guild of natural enemies can reduce the density of a widespread group of herbivorous pests and, in turn, increase the yield of an economically important crop. We performed an experiment in large field enclosures where we manipulated the presence/absence of three of the most important natural enemies (the coccinellid beetle Harmonia axyridis, the damsel bug Nabis sp., and the parasitic wasp Aphidius ervi) of pea aphids (Acyrthosiphon pisum) that feed on alfalfa (Medicago sativa). When all three enemy species were together, the population density of the pea aphid was suppressed more than could be predicted from the summed impact of each enemy species alone. As crop yield was negatively related to pea aphid density, there was a concomitant non‐additive increase in the production of alfalfa in enclosures containing the more diverse enemy guild. This trophic cascade appeared to be influenced by an indirect interaction involving a second herbivore inhabiting the system – the cowpea aphid, Aphis craccivora. Data suggest that high relative densities of cowpea aphids inhibited parasitism of pea aphids by the specialist parasitoid, A. ervi. Therefore, when natural enemies were together and densities of cowpea aphids were reduced by generalist predators, parasitism of pea aphids increased. This interaction modification is similar to other types of indirect interactions among enemy species (e.g. predator–predator facilitation) that can enhance the suppression of agricultural pests. Results of our study, and those of others performed in agroecosystems, complement the broader debate over how biodiversity influences ecosystem functioning by specifically focusing on systems that produce goods of immediate relevance to human society.     

Intraspecific heritable variation in life-history traits can alter the outcome of interspecific competition among insect herbivores

Competition is one of the most important biotic factors determining the structure of ecological communities. In this study, we show that there is variation in competitive ability between two clones of the pea aphid, Acyrthosiphon pisum, both of which out-compete a clone of the vetch aphid, Megoura viciae, in the laboratory. We tested whether this variation in competitive ability would alter the outcome of interspecific competition in the field. While one pea aphid clone followed the pattern set in the laboratory, out-competing the Megoura viciae clone, another showed the reverse effect with Megoura viciae dominating. These differences appear to be the result of variation in early population growth rate between the pea aphid clones, rather than predation, although predation did lead to the eventual extinction of colonies. We also questioned whether intra- and interspecific differences in predator escape behaviour could affect the outcome of competition in the field. All three clones responded similarly to the presence of foraging hoverfly larvae (Episyrphus balteatus), but the Megoura viciae clone dropped from the plant significantly less often in response to the presence of a foraging two-spot ladybird (Adalia bipunctata). This work provides evidence that intraspecific variation in competitive ability can alter the outcome of interspecific competitive interactions in nature and suggests that species–specific behavioural traits may have the potential to modify the outcome of these interactions.     

Influence of Experience on the Response of Bathyplectes curculionis (Hymenoptera: Ichneumonidae), a Nonaphidophagous Parasitoid, to Aphid Odor

Bathyplectes curculionis (Thomson) is an introduced natural enemy of the alfalfa weevil in North America. The wasp requires carbohydrate foods as an adult. Adult wasps have increased longevity and fecundity when provided access to pea aphid, Acyrthosiphon pisum (Harris), honeydew in the laboratory, and adults respond positively to the presence of pea aphids in alfalfa fields. However, it is unknown how these wasps find aphid honeydew in the field. In a series of Y-tube olfactometer experiments, we evaluated the response of naïve and experienced adult female B. curculionis to odors from pea aphids, alfalfa, and pea aphids on alfalfa. Naïve adult females did not respond positively to pea aphid odor even when hungry. But adult females were able to learn aphid odor, and the mechanism of learning appears to be associative rather than by sensitization. Naïve females also showed no preference for alfalfa odor but learned alfalfa odor through sensitization. The wasps did not distinguish between alfalfa with aphids and alfalfa without aphids, even after exposure to aphids or alfalfa with aphids. However, they preferred pea aphid odor to alfalfa odor after a feeding experience in the presence of pea aphid odors. But after exposure to mixed odors of aphids and alfalfa while feeding, B. curculionis females preferred the odor of alfalfa to the odor of pea aphids. These results suggest that alfalfa odors mask or override aphid odors when aphids are associated with alfalfa (as happens naturally), thus interfering with the wasp's ability to respond to learned aphid odors. Therefore, although the wasps are capable of learning to find pea aphids and their honeydew in a simplified laboratory setting, it appears unlikely that they do so in the field.     

Combining intercropping with semiochemical releases: optimization of alternative control of Sitobion avenae in wheat crops in China

Overreliance on pesticides has large environmental and human health costs that compel researchers and farmers to seek alternative management tactics for crop pests. For insect pests, increasing crop species diversity via intercropping and using semiochemicals to alter local arthropod populations have separately proven effective at reducing pest densities. Here, we combine these two tactics in an effort to gain better control of Sitobion avenae (Fabricius) (Hemiptera: Aphididae), the English grain aphid, a major pest of cereal production worldwide. We conducted field experiments over 2 years testing the effectiveness of combining intercropping of wheat and oilseed rape with release of methyl salicylate (MeSA). We found that maximum and mean aphid densities were highest in wheat monocultures, significantly lower in intercropped plots and MeSA plots, and lowest when intercropping and MeSA release were combined by obtaining highest densities of predatory lady beetles and parasitoids rates. Importantly, grain yield and quality showed a similar pattern: they were highest for combined intercropped/MeSA plots, intermediate in plots with intercropping or MeSA alone, and lowest in control monoculture plots. Our results suggest that combining these two tactics holds significant promise for improved management of aphid populations and emphasize the need to integrate alternative pest control approaches to optimize sustainable insect pest management.     

