Host choice and host leaving in Rhopalosiphum padi (Hemiptera: Aphididae) emigrants and repellency of aphid colonies on the winter host

Host choice and winter-host leaving in emigrants of bird cherry–oat aphid, Rhopalosiphum padi (Linnaeus), were investigated in the laboratory. In settling choice tests, emigrants collected from the winter host, Prunus padus, preferred this plant over a summer host, oats. Emigrants which had left P. padus for up to 24 h did not express a preference as a group, and those which had left for 24–48 h preferred oats. Eighty seven percent of emigrants caged as fourth-instar nymphs on P. padus leaves abandoned the host by the second day of adult life, and apparently did not subsequently return to the leaf. In an olfactometer, P. padus leaves which had supported spring generations of R. padi were repellent to emigrants. Volatiles were entrained from uninfested and R. padi-infested P. padus using cut twigs in the laboratory as well as intact twigs on a tree in the field. Entrainment extracts from uninfested P. padus had no effect on emigrants in the olfactometer, whereas those from twigs infested with nymphal emigrants were repellent. The study indicates that in R. padi, host-alternation is driven by behavioural changes which occur in individuals as well as between morphs

Sustainable plant protection for increased food security in a changing climate

The global climate is changing. Rising temperatures in temperate regions are making headlines, but there are a host of changes that may have even greater impact on a global scale, particularly in regions where food security is already delicately balanced. Rising sea levels, changing patterns of rainfall, availability of water and increasing concentration of carbon dioxide in the atmosphere are all likely to affect the biotic environment upon which we depend

Responses of aphid parasitoids to aphid sex pheromones: laboratory and field studies

The behavioural responses of aphid parasitoids (Hymenoptera: Braconidae: Aphidiinae) to aphid sex pheromones were investigated in the laboratory and field. In a wind tunnel bioassay, Aphidius eadyi, Aphidius rhopalosiphi, Diaeretiella rapae, Ephedrus plagiator, Praon myzophagum and Praon volucre responded to the aphid sex pheromone components nepetalactone and nepetalactol. P. myzophagum reared on two different host aphid species showed different responses to combinations of nepetalactone and nepetalactol in the wind tunnel, indicating that long term laboratory rearing may influence parasitoid responses to aphid sex pheromones. The ability of two aphid parasitoids to learn aphid sex pheromones through prior exposure in the presence of host aphids was investigated. The generalist E. plagiator showed evidence of associative learning, whereas the specialist Aphidius ervi did not. When A. ervi was exposed to the pheromone without contact with host aphids, the parasitoid response was reduced by habituation. Exposure to aphid sex pheromone during laboratory host attack trials had no effect on the host attack behaviour of A. ervi. In laboratory cage experiments, aphid sex pheromone lures increased the retention of A. rhopalosiphi, but not by Praon volucre, on aphid-infested plants. In a wind tunnel bioassay, aphid sex pheromone enhanced the attraction of A. ervi to a plant-host complex. In the field, aphid sex pheromone lures increased parasitisation rates by A. rhopalosiphi and P. volucre on aphid-infested potted plants. A series of potted plant experiments indicated that the pheromone may increase parasitisation of aphids by A. rhopalosiphi, but not P. volucre, at a distance of 1m away from the lure. The effect of baiting plots of winter wheat with aphid sex pheromone was investigated in two field experiments. In 1996, the number of parasitoid mummies was higher in baited plots than in unbaited plots, and the synchrony between aphid and parasitoid populations was closer in baited plots. In 1997, aphid sex pheromone had no effect on parasitisation levels. The results are discussed in the context of developing a novel aphid control strategy based on the use of aphid sex pheromones to manipulate parasitoid populations

Responses of aphid parasitoids to aphid sex pheromones: laboratory and field studies

