Alex Stewart-Jones

Comparison of Glass Vessels and Plastic Bags for Enclosing Living Plant Parts for Headspace Analysis

Plants release volatile chemicals into their surrounding air space that can affect the physiology of neighboring plants and influence the behavior of insects. In studying these interactions, it is desirable to collect volatiles from plants that have not been excised and are growing under as natural conditions as possible. We compared a vessel of borosilicate glass and Nylon-6 or polyester [poly(ethyleneterephthalate) or PET] cooking bags for enclosing plants during collection of volatiles. A push–pull airflow system was used, and volatiles were trapped on Tenax TA and analyzed by gas chromatography after thermal desorption. Low levels of impurities were found for the glass vessel and polyester bags. Nylon bags contained higher levels and more impurities. Recoveries of standards of 10 plant volatiles were measured in static and dynamic systems. In a static air system, there was good recovery only from the glass vessel. In a dynamic system, there was generally good recovery from both the glass vessel and polyester bags. Recoveries of α-pinene and (Z)-jasmone were poor throughout. The former was shown to have a very low breakthrough volume on the Tenax TA adsorbent, and the latter may be strongly adsorbed on glass. All three materials were essentially transparent in the IR and visible (photosynthetic) range but with significantly different absorptions in the UV range. In a simulated dynamic entrainment in full sunlight, internal vessel temperatures were higher than ambient by up to 9.5°C in the glass vessel and 7.5°C in the polyester bag. Lower increases in temperature relative to ambient (<1°C) were recorded when entrainments were conducted in the shade. In a field trial, the profiles of volatiles collected from an apple tree infested with rosy apple aphid using a glass vessel and a polyester bag were similar. Polyester bags are recommended as more convenient than glass vessels for the enclosure of plants during the collection of volatiles.

Varying responses of insect herbivores to altered plant chemistry under organic and conventional treatments

The hypothesis that plants supplied with organic fertilizers are better defended against insect herbivores than those supplied with synthetic fertilizers was tested over two field seasons. Organic and synthetic fertilizer treatments at two nitrogen concentrations were supplied to Brassica plants, and their effects on the abundance of herbivore species and plant chemistry were assessed. The organic treatments also differed in fertilizer type: a green manure was used for the low-nitrogen treatment, while the high-nitrogen treatment contained green and animal manures. Two aphid species showed different responses to fertilizers: the Brassica specialist Brevicoryne brassicae was more abundant on organically fertilized plants, while the generalist Myzus persicae had higher populations on synthetically fertilized plants. The diamondback moth Plutella xylostella (a crucifer specialist) was more abundant on synthetically fertilized plants and preferred to oviposit on these plants. Glucosinolate concentrations were up to three times greater on plants grown in the organic treatments, while foliar nitrogen was maximized on plants under the higher of the synthetic fertilizer treatments. The varying response of herbivore species to these strong differences in plant chemistry demonstrates that hypotheses on defence in organically grown crops have over-simplified the response of phytophagous insects.

Fertilizer affects the behaviour and performance of Plutella xylostella on brassicas

1 Foliar nitrogen concentration, which can be manipulated in crop plants by fertilizer supply, has long been recognized as a major factor in phytophagous insect abundance and performance. More recently, the type of fertilizer supplied has been shown to influence the abundance of some herbivore species. The diamondback moth Plutella xylostella is a global pest of Brassica crops. Although it has been the subject of numerous studies on host‐plant resistance and pest control, few studies have addressed the effect of abiotic factors, such as nutrient supply, on its performance and behaviour.

Structure, ratios and patterns of release in the sex pheromone of an aphid, Dysaphis plantaginea.