Assessment of fennel aphids (Hemiptera: Aphididae) and their predators in fennel intercropped with cotton with colored fibers

The fennel aphid, Hyadaphis foeniculi (Passerini) (Hemiptera: Aphididae) is a major pest of fennel, Foeniculum vulgare Miller in northeast region of Brazil. We hypothesize that intercropping can be used as an alternative pest management strategy to reduce aphid yield loss in fennel. Thus, we investigated the severity of fennel plant damage in relation to infestation by the fennel aphid and predation by Cycloneda sanguinea (L.) (Coleoptera: Coccinellidae) (spotless lady beetle), green lacewing, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae), and Scymnus spp. (Coleoptera: Coccinellidae) in sole fennel plots and plots of fennel intercropped with cotton with colored fibers. The fennel aphid populations in nontreated plots were significantly larger in sole fennel plots than in intercropped plots. The highest densities of C. sanguinea, green lacewings and Scymnus spp., associated with the suppression of fennel aphid populations was found in fennel in the intercropping systems. Fennel aphids reduced the fennel seed yield by 80% in the sole fennel plots compared with approximately 30% for all intercropping systems. The results obtained in this research are of practical significance for designing appropriate strategies for fennel aphid control in fennel-cotton intercropping systems. In summary, intercropping fennel with cotton with colored fibers apparently promoted biocontrol of fennel aphid in fennel.     

Has the Attraction of Predatory Coccinellids to Cornicle Droplets Constrained Aphid Alarm Signaling Behavior?

When attacked by a predator, aphids of many species secrete cornicle droplets, containing an alarm pheromone, that results in the dispersal of nearby conspecifics. As females are parthenogenetic, alarm signaling functions to enhance the survival of clone-mates. Enigmatically, however, aphids are physically able to, but usually do not emit alarm pheromone when initially detecting a predator, but rather signal only when captured by a predator. We hypothesized that cornicle droplets may be attractive to natural enemies and result in an increased risk of predation for the signaler, thereby selecting for prudent alarm signalers. We tested this hypothesis by investigating the olfactory cues that the multicolored Asian ladybird beetle, Harmonia axyridis Pallas, uses to locate pea aphids, Acyrthosiphon pisum. In choice tests, H. axyridis were attracted to odors from pea aphid colonies, whether feeding or not feeding on a host plant leaf, but were not attracted to cornicle droplets containing alarm pheromone. Further, individual pea aphids emitting cornicle droplets were not located more often or in a shorter period of time by beetles than aphids not emitting cornicle droplets. Thus, the cost of emitting early alarm signals is not prohibitively high in regards to the attraction of predators such as H. axyridis.     

Tri-trophic interactions involving pest aphids,predatory 2-spot ladybirds and transgenic potatoes expressing snowdrop lectin for aphid resistance

Transgenic crops genetically engineered for enhanced insect resistance should be compatible with other components of IPM for the pest resistance to be durable and effective. An experimental potato line was genetically engineered to express an anti-aphid plant protein (snowdrop lectin, GNA), and assessed for possible interactions of the insect resistance gene with a beneficial pest predator. These extended laboratory studies are the first to demonstrate adverse tri-trophic interactions involving a lectin- expressing transgenic crop, a target pest aphid and a beneficial aphidophagous predator. When adult 2-spot ladybirds (Adalia bipunctata[L.]) were fed for 12 days on peach-potato aphids (Myzus persicae Sulzer) colonising transgenic potatoes expressing GNA in leaves, ladybird fecundity, egg viability and longevity significantly decreased over the following 2–3 weeks. No acute toxicity due to the transgenic plants was observed, although female ladybird longevity was reduced by up to 51%. Adverse effects on ladybird reproduction, caused by eating peach-potato aphids from transgenic potatoes, were reversed after switching ladybirds to feeding on pea aphids from non-transgenic bean plants. These results demonstrate that expression of a lectin gene for insect resistance in a transgenic potato line can cause adverse effects to a predatory ladybird via aphids in its food chain. The significance of these potential ecological risks under field conditions need to be further evaluated.     

