Jim Hardie

The aphids' peripheral perception of plant volatiles

In aphids the olfactory receptor neurones are housed in placoid sensilla on the antennae, the so-called distal and proximal primary rhinaria on segments six and five, respectively, and, in winged aphids, the secondary rhinaria on segments three and four (Shambaugh et al, 1978; Bromley et al., 1979). The overall responses of these receptors to plant odours can be studied by the electroantennogram (EAG) technique. In this way, it can be shown which plant volatiles are detected and, thus, are likely to play a role in the discrimination between the odour blends of host and non-host plant species during orientation and landing. EAGs in response to plant volatiles have been recorded in a number of aphid species, i.e., the English grain aphid Sitobion avenae (F.) and the rose-grain aphid Metopolophium dirhodum (Walk.) (Yan & Visser, 1982; Visser & Yan, 1995), the black bean aphid Aphis fabae Scop. (Hardie et al., 1995), and the vetch aphid Megoura viciae Buckton (Visser & Piron, 1994, 1995).

Aphid responses to non-host epicuticular lipids.

Initiation of stylet penetration by aphids depends on the sensory assessment of a number of plant surface features including colour, texture and phytochemicals (volatile and non-volatile). Video recording behaviour of the black bean aphid, Aphis fabae, showed that these insects rapidly inserted their stylets following contact with host plants (beans), but were reluctant to penetrate non-hosts (oats). However, when epicuticular waxes were stripped from oats, using cellulose acetate, aphids penetrated the plant surface significantly earlier than on oats with the wax layer intact. Chloroform extraction of epicuticular lipids, followed by coupled gas chromatography-mass spectrometry, revealed a complex blend of wax components on beans, whereas one compound (1-hexacosanol) predominated on oats. Epicuticular lipids were applied to artificial (glass) substrates in order to investigate their behavioural activity. Initiation of a stylet penetration attempt by A. fabae was delayed when the oat extract or pure 1-hexacosanol were applied, but the bean extract had no behavioural effect. The results suggest that epicuticular lipids play an important role in the early stages of host-plant selection by A. fabae.

ROLE OF THE HELPER COMPONENT IN VECTOR-SPECIFIC TRANSMISSION OF POTYVIRUSES

Four aphid species were tested for their ability to transmit tobacco etch (TEV) and turnip mosaic (TuMV) potyviruses. Myzus persicae and Aphis gossypii transmitted both viruses efficiently from infected plants, whereas Lipaphis erysimi transmitted only TuMV and Myzus ascalonicus was a poor or non-transmitter of either virus. Similar electrically monitored probing patterns were produced by M. persicae, L. erysimi and M. ascalonicus, ruling out behavioural differences as the cause of differential transmission. Transmission results similar to those from infected plants were obtained when these aphids acquired homologous virus/helper component (HC) mixtures through membranes. With heterologous virus/HC mixtures, M. persicae remained a highly efficient vector and M. ascalonicus a non-vector, but L. erysimi became an efficient vector of TEV if acquired in the presence of TuMV HC and A. gossypii transmitted both viruses less efficiently when acquired with TuMV HC. Transmission was highly correlated with the retention of virus in the stylets, as determined by autoradiography of 125I-labelled virions. The results show that constituent(s) of or in the food canal of different aphid species differ in their ability to interact with specific HCs, leading to qualitative or quantitative differences in ability to retain and subsequently transmit specific potyviruses.

Genotypic variation among different phenotypes within aphid clones

Most aphid species (Hemiptera : Aphididae) are parthenogenetic between periods of sexual reproduction. They are also highly polyphenic, with different adult morphs occurring in the life cycle, viz. winged, wingless, asexual and sexual. It is assumed that aphids born in a parthenogenetic clonal lineage are genetically identical regardless of the final adult form (with the exception of sexual forms). Using the randomly amplified polymorphic DNA–polymerase chain reaction (RAPD–PCR) we have found that different asexual adult phenotypes (winged and wingless) of some clones of two cereal aphid species (the grain aphid,Sitobion avenae (F.) and the bird–cherry aphid, Rhopalosiphum padi (L.)) may be distinguished by the presence or absence of one or more RAPD–PCR bands. In three of nine clones examined, such differences were found, and Southern blotting and hybridization of the discriminating bands confirmed these to be of aphid origin, rather than due to endosymbiotic bacteria or contaminating fungi. The main 248 and 296 bp bands, in the two species, respectively, were sequenced and found to be A/T rich. The smaller band showed 57 per cent homology with white striated muscle over a stretch of 90 bp. Genomic DNA treated with dimethyl sulphoxide to remove secondary structures still showed differences in RAPD–PCR profiles between winged and wingless morphs within the unusual clones. This discovery may be widespread and therefore it is important to understand the phenomenon in relation to clonal organisms.

The potential of semiochemicals for control of Phorodon humuli (Homoptera: Aphididae)

Field experiments employing yellow water-traps with vials releasing methyl salicylate, butyl isothiocyanate, 4-pentenyl isothiocyanate and diethyltoluamide were conducted during the spring migration of Phorodon humuli (Schrank), with the aim of identifying substances which might be used in the field to deter landing on hop plants. Methyl salicylate and the two isothiocyanates reduced trap catches of P. humuli. During the spring of 1994 a slow-release formulation of methyl salicylate and a beta-acid-rich hop resin sprayed on to hop plants did not reduce aphid infestations significantly. In autumn cis,cis-nepetalactol the main component of P. humulis sex pheromone, prepared by various synthetic routes, increased trap catches of males and gynoparae equally. Catches of males in pheromone traps situated in a hop garden decreased with increasing trap height. Catches of males in traps charged with increasing doses of the cis,cis-nepetalactol peaked at 1 mg and then plateaued, whereas catches of gynoparae peaked similarly at 1 mg and then decreased. The effects of kairomones from an extract of the primary host, sex pheromone and a visual cue from yellow compared with clear water-traps were additive. The prospects for developing a semiochemicals-based control strategy against P. humuli, using some or all of the above elements, are discussed.