The behavioural responses of aphid parasitoids (Hymenoptera: Braconidae: Aphidiinae) to aphid sex pheromones were investigated in the laboratory and field. In a wind tunnel bioassay, Aphidius eadyi, Aphidius rhopalosiphi, Diaeretiella rapae, Ephedrus plagiator, Praon myzophagum and Praon volucre responded to the aphid sex pheromone components nepetalactone and nepetalactol. P. myzophagum reared on two different host aphid species showed different responses to combinations of nepetalactone and nepetalactol in the wind tunnel, indicating that long term laboratory rearing may influence parasitoid responses to aphid sex pheromones. The ability of two aphid parasitoids to learn aphid sex pheromones through prior exposure in the presence of host aphids was investigated. The generalist E. plagiator showed evidence of associative learning, whereas the specialist Aphidius ervi did not. When A. ervi was exposed to the pheromone without contact with host aphids, the parasitoid response was reduced by habituation. Exposure to aphid sex pheromone during laboratory host attack trials had no effect on the host attack behaviour of A. ervi. In laboratory cage experiments, aphid sex pheromone lures increased the retention of A. rhopalosiphi, but not by Praon volucre, on aphid-infested plants. In a wind tunnel bioassay, aphid sex pheromone enhanced the attraction of A. ervi to a plant-host complex. In the field, aphid sex pheromone lures increased parasitisation rates by A. rhopalosiphi and P. volucre on aphid-infested potted plants. A series of potted plant experiments indicated that the pheromone may increase parasitisation of aphids by A. rhopalosiphi, but not P. volucre, at a distance of 1m away from the lure. The effect of baiting plots of winter wheat with aphid sex pheromone was investigated in two field experiments. In 1996, the number of parasitoid mummies was higher in baited plots than in unbaited plots, and the synchrony between aphid and parasitoid populations was closer in baited plots. In 1997, aphid sex pheromone had no effect on parasitisation levels. The results are discussed in the context of developing a novel aphid control strategy based on the use of aphid sex pheromones to manipulate parasitoid populations

Seed treatment with plant-defense elicitors decreases the abundance of ammonia oxidizers associated with winter wheat roots

Priming with elicitors, including phytohormones, is known to induce physiological changes in plants that affect resource allocation and nutrient uptake strategies. However, effects of these changes on belowground microbial communities, particularly those involved in nitrogen (N) transformations, remain poorly characterized. Here, we tested the hypothesis that treating seeds with elicitors would affect N-transforming communities associated to roots and rhizosphere, influencing soil N availability and thus plant performance. We measured the effects of treating winter wheat seeds with cis-jasmone, methyl jasmonate or methyl salicylate on plant traits and on the genetic potential for ammonia oxidation by ammonia-oxidizing bacteria (AOB) and archaea (AOA), the initial inorganic N transformation process resulting in N loss. Elicitors reduced the genetic potential for ammonia oxidation by AOB in root-associated microbial communities and increased traits reflecting N uptake in winter wheat. Proposed mechanisms include increased exploitative competition for ammonium, limiting substrate availability for microorganisms, and induced biological nitrification inhibition capacity actively suppressing ammonia oxidizers. There were minor effects on the composition of the AOB and AOA communities, but an indicator species analysis showed that several AOB and a few AOA genotypes were indicative of elicitor treatments. Seed treatment with elicitors may represent an overlooked strategy to minimize nitrification to mitigate N losses from arable soils

Olfactory learning of plant genotypes by a polyphagous insect predator

Olfactory learning may allow insects to forage optimally by more efficiently finding and using favourable food sources. Although olfactory learning has been shown in bees, insect herbivores and parasitoids, there are fewer examples from polyphagous predators. In this study, olfactory learning by a predatory coccinellid beetle is reported for the first time. In laboratory trials, adults of the aphidophagous ladybird Coccinella septempunctata did not prefer the odour of one aphid-infested barley cultivar over another. However, after feeding on aphids for 24 h on a cultivar, they preferred the odour of that particular cultivar. The mechanism appeared to be associative learning rather than sensitisation. Although inexperienced ladybirds preferred the odour of an aphid-infested barley cultivar over uninfested plants of the same cultivar, after feeding experience on a different cultivar this preference disappeared. This may indicate the acquisition and replacement of olfactory templates. The odour blends of the different aphid-infested barley cultivars varied qualitatively and quantitatively, providing a potential basis for olfactory discrimination by the ladybird. The results show that predatory coccinellids can learn to associate the odour of aphid-infested plants with the presence of prey, and that this olfactory learning ability is sensitive enough to discriminate variability between different genotypes of the same plant

Landing Preference and Reproduction of Rhopalosiphum padi (Hemiptera: Aphididae) in the Laboratory on Three Maize, Potato, and Wheat Cultivars

The bird cherry-oat aphid Rhopalosiphum padi (L.) transmits the nonpersistent Potato virus Y (PVY) to seed potatoes. Planting a nonvirus host plant around the main crop can reduce PVY incidence, because aphids tend to land in high numbers at the edge of a field and the crop border acts as a virus sink. This study determined R. padi landing and settling preferences and reproductive rates on three cultivars each of maize and wheat compared with potato in the laboratory as a basis for identifying an attractive crop border plant. Aphids were reared on maize and wheat to control for bias due to previous experience. Irrespective of origin, alates preferred to land almost exclusively on maize and wheat rather than on potato cultivars in choice experiments. Aphid settling on the maize and wheat cultivars depended on aphid origin. In no-choice experiments, R. padi produced the highest number of offspring on the wheat cultivars, irrespective of origin. Plant nitrogen content and trichome density did not influence R. padi reproduction. The study demonstrates that host plant preference of aphids may vary between plant cultivars and can therefore influence the effectiveness of a crop border. The high landing rate but low reproduction suggest that maize cultivars '6Q-121' and '78-15B' could be suitable crop border plants in regions where R. padi is abundant. Before testing potential crop border plants in the field, cultivars should be screened using aphid landing, settling and reproduction as selection criteria

