Michael A. Birkett

Aphid alarm pheromone produced by transgenic plants affects aphid and parasitoid behavior

The alarm pheromone for many species of aphids, which causes dispersion in response to attack by predators or parasitoids, consists of the sesquiterpene (E)-β-farnesene (Eβf). We used high levels of expression in Arabidopsis thaliana plants of an Eβf synthase gene cloned from Mentha × piperita to cause emission of pure Eβf. These plants elicited potent effects on behavior of the aphid Myzus persicae (alarm and repellent responses) and its parasitoid Diaeretiella rapae (an arrestant response). Here, we report the transformation of a plant to produce an insect pheromone and demonstrate that the resulting emission affects behavioral responses at two trophic levels.

Identification of Human-Derived Volatile Chemicals that Interfere with Attraction of Aedes aegypti Mosquitoes

It is known that human individuals show different levels of attractiveness to mosquitoes. In this study, we investigated the chemical basis for low attractiveness. We recorded behaviors of Aedes aegypti toward the hands of human volunteers and toward the volatile chemicals produced by their bodies. Some individuals, and their corresponding volatiles, elicited low upwind flight, relative attraction, and probing activity. Analyzing the components by gas chromatography coupled to electrophysiological recordings from the antennae of Aedes aegypti, enabled the location of 33 physiologically relevant compounds. The results indicated that higher levels of specific compounds may be responsible for decreased “attractiveness.” In behavioral experiments, five of the compounds caused a significant reduction in upwind flight of Aedes aegypti to attractive human hands. Thus, unattractiveness of individuals may result from a repellent, or attractant “masking,” mechanism.

cis-Jasmone induces Arabidopsis genes that affect the chemical ecology of multitrophic interactions with aphids and their parasitoids.

It is of adaptive value for a plant to prepare its defenses when a threat is detected, and certain plant volatiles associated with insect damage, such as cis-jasmone (CJ), are known to switch-on defense metabolism. We used aphid and aphid parasitoid responses to Arabidopsis thaliana as a model system for studying gene expression and defense chemistry and its impact at different trophic levels. Differential responses to volatiles of induced Arabidopsis occurred for specialist and generalist insects: the generalist aphid, Myzus persicae, was repelled, whereas the specialist, Lipaphis erysimi, was attracted; the generalist aphid parasitoid Aphidius ervi was attracted, but the specialist parasitoid Diaeretiella rapae was not affected. A. ervi also spent longer foraging on induced plants than on untreated ones. Transcriptomic analyses of CJ-induced Arabidopsis plants revealed that a limited number of genes, including a gene for a cytochrome P450, CYP81D11, were strongly up-regulated in the treated plants. We examined transgenic Arabidopsis lines constitutively overexpressing this gene in bioassays and found insect responses similar to those obtained for wild-type plants induced with CJ, indicating the importance of this gene in the CJ-activated defense response. Genes involved in glucosinolate biosynthesis and catabolism are unaffected by CJ and, because these genes relate to interactions with herbivores and parasitoids specific to this family of plants (Brassicaceae), this finding may explain the differences in behavioral response of specialist and generalist insects.

The role of volatile semiochemicals in mediating host location and selection by nuisance and disease-transmitting cattle flies.