Sex pheromone and visual trap interactions in mate location strategies and aggregation by host-alternating aphids in the field

Abstract. Field observations were made on the responses of males and gynoparae of three host‐alternating aphid species, the blackberry‐cereal aphid, Sitobion fragariae (Walker), the bird cherry‐oat aphid, Rhopalosiphum padi (L.) and the damson‐hop aphid, Phorodon humuli (Schrank) to species‐specific sex pheromones released from transparent and coloured water traps.Pheromone traps caught significantly more males than did control traps without pheromone, whereas transparent, light green, yellow and orange traps caught most insects.Measurements of the distance over which sex pheromones function indicated that male P.humuli detect the pheromone 2–6 m from the source and can fly upwind to a source in wind speeds of 0.7 m s‐1.In all three species significantly more gynoparae were caught in pheromone traps than in control traps, suggesting that pheromone released by adult sexual females may assist late‐flying gynoparae to locate a suitable host plant on which to deposit their progeny.The response is relatively stronger for males than gynoparae, but the pheromones appear to act as both sex and aggregation pheromones.

An improved aphid electroantennogram

Excised antennae have been used to record aphid electroantennograms (EAGs) but these preparations have small, rapidly declining responses and a short usable life. An improved EAG technique is described and evaluated using alate virginoparae of the black bean aphid, Aphis fabae, and a series of plant volatiles. EAG recording with fine-tip glass electrodes inserted into the surface of the intact antenna of a whole insect preparation gave stable EAG peaks of a few mV and EAG peak amplitude decreased by only 22% over an 8 hour recording period. The EAG responses showed typical dose-dependent characteristics. There was variation in EAGs from different preparations, so normalisation of the EAG responses against a standard stimulus is still necessary. The thirty compounds tested elicited an EAG response profile largely similar to that reported previously from excised antennae. EAG responses recorded with the electrode at three different positions, between the 5th-6th (A), 4th-5th (B), and 3rd-4th (C) antennal segments, were smaller when recorded closer to the head. Position B produced larger EAG responses than those at C although there was no olfactory receptor between position B and C.

Plant virus and parasitoid interactions in a shared insect vector/host

Interactions between barley yellow dwarf luteovirus (BYDV) and the aphid parasitoid, Aphidius ervi Haliday (Hymenoptera: Aphidiidae), were investigated while sharing the vector/host, Sitobion avenae (F.) (Homoptera: Aphididae). Aphids, which were parasitized during their second larval stadium, had access to virus‐infected plants before, immediately after, or several days after parasitoid attack. The larval development of A. ervi in S. avenae was significantly delayed when virus acquisition took place before or shortly after the parasitoid had hatched, but not when the parasitoid was at the second larval stage during virus acquisition. Similarly, the presence of BYDV led to a significantly higher aphid mortality when they acquired virus up to and including the time that A. ervi was at the first larval stage. Adult female parasitoids deposited fewer eggs in viruliferous aphids. Virus transmission was not reduced by parasitization, and in some experiments aphids which were subjected to parasitoid attack transmitted BYDV more efficiently than unattacked insects.

Melatonin and the pea aphid, Acyrthosiphon pisum.

Pea aphids, Acyrthosiphon pisum, were fed on artificial diet containing various concentrations of melatonin. Under long-day conditions (16h light:8h dark) their progeny included males and virginoparous/oviparous (asexual/sexual) intermediate females, which normally occur only in short days or around critical night-length. Endogenous melatonin in pea aphids was measured by radioimmunoassay and verified by parallelism with a melatonin standard curve and by thin layer chromatography. However, melatonin titres showed large variations and although they tended to be higher during the scotophase than during the photophase they were not significantly different. The possibility of melatonin being involved in photoperiodism is discussed.

Sex pheromone stereochemistry and purity affect field catches of male aphids

Male blackberry-cereal aphids, Sitobion fragariae (Walker), were caught in the ield by water traps releasing synthetic (4aS, 7S, 7aR)-nepetalactone, the major sex pheromone component. The presence of the enantiomer (4aR,7R,7aS)-nepetalactone reduced catches and a plant-extracted (4aS,7S,7aR)-nepetalactone was less effective than the 99% (7S)-lactone. A more extensive trial with the bird cherry-oat aphid, Rhopalosiphum padi, involving the pheromone comprising the stereochemically related (4aS,7S,7aR)-nepetalactol showed a similar trend. The (7R)-isomer caught fewer males than the (7S)-isomer, but in this case the addition of high-purity (7S)-lactol to make a 50% blend caught as many males as pure (7S)-lactol. With plant-derived lactol, further purification did not significantly increase the catch. It is suggested that trace compounds associated with reduced enantiomeric purity in terms of the (7S)-configuration or from plant sources reduce activity of the sex pheromone components. Male damson-hop aphids, Phorodon humuli (Schrank), were also caught in the synthetic lactol traps, and it is suggested that this is due to traces of the (4aR,7S,7aS)-nepetalactols, which comprise the sex pheromone for this species. The significance for aphid chemical ecology studies and pest control strategies is discussed.