SUMMARY Insect communication is primarily via chemicals. In Aphidinae aphids, the structure and ratio of iridoid (monoterpenoid) chemicals are known to be important components of the sex pheromone. However, for enhanced species specificity, it has been suggested that release of sex pheromone might be restricted to a narrow time period within the diel cycle. Here, we determine the structure, ratios and release patterns of iridoid chemicals produced by a serious global pest, the rosy apple aphid, Dysaphis plantaginea. Volatiles were collected from batches of oviparae (sexual females) and chemicals identified by gas chromatography, mass-spectrometry and microscale NMR spectroscopy. (1R,4aS,7S,7aR)-Nepetalactol and (4aS,7S,7aR)-nepetalactone were detected in a 3.7:1 ratio. To investigate timing of release, we constructed a sequential sampling device that allowed volatile chemicals to be captured hourly from 95 same-aged oviparae over 20 consecutive days. Release patterns of the two sex pheromone components show that D. plantaginea oviparae release high levels of the two components during photophase and low levels during scotophase. Release of the two components increased significantly during the first 3 h of photophase and thereafter remained at a high level until the onset of scotophase. The ratio of (1R,4aS,7S,7aR)-nepetalactol to (4aS,7S,7aR)-nepetalactone released did not change significantly between days two to 14 of the adult stadium, but from the 15th day onward there was a significant decrease in the relative amount of (1R,4aS,7S,7aR)-nepetalactol. Pheromone release was greatest on the eighth day of the adult stadium, with up to 8.4 ng of pheromone released per ovipara per hour. This is the first report on the full structural identification and ratios of volatile iridoid components collected from D. plantaginea oviparae and is also the most detailed temporal study on sex pheromone release from any aphid species. The lack of a temporally narrow and distinct period of very high sex pheromone release suggests that alternative mechanisms or factors for species recognition and isolation may be important. Findings are discussed broadly in relation to the biology of the aphid.

Dolichodial: a new aphid sex pheromone component?

The sex pheromones of many aphid species from the subfamily Aphididae comprise a mixture of the iridoids (cyclopentanoids) (1R,4aS,7S,7aR)-nepetalactol and (4aS,7S,7aR)-nepetalactone. In this paper, we investigate whether other chemicals, in addition to nepetalactol and nepetalactone, are released from Dysaphis plantaginea (rosy apple aphid) oviparae as part of their sex pheromone. Four compounds present in an air entrainment sample collected from D. plantaginea oviparae feeding on apple (Malus silvestris c.v. Braburn) elicited electrophysiological responses from male D. plantaginea. Active peaks were tentatively identified by gas chromatography (GC) coupled with mass spectrometry, with identification confirmed by peak enhancement with authentic compounds on GC columns of different polarities. The electroantennography-active chemicals were (1R,4aS,7S,7aR)-nepetalactol, (4aS,7S,7aR)-nepetalactone, (1S,2R,3S)-dolichodial, and phenylacetonitrile. (1S,2R,3S)-Dolichodial elicited a behavioral response from male D. plantaginea and naïve-mated female parasitoids, Aphidius ervi. This is the first report of electrophysiological and behavioral responses from any aphid morph to (1S,2R,3S)-dolichodial. Whether or not (1S,2R,3S)-dolichodial is a third component of the aphid sex pheromone is discussed.

Organic and conventional fertilizer effects on a tritrophic interaction: parasitism, performance and preference of Cotesia vestalis

Interest in sustainable farming methods that rely on alternatives to conventional synthetic fertilizers and pesticides is increasing. Sustainable farming methods often utilize natural populations of predatory and parasitic species to control populations of herbivores, which may be potential pest species. We investigated the effects of several types of fertilizer, including those typical of sustainable and conventional farming systems, on the interaction between a herbivore and parasitoid. The effects of fertilizer type on percentage parasitism, parasitoid performance, parasitoid attack behaviour and responses to plant volatiles were examined using a model Brassica system, consisting of Brassica oleracea var capitata, Plutella xylostella (Lepidoptera) larvae and Cotesia vestalis (parasitoid). Percentage parasitism was greatest for P. xylostella larvae feeding on plants that had received either a synthetic ammonium nitrate fertilizer or were unfertilized, in comparison to those receiving a composite fertilizer containing hoof and horn. Parasitism was intermediate on plants fertilized with an organically produced animal manure. Male parasitoid tibia length showed the same pattern as percentage parasitism, an indication that offspring performance was maximized on the treatments preferred by female parasitoids for oviposition. Percentage parasitism and parasitoid size were not correlated with foliar nitrogen concentration. The parasitoids did not discriminate between hosts feeding on plants in the four fertilizer treatments in parasitoid behaviour assays, but showed a preference for unfertilized plants in olfactometer experiments. The percentage parasitism and tibia length results provide support for the preference–performance hypothesis.

Luring houseflies (Musca domestica) to traps: do cuticular hydrocarbons and visual cues increase catch?

