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Dense colonies on ear sheaths and leaves of maize. Carter et al. (1980) reviewed feeding patterns on various cereals. Honeydew and resultant sooty mould growth may be visible.
Virus-carrying aphids can be identified using ELISA techniques (Torrance, 1987). Infected plants can be identified using similar techniques. Barley yellow dwarf virus symptoms are described by D'Arcy and Bennett (1995).
Chemical Control and IPM
A wide range of insecticides provide effective control against R. padi. Systemic insecticides are reported to be more effective than non-systemic ones. Seed treatment of winter cereals can provide protection from aphid infestation in the early developmental stage and prevent the spread of Barley yellow dwarf virus (BYDV). Threshold numbers for spraying and forecast systems are now commonly used, so insecticides are only sprayed during significant infestations. Smeets et al. (1994) described EPIPRE (EPIdemic PREdiction and PREvention), a computer-based advisory system for pest and disease management in spring and winter wheat in the Netherlands and Belgium. Aphid monitoring systems have now been set up in 19 European countries, co-ordinated by the European Union-funded thematic network ‘EXAMINE’ (Exploitation of Aphid Monitoring in Europe) to provide data towards the studies of the impact of global change (Harrington et al., 2004). The data, such as first flight records and abundance, could be used by the agronomists for prevention and control plan preparation.
R. padi and BYDV control should start with the control of volunteer cereal plants and overwintering stubble, which provide optimal feeding ground for aphids and so should be destroyed by desiccation before the preparation of the field for the new crop. Delaying the drilling of winter cereals limits the opportunity for colonisation, which would reduce the spread of BYDV by up to half and prevent the settlement of migrating aphids. Aphid and/or virus resistant varieties would be an ideal choice to prevent aphid settlement, feeding damage and virus transmission. Natural enemies can be encouraged by wildflower strips (HGCA, 2014). The effects of intercropping on R. padi were investigated by Helenius (1990).
Zuniga et al. (1986) previously described the predators and parasitoids introduced into Chile as biological control agents of cereal aphids. These included parasitoids introduced from France: Aphidius ervi, A. rhopalosiphi and A. uzbekistanicus. However, the controlling effect of the parasitoid introductions has been smaller for R. padi than with other cereal aphids. In the former USSR, the Cecidomyiid Aphidoletes aphidimyza was reared for release as a biological control agent. Feng et al. (1990b) investigated the potential of two fungal pathogens, Verticillium lecanii and Beauveria bassiana, as biological control agents for R. padi and other cereal aphids.
Leaf wax in barley and leaf pubescence in wheat have been suggested as valuable resistance mechanisms (Roberts and Foster, 1983; Tsumauki et al., 1989).
In a range of wheat cultivars, the levels of alanine, histidine and threonine accounted for a large amount of variation in aphid fecundity (Kazemi and van Emden, 1992). Gramine content in barley is related to resistance, and is thought to affect feeding behaviour. Weibull (1988) suggested, based on several years' data from Swedish studies, that 55-85% of variation in resistance to R. padi in oats and barley may be explained by the composition and concentration of free amino acids in the phloem. Varietal resistance to R. padi was negatively correlated with the soluble sugar concentration in wheat plants in a Chinese study (Zhou et al., 1999).
Maize that has been genetically modified to express Bacillus thuringiensis (Bt) toxin, chiefly against European corn borer, is being increasingly grown. No significant differences in the growth or abundance of R. padi on conventional and Bt maize crops have been found (Lozzia et al., 1998; Manachini et al., 1999).
Hydroxamic acids are the main group of secondary metabolites involved in the resistance of certain cereals against bacteria, fungi and several insects including aphids (Thackray et al., 1991; Nicol et al., 1992; Rustamani et al., 1996). Hydroxamic acids have a feeding deterrent effect (Givovich and Niemeyer, 1991), are able to reduce aphid reproduction and have been shown to be toxic in an artificial diet (Escobar et al., 1999; Niemeyer et al., 1992). Previous studies of hydroxamic acids in wheat have shown that the range of levels present in the tetraploid and hexaploid varieties tested was insufficient to have a negative effect on R. padi behaviour or development (Elek et al., 2013). The analysis of the fluid extracted from the apoplast of tetraploid and hexaploid varieties revealed a non-toxic level of hydroxamic acids, which suggesteds that aphids may able to avoid the toxic compounds by carefully driving through the stylet, thereby causing minimal cell damage. The diploid Aegilops speltoides contains very high levels of hydroxamic acids and showed adverse effects on aphid host selection and reproduction (Elek et al., 2013); the level in the apoplast fluid was similar to the level found to reduce the survival rate in an artificial diet (Elek et al., 2013; 2014). Aegilops speltoides could be a source of R. padi-resistant wheat varieties.
R. padi is a serious pest of cereals, causing direct feeding damage and transmitting viruses.
Its pest status in Europe is shown on maps of affected crops by Zadoks and Rijsdijk (1984). Feeding damage alone can result in losses of 15% in cereal yields, although its importance in many locations is due to virus transmission. Its prevalence as a pest in Northern Europe was linked with the spring planting of cereals and the abundance of its primary host (Leather et al., 1989). In a US study, the greatest yield loss occurred when aphids fed during the seedling (2-3 leaf) stage in autumn; mean densities of 25-30 aphids/stem caused reductions of 50% in some components of yield at this stage (Kieckhefer and Kantack, 1988). In wheat, at high yield levels, direct feeding damage is often of less importance than the indirect effects, such as honeydew deposits, which reduce photosynthesis, induce sooty mould production and cause premature leaf senescence.
R. padi is the most important vector of Barley yellow dwarf virus (other synonyms include maize leaf fleck, rice 'giallume', yellow disease and rice yellows), the most widespread and economically important disease of small-grain cereals (D'arcy and Bennett, 1995). UK observations revealed the disease can reduce the barley yield by up to 75% and wheat yield by up to 30%, with infection largely being transmitted by disease-carrying aphids in the autumn (Jones, 2014).
Recent studies have shown that climate change has an effect on crop yield and on the relative importance of pests and pathogens (Esterling et al., 2007). Milder winters have been shown to improve the chance of survival of aphids on cereal crops in their active form, giving them an opportunity to feed and reproduce longer. Higher mean winter temperature would also lead to earlier aphid migration which can increase the severity of damage in the spring cereal crops (Harrington et al., 2001; Harrington et al., 2007).
R. padi is also responsible for transmitting many less importamt non-persistent viruses, including Abaca mosaic virus (Sugarcane mosaic virus), Onion yellow dwarf virus, Maize dwarf mosaic virus, Ryegrass mosaic virus, Wheat streak mosaic virus, Cynosurus mottle virus and Potato virus Y.