Natural enemy composition rather than richness determines pest suppression

Natural enemy (NE) biodiversity is thought to play an important role in agricultural pest suppression. However, the relative importance of the number of NE species (species richness), versus the particular combinations of species (species composition), in determining aphid suppression and ultimately crop yields, remains poorly understood. We tested the effects of NE richness and composition on pea aphids Acyrthosiphon pisum (Harris) and broad bean plants Vicia faba (Linn.). We used the larvae of two predator species, the ladybird Adalia bipunctata (Linn.) and the green lacewing Chrysopa carnea (Stephens), and the parasitic wasp Aphidius ervi (Haliday) as enemies. NEs generally reduced aphid density but did not increase final plant biomass, despite a significant negative correlation between aphid density and plant biomass. Among NE treatments, species richness had an inconsistent effect on aphid density. The composition of NEs within richness levels also affected final aphid density: the ladybird was a key species among the treatments in controlling aphid density and was especially effective in combination with the parasitoid. This ladybird/parasitoid combination also appeared to drive the higher level of suppression observed at the two, relative to three, species richness levels. Although these three species of aphid NEs are commonly used in aphid control, this is the first study, to our knowledge, that simultaneously examined these three species and highlighted the composition effect between the A. bipunctata and A. ervi. In conclusion, increasing NE species richness had an inconsistent effect on aphid density. Meanwhile, the presence of a key species (the ladybird) and its combination with a parasitoid was an important determinant of aphid biological control.     

Detection and decay rates of prey and prey symbionts in the gut of a predator through metagenomics

DNA methods are useful to identify ingested prey items from the gut of predators, but reliable detection is hampered by low amounts of degraded DNA. PCR‐based methods can retrieve minute amounts of starting material but suffer from amplification biases and cross‐reactions with the predator and related species genomes. Here, we use PCR‐free direct shotgun sequencing of total DNA isolated from the gut of the harlequin ladybird Harmonia axyridis at five time points after feeding on a single pea aphid Acyrthosiphon pisum. Sequence reads were matched to three reference databases: Insecta mitogenomes of 587 species, including H. axyridis sequenced here; A. pisum nuclear genome scaffolds; and scaffolds and complete genomes of 13 potential bacterial symbionts. Immediately after feeding, multicopy mtDNA of A. pisum was detected in tens of reads, while hundreds of matches to nuclear scaffolds were detected. Aphid nuclear DNA and mtDNA decayed at similar rates (0.281 and 0.11 h^?1 respectively), and the detectability periods were 32.7 and 23.1 h. Metagenomic sequencing also revealed thousands of reads of the obligate Buchnera aphidicola and facultative Regiella insecticola aphid symbionts, which showed exponential decay rates significantly faster than aphid DNA (0.694 and 0.80 h^?1, respectively). However, the facultative aphid symbionts Hamiltonella defensa, Arsenophonus spp. and Serratia symbiotica showed an unexpected temporary increase in population size by 1–2 orders of magnitude in the predator guts before declining. Metagenomics is a powerful tool that can reveal complex relationships and the dynamics of interactions among predators, prey and their symbionts.     

Aphid population dynamics: does resistance to parasitism influence population size?

1. Insect population size is regulated by both intrinsic traits of organisms and extrinsic factors. The impacts of natural enemies are typically considered to be extrinsic factors, however insects have traits that affect their vulnerability to attack by natural enemies, and thus intrinsic and extrinsic factors can interact in their effects on population size. 2. Pea aphids Acyrthosiphon pisum Harris (Hemiptera: Aphididae) in New York and Maryland that are specialised on alfalfa are approximately two times more physiologically resistant to parasitism by Aphidius ervi Haliday (Hymenoptera: Braconidae) than pea aphids specialised on clover. To assess the potential influence of this genetically based difference in resistance to parasitism on pea aphid population dynamics, pea aphids, A. ervi, and other natural enemies of aphids in clover and alfalfa fields were sampled. 3. Rates of successful parasitism by A. ervi were higher and pea aphid population sizes were lower in clover, where the aphids are less resistant to parasitism. In contrast, mortality due to a fungal pathogen of pea aphids was higher in alfalfa. Generalist aphid predators did not differ significantly in density between the crops. 4. To explore whether intrinsic resistance to parasitism influences field dynamics, the relationship between resistance and successful field parasitism in 12 populations was analysed. The average level of resistance of a population strongly predicts rates of successful parasitism in the field. The ability of the parasitoid to regulate the aphid may vary among pea aphid populations of different levels of resistance.     

Intercropping impacts the host location behaviour and population growth of aphids

Increasing intrafield plant diversity has been shown to regulate pest populations in various agroecosystems. Among the suggested mechanisms for this bottom-up pest control, the disruptive crop hypothesis states that herbivores' abilities to locate and colonize their host plants are reduced by the presence of non-host plants. Under laboratory conditions, we evaluated how intercropping wheat and legumes modifies the behaviour of apterous cereal aphids, Sitobion avenae (Fabricius) (Hemiptera: Aphididae), in terms of host plant location and population growth. We compared two intercropping systems – soft winter wheat, Triticum aestivum L. (Poaceae), associated with winter pea, Pisum sativum L., or with white clover, Trifolium repens L. (both Fabaceae) – and sole stands of soft winter wheat. Aphids needed more time to locate their wheat host plant and then spent less time on wheat when it was intercropped with clover. At the population level, and accounting for host plant biomass, only intercropping wheat with clover significantly reduced aphid densities on wheat, as this was particularly disruptive to S. avenae behaviour and population growth. Our laboratory study points out that the species used as non-host plants and their density are important parameters that should be taken into account in field studies on intercropping systems.