Abstract Houseflies (Musca domestica L.) are a major pest species of livestock units and landfill sites. Insecticide resistance has resulted in an increased emphasis on lure‐and‐kill control methods, but the success of this approach relies on the effective attraction of houseflies with olfactory or visual stimuli. This study examined the efficacy of olfactory (cuticular hydrocarbons) or visual (colours and groups of flies) attractants in a commercial poultry unit. Despite simulating the cuticular hydrocarbon profiles of male and female houseflies, we found no significant increase in the number of individuals lured to traps and no sex‐specific responses were evident. The use of target colours selected to match the three peaks in housefly visual spectral sensitivity yielded no significant increase in the catch rate of traps to which they were applied. This study also demonstrated that male and female flies possess significantly different spectral reflectance (males are brighter at 320–470 nm; females are brighter at 470–670 nm). An experiment incorporating groups of recently killed flies from which cuticular hydrocarbons were either removed by solvent or left intact also failed to show any evidence of olfactory or visual attraction for houseflies of either sex. This study concluded that variations of the most commonly applied methods of luring houseflies to traps in commercial livestock units fail to significantly increase capture rates. These results support commonly observed inconsistencies associated with using olfactory or visual stimuli in lure‐and‐kill systems, possibly because field conditions lessen the attractant properties observed in laboratory experiments.

Solvent extraction of cues in the dust and frass of Prostephanus truncatus and analysis of behavioural mechanisms leading to arrestment of the predator Teretrius nigrescens

Abstract.  Teretrius nigrescens is a predatory beetle released into Africa for classical biological control of Prostephanus truncatus, currently the most serious insect pest of stored maize on the continent. T. nigrescens is arrested by the dust and frass produced by P. truncatus boring into maize and it has been concluded that this effect is most likely to be caused by chemicals in the dust/frass. A bioassay is presented in which EthoVision software (Noldus Information Technology, the Netherlands) captures and analyses the movement of individual insects within arenas containing zones to which solvent extracts are applied. The results demonstrate that cues enabling adult T. nigrescens to discriminate between maize flour and dust/frass are extractable in hexane, methanol and chloroform, but not water. Such discrimination is manifested by an increase in residence time, distance covered and path sinuosity in the dust/frass zone. Analysis of behaviour demonstrates cues that trigger inverse orthokinetic mechanisms contributing to arrestment in adult T. nigrescens. An increase in angular velocity would also suggest a direct klinokinetic component, although this parameter could be affected by postulated tropotactic and/or transverse klinotactic and/or longitudinal klinotactic mechanisms occurring at the edge of the application zone. Development of this bioassay, for the first time, permits responses of larval T. nigrescens to prey cues to be assessed. The results show that larvae apparently respond to cues extracted from dust/frass in a stronger manner than adults, with significant increases in the same descriptive parameters of time, distance and path sinuosity. However, unlike in the adults, there is no orthokinesis. It is proposed that strong larval response is attributable to either direct klinokinesis and/or tropotaxis interacting with transverse klinotaxis. Observations of larval backtracking at the zone boundary may also indicate longitudinal klinotaxis. The reasons for the differences in the adult and larval response to prey cues are considered, and how the described behavioural mechanisms combine to improve prey foraging is discussed.

Neem chemicals disturb the behavioral response of Liriomyza huidobrensis to conspecific-induced potato volatiles*

Plants respond to insect attack by releasing complex blends of phytogenic volatile organic compounds. Selection of a host plant by the leafminer Liriomyza huidobrensis is influenced by such volatile chemicals (allelochemicals) released by the host plant. In the case of potato plants, the allelochemicals identified are mainly volatile monoterpenes and sesquiterpenes. A potato plant that is already infested by other individuals of the same species of insects shows a quantitative difference in emission of such allelochemicals, which in turn plays an important role in the host preference behavior exhibited by the insect. Olfactometer tests (using whole, potted potato plants as well as excised leaves) showed that L. huidobrensis adults were more attracted to already infested plants for feeding and egg-laying than they were to uninfested plants, demonstrating that volatile chemicals from infested plants function as kairomones in this insect-plant relationship. However, when the potato plants were grown in soil treated with aqueous neem extract, this relationship was disturbed and the insects did not exhibit any consistent feeding or egg-laying response to neem-treated infested or uninfested host plants. This indicates that neem compounds disturb secondary metabolite production in the potato plant, which in turn has an influence on insects that use volatile plant allelochemicals as cues for host identification and location. Results of this study can help in a better understanding of neem as an environment-friendly botanical pesticide for use in sustainable agriculture.