Abstract.  The role of volatile semiochemicals in mediating the location and selection within herds of Holstein‐Friesian heifers by nuisance and disease‐transmitting cattle flies was investigated using coupled gas chromatography–electrophysiology (GC–EAG), coupled gas chromatography–mass spectrometry (GC–MS), electrophysiology (EAG), laboratory behaviour and field studies. Using volatile extracts collected by air entrainment from heifers in the Netherlands, a number of active peaks were located by coupled GC–EAG for Musca autumnalis (de Geer) (Diptera: Muscidae) and Haematobia irritans (L.) (Diptera: Muscidae). Volatile samples were also collected from two heifers in Denmark shown in previous counting experiments to differ significantly in their fly loads. Coupled GC–EAG using Ha. irritans antennae revealed differences in the EAG response to the samples, with additional EAG activity in the sample collected from the heifer with the lower fly load. To identify more EAG active compounds, volatiles were also collected from 48‐h‐old urine by air entrainment. In total, 23 compounds were located and identified by coupled GC–EAG and GC–MS. Further electrophysiological testing of these compounds with five fly species [M. autumnalis, Ha. irritans, Hydrotaea irritans (L.) (Diptera: Muscidae), Stomoxys calcitrans (L.) (Diptera: Musicidae) and Wohlfahrtia magnifica (Schiner) (Diptera: Sarcophagidae)] showed that only some of the compounds were physiologically active across the range of flies tested. These included 1‐octen‐3‐ol, 6‐methyl‐5‐hepten‐2‐one, (Z)‐3‐hexen‐1‐ol, naphthalene, and all EAG active compounds identified from urine. Compounds showing significant EAG activity were tested for behavioural activity using a wind‐tunnel designed for measuring upwind flight behaviour. At certain concentrations, 1‐octen‐3‐ol, 6‐methyl‐5‐hepten‐2‐one and 3‐octanol increased upwind flight, whereas naphthalene, propyl butanoate and linalool reduced upwind flight. In field studies using small herds of heifers ranked according to their fly load, individual slow‐release formulations of 1‐octen‐3‐ol and 6‐methyl‐5‐hepten‐2‐one, when applied to low and high fly loading heifers, reduced fly loads on these individuals. This study provides evidence for the hypothesis that the natural differential attractiveness within herds of Holstein‐Freisian heifers, i.e. a single host species, for cattle flies is partly due to differences in volatile semiochemicals emitted from the host. It is suggested that this phenomenon applies to other vertebrate host species and their associated insect pests.

Response of the Seven-spot Ladybird to an Aphid Alarm Pheromone and an Alarm Pheromone Inhibitor is Mediated by Paired Olfactory Cells

Electrophysiological responses of adult seven-spot ladybirds, Coccinella septempunctata, to (E)-β-farnesene, an aphid alarm pheromone, and (−)-β-caryophyllene, a plant-derived alarm pheromone inhibitor, were investigated by recording from single olfactory cells (neurons) on the antenna. Cells having high specificity for each of the two compounds were identified. Furthermore, these two cell types were frequently found in close proximity, with a larger amplitude consistently recorded for the cell responding specifically to (E)-β-farnesene. Preliminary behavioral studies in a two-way olfactometer showed that walking adults were significantly attracted to (E)-β-farnesene; this activity was inhibited with increasing proportions of (−)-β-caryophyllene. The possible ecological significance of colocation or pairing of olfactory cells for semiochemicals with different behavioral roles is discussed.

Identification of Volatile Compounds Used in Host Location by the Black Bean Aphid, Aphis fabae

Behavioral and electrophysiological responses of winged Aphis fabae to volatiles of faba bean, Vicia faba (var. Sutton dwarf), plants were studied and semiochemicals used in host location were identified. In olfactometer bioassays, aphids spent significantly more time in the region of the olfactometer where V. faba volatiles from an intact plant were present than in control regions with clean air. This response also occurred when an air entrainment sample of a V. faba plant was used as the odor source. Coupled gas chromatography–electroantennography revealed the presence of 16 electrophysiologically active compounds in the air entrainment sample. Fifteen of these were identified as (Z)-3-hexen-1-ol, 1-hexanol, (E)-2-hexenal, benzaldehyde, 6-methyl-5-hepten-2-one, octanal, (Z)-3-hexen-1-yl acetate, (R)-(−)-linalool, methyl salicylate, decanal, undecanal, (E)-caryophyllene, (E)-β-farnesene, (S)-(−)-germacrene D, and (E,E,)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. An olfactometer response was observed to a 15-component synthetic blend that comprised all identified compounds at the same concentration and ratio as in the natural sample, with the aphids spending significantly more time in the treated regions of the olfactometer where volatiles were present than in the control regions. These data are discussed in the context of insect host location and crop protection.