Effects of Methyl Salicylate on Host Plant Acceptance and Feeding by the Aphid Rhopalosiphum padi

Methyl salicylate (MeSA) is a volatile shown to act as an inducer of plant defense against pathogens and certain herbivores, particularly aphids. It has been shown to have potential for aphid pest management, but knowledge on its mode of action is lacking, particularly induced plant-mediated effects. This study investigated the effects of exposing plants to MeSA on the host searching, host acceptance and feeding behavior of the bird cherry-oat aphid Rhopalosiphum padi. Barley plants were exposed to volatile MeSA for 24 h, after which biological effects were tested immediately after the exposure (Day 0), and then 1, 3 and 5 days after the end of the exposure. Aphid settling on MeSA-exposed plants was significantly reduced on days 0, 1 and 3, but not on day 5. In olfactometer tests, aphids preferred the odor of unexposed plants on days 1 and 3, but not on day 0 or 5. Analysis of volatiles from exposed and unexposed plants showed higher levels of MeSA from exposed plants, most likely absorbed and re-released from plant surfaces, but also specific changes in other plant volatiles on days 0, 1 and 3. High doses of MeSA did not affect aphid orientation in an olfactometer, but lower doses were repellent. Analysis of aphid feeding by Electronic penetration graph (EPG) showed that MeSA exposure resulted in resistance factors in barley plants, including surface factors and induced systemic factors in other tissues including the phloem. The results support the potential of MeSA as a potential tool for management of aphid pests

Concentration- and time-dependent effects of isothiocyanates produced from Brassicaceae shoot tissues on the pea root rot pathogen Aphanomyces euteiches

Isothiocyanates (ITCs) hydrolyzed from glocosinolates (GSLs) in Brassicaceae tissue are toxic to soil organisms. In this study, the effect of aliphatic and aromatic ITCs from hydrated dy Brassicaceae shoot tissues on the mycelium and oospores of the pea root rot pathogen Aphanomyces euteiches was investigated. The profile and concentrations of GSLs in two test Brassicaceae species, Sinapis alba and Brassica juncea, and the ITCs from the dominant hydrolyzed parent GSLs were monitored. The concentrations of dominant ITCs and pathogen exposure time were evaluated in in vitro experiments. The greatest effect on the pathogen was observed from aliphatic ITCs hydrolyzed from B. juncea tissue, and the effect depended on the ITS concentration and exopsure time. ITCs were more effectively hydrolyzed from B. juncea GSLs than from S. alba GSLs; i.e., the ITC/GSL ratio was higher in B. juncea than in S. albatissue, giving a different release pattern. The release of phenylethyl isothiocyanate, which was common to both species, followed a pattern similar to that of the dominant ITC in each crop speices. This suggests that trait other than GSL content, e.g., plant cell structure, may affect the release of ITCs ans should therefore influence the choice of speices used for biofumigation purposes

Pest suppression in cultivar mixtures is influenced by neighbor-specific plant-plant communication

Increased plant genotypic diversity in crop fields can promote ecosystem services including pest control, but understanding of mechanisms behind herbivore population responses to cultivar mixtures is limited. We studied aphid settling on barley plants exposed to volatiles from different cultivars, aphid population development in monocultures and two-cultivar mixtures, and differences in volatile composition between studied cultivars. Aphid responses to one cultivar in a mixture were neighbor-specific and this was more important for pest suppression than the overall mixture effect, aphid colonization patterns, or natural enemy abundance. Aphid populations decreased most in a mixture where both cultivars showed a reduced aphid-plant acceptance after reciprocal volatile exposure in the laboratory, and reduced population growth compared to monocultures in the field. Our findings suggest that herbivore population responses to crop genotypic diversity can depend on plant-plant volatile interactions, which can lead to changes in herbivore response to individual cultivars in a mixture, resulting in slower population growth. The impact of plant-plant interaction through volatiles on associated herbivore species is rarely considered, but improved understanding of these mechanisms would advance our understanding of the ecological consequences of biodiversity and guide development of sustainable agricultural practices. Combining cultivars in mixtures based on how they interact with each other is a promising strategy for sustainable pest management