Effects of treatments for electrical penetration graph recordings on behaviour and biology of Aphis craccivora (Aphididae)

Abstract. Two laboratory studies were conducted to investigate effects of treatments for direct "current electrical penetration graph (DC-EPG) recordings or 'tether effect', on behaviour and reproductive performance of cowpea aphid Aphis craccivora Koch. The experiments constituted a control study in application of DC-EPGs to analyse cowpea aphid feeding behaviour and host plant resistance mechanisms. Resistant (ICV-12) and susceptible (ICV-1) cultivars of cowpea Vigna unguiculata (L.) Walp were used. EPG treatments included two groups of aphids: tethered aphids that were exposed to DC electricity via an attachment of a thin, flexible gold wire on their dorsum using a droplet of adhesive silver paint, and 'free' (untethered) aphids with a dorsal spot of silver paint only. EPGs of the tethered aphids were recorded continuously for c. ! h, whereas from the 'free' aphids recordings were done only for brief periods of 2–5 min, by temporarily contacting a gold wire to the spot of silver paint. Waveform signals generated from resistance fluctuations and electromotive forces, and representing aphid stylet penetration behaviour were recorded. A separate experiment was conducted to investigate effects of EPG treatments on aphid survivorship and population growth. Overall, EPG treatments did not significantly affect aphid stylet penetration behaviour or life-table parameters. However, effects of crop cultivar on those characteristics were significant. Waveform E2, which denotes aphid ingestion in phloem sieve elements, and non-penetration behaviour were important indicators of aphid resistance in ICV-12. Also, apart from the number of aphid generations, other life-table parameters were useful indicators of ICV-12 resistance. Thus, DC-EPGs provided a reliable technique for studying aphid stylet behaviour, and investigation of aphid resistance in cowpeas.     

Change in response of Rhopalosiphum padi spring migrants to the repellent winter host component methyl salicylate

Olfactometry showed that the response of spring migrants of the bird cherry-oat aphid, Rhopalosiphum padi (L.) (Homoptera: Aphididae), to the repellent winter host volatile methyl salicylate changes with age of the adult aphid. Between three and four days after becoming adult, and having left the winter host Prunus padus L., aphids lost their negative response to the chemical. The change in response was not associated with contact with a summer host, oats. In a settling choice bioassay, migrants avoided oats which had been exposed to volatile methyl salicylate. Aphids with removed antennal tips did not avoid the exposed plant, indicating that plant choice was influenced by cues from the plant surface. The results are discussed in relation to the use of methyl salicylate in integrated control.     

Effects of 20 fungicides on the infectivity of conidia of the aphid entomopathogenic fungus Erynia neoaphidis

The effect of 20 fungicides on theinfectivity of conidia of the entomopathogenicfungus, Erynia neoaphidis, were assessedin the laboratory. After projection on broadbean leaves, conidia were treated withfungicides applied at their recommended fieldrate. Afterwards, the infectivity of theseinocula was assessed using an aphid bioassay.Four fungicides, carbendazim, kresoxym-methyl, nuarimol and thiophanate-methyl reduced the infectivity of the conidia by less than 25% and can be considered harmless for this aphid pathogen. Propiconazole was a little more toxic, with 37% reduction. Other products reducedinfectivity by between 50% and 100%. These are, from the least to the most toxic:flutriafol, prochloraz, epoxyconazole,iprodione, hexaconazole, triadimenol,azoxystrobine, cyproconazole, cyprodynil,flusilazole and tridemorph. Chlorothalonil,fenpropimorph, spiroxamine and tebuconazoletotally inhibited infectivity of the fungi. Analysis of the results according to chemicalclass showed that the benzimidazoles were theleast toxic for E. neoaphidis and themorpholines the most toxic. Effects oftriazoles and strobilurines were variable, withreduction ranging from 37% to 100% fortriazoles and from 17% to 68% forstrobilurines.