Volatiles from whitefly-infested plants elicit a host-locating response in the parasitoid, Encarsia formosa.

The blend of volatile compounds emitted by bean plants (Phaseolus vulgaris) infested with greenhouse whitefly (Trialeurodes vaporariorum) has been studied comparatively with undamaged plants and whiteflies themselves. Collection of the volatiles and analysis by gas chromatography revealed more than 20 compounds produced by plants infested with whitefly. Of these, 4 compounds, (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, 3-octanone, and one unidentified compound were emitted at higher levels than from the undamaged control plants. Synthetic (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, or 3-octanone all elicited a significant increase in oriented flight and landing on the source by the parasitoid, Encarsia formosa, in wind tunnel bioassays. Two-component mixtures of the compounds and the three-component mixture all elicited a similar or, in most cases, a better response by the parasitoid, the most effective being a mixture of (Z)-3-hexen-1-ol and 3-octanone. These results demonstrate that E. formosa uses volatiles from the plant-host complex as olfactory cues for host location.

Electroantennogram and behavioural responses of the malaria vector Anopheles gambiae to human-specific sweat components.

Abstract. Afrotropical malaria vectors of the Anopheles gambiae complex (Diptera: Culicidae), particularly An. gambiae sensu stricto, are attracted mainly to human hosts. A major source of human volatile emissions is sweat, from which key human‐specific components are the carboxylic acids (E)‐ and (Z)‐3‐methyl‐2‐hexenoic acid and 7‐octenoic acid. Electrophysiological studies on the antennae of An. gambiae s.s. showed selective sensitivity to these compounds, with a threshold at 10−6 g comparable to that of known olfactory stimulants 1‐octen‐3‐ol, p‐cresol, isovaleric acid, and lower than threshold sensitivity to l‐lactic acid and the synthetic mosquito repellent N,N‐diethyltoluamide (DEET). A combination of the acids released at concentrations > 10−5 g in wind tunnel bioassays significantly reduced the response to CO2, the major attractant released by human hosts, for strains of An. gambiae s.s. originating from East and West Africa.

ANTENNAL ELECTROPHYSIOLOGICAL RESPONSES OF THREE PARASITIC WASPS TO CATERPILLAR-INDUCED VOLATILES FROM MAIZE (Zea mays mays), COTTON (Gossypium herbaceum), AND COWPEA (Vigna unguiculata)

Many parasitic wasps are attracted to volatiles that are released by plants when attacked by potential hosts. The attractiveness of these semiochemicals from damaged plants has been demonstrated in many tritrophic systems, but the physiological mechanisms underlying the insect responses are poorly understood. We recorded the antennal perception by three parasitoids (Cotesia marginiventris, Microplitis rufiventris, and Campoletis sonorensis) to volatiles emitted by maize, cowpea, and cotton plants after attack by the common caterpillar pest Spodoptera littoralis. Gas chromatography-electroantennography (GC-EAG) recordings showed that wasps responded to many, but not all, of the compounds present at the physiologically relevant levels tested. Interestingly, some minor compounds, still unidentified, elicited strong responses from the wasps. These results indicate that wasps are able to detect many odorant compounds released by the plants. It remains to be determined how this information is processed and leads to the specific behavior of the parasitoids.

Ladybird beetle odour identified and found to be responsible for attraction between adults

Abstract. The distinctive odour of the seven-spot ladybird beetle, Coccinella septempunctata, had not previously been identified unequivocally, although it was considered likely to arise from olfactorily potent pyrazines. The component responsible was located by human organoleptic evaluation of the gas chromatography effluent from ladybird volatiles and was fully characterized as 2-isopropyl-3-methoxypyrazine. Although insects may not be expected to have similar olfactory sensitivities to human beings, this compound was found to fulfil a pheromonal role in the attraction between adult C. septempunctata, typical of many ladybird species. Thus, in a behavioural assay, both males and females were attracted by amounts of this pyrazine equivalent to the ladybird volatiles.