| Abstract |
It is only recently that the Russian wheat aphid [Diuraphis noxia (Mordvilko)] has become a serious pest of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) outside its primary range of the mountains of Iran-Turkestan. Originally named and recorded from southern Russia in 1900, it appeared as a pest of wheat in South Africa in 1978. It has since spread to the Americas where it has caused considerable economic losses for small-grain growers, and if it spread to Australia, where conditions are favorable for the pest, it would be a threat to the wheat industry there. During feeding, the aphid has been reported to inject a toxin into its host plant, which is thought to be associated with reduced photosynthetic capacity resulting from damage to the chloroplast membrane. Diuraphis noxia feeds deep within the leaf whorl, inside tightly rolled leaves, and is consequently difficult to control with contact insecticides although systemics work weIl. Natural insect enemies (see Appendix 1), moreover, find it difficult to attack the aphid because of the rolled leaves. Host-plant resistance represents an attractive and environmentally sound method of management. This report describes a field-based screening method that uses artificial aphid infestation in screening nurseries to facilitate selection of resistant germplasm. Resistance to D. noxia was identified in barley, triticale, tetraploid (T. dicoccon Schrank) and diploid wheat-related species (T. boeticum Boiss., T..monococcum L., T. urartu Thum.), and landrace bread wheats from Turkey. The functional mechanisms of resistance to D. noxia in barley are antibiosis and tolerance. The antibiosis component is evident as reduced fecundity in the aphid. The tolerance component is potentially useful to plant breeders as it does not put the aphid under selection pressure, which might otherwise encourage development of new biotypes. The resistance operating in one selected barley genotype was determined to be specifie for D. noxia; there was no discernible resistance to other major cereal aphids in that barley genotype. The conditioning host plant, Le., the plant on which the D. noxia was born and developed, was found to influence the expression of antixenosis and antibiosis on subsequent host plants. Selected barleys, including resistant and susceptible accessions, were assessed for productivity in the field following artificial infestation in small plots, which simulated commercial planting conditions. Susceptibility to D. noxia, based on damage scores from hill plots, was indicative of yield lossin larger plots. Resistant accessions, compared with susceptible ones, when infested with D. noxia, maintained greater plant height, more spikes per unit area, higher straw weight, and higher grain weight. The genetic mechanisms of resistance to D. noxia were investigated in the greenhouse and in the field using two symptomatically resistant and two symptomatically susceptible spring barley genotypes. A single major gene was determined to govern inheritance of resistance in the two resistant barley genotypes. This trait should be relatively easy to transfer to other genotypes. Greater gene diversity for resistance may, however, reduce reliance on a single major gene resistance, and possibly reduce pressure on the aphid to overcome the antibiotic component of the resistance through development of new biotypes. Notes were taken on the predators and parasitoids of D. noxia in the High Valley of Mexico, with a possible view towards integrated pest management. Biocontrol alone, however, is unlikely to be an efficient management method as many of the natural enemies are generalists, and the aphid's cryptic feeding habits make it relatively less accessible to predators than other major cereal aphids. The naturally occurring insect enemies of D. noxia include several species of coccinellid, syrphids, chrysopids, and parasitoid wasps. The ideal method of controlling D. noxia would comprise plant resistance and biocontrol in an integrated approach. Diversity in D. noxia at the molecular level was researched by generating DNA fingerprints from clones of the aphid from Mexico, USA, Canada, France, South Africa, Chile, and Syria. Amethod using PCR (polymerase chain reaction)-based RAPD (random amplified polymorphie DNA) was developed for producing DNA fingerprints of the aphids. With the exception of D. noxia from Syria, no variation was found in the DNA fingerprints of the various clones following the use of 18 random decamer primers. This indicates that plant breeders might not yet have to face problems associated with biotypie variation in the aphid, which could confound the results of breeding for host-plant resistance. |
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