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Although D. noxia feeds on leaves and flowers/seedheads of grasses, it appears to inject a polypeptide toxin that affects the entire plant (Hewitt et al., 1984). Colonies are found most frequently on the youngest leaves or on newly emerged flowers/seedheads. Even very small colonies of D. noxia cause characteristic white to purple streaking and leaf-rolling on wheat and barley leaves. The streaks usually extend most of the long axis of the leaf and are irregularly distributed across the short axis of the leaf. Rolling extends in severity from simple folding of the leaf along the mid-vein, to one side of the leaf rolled in upon itself, to the whole leaf being tightly rolled around the aphid colony. Large colonies can roll the flag leaf to the point where the tip of the inflorescence becomes trapped, giving it a fish-hook shape. Young plants are often stunted and even killed. Plants attacked after flowering show few to no obvious symptoms. Duration of infestation may have more impact than aphid density on yield loss (Burd and Burton, 1992; Kieckhefer and Gellner, 1992).
Walker and Peairs (1994) reviewed cultural control methods for D. noxia. These include planting date, planting density, furrow orientation, irrigation, fertilization, grazing, manipulation of crop residue, and alternative host plants.
Delaying planting beyond the period at the beginning of a season when alate D. noxia are abundant can reduce initial infestations. However, other agronomic constraints prevent large shifts in planting date. Although suction trap catches indicated sufficient autumn dispersal in Idaho (USA) to warrant delayed planting to avoid D. noxia infestation (Halbert et al., 1990), in more southerly regions, where early spring densities of alate D. noxia are higher, late-planted wheat may suffer more from spring infestations. D. noxia density is sometimes higher where plant density is lower (Walker and Peairs, 1994), so that increasing plant density could reduce D. noxia infestation.
North-South-oriented irrigation furrows in some cases give lower winter survival and slower development of D. noxia than East-West-oriented furrows because of reduced solar insolation (Hammon and Peairs, 1992). Thus, running irrigation furrows from north to south where possible in regions with marginal winter survival for D. noxia could reduce spring infestations and damage.
Nitrogen fertilization alone did not affect D. noxia numbers in several studies (Walker et al., 1991; Archer et al., 1995), although there was a shift to earlier reproduction with high-nitrogen fertilized wheat (Moon et al., 1995). On the other hand, Larson et al. (1990) found that nitrogen and sulphur together increased D. noxia infestation and Walker and Peairs (1994) found the opposite. It appears that individual nutrients can have varied effects depending on other nutrients, irrigation, and perhaps other variables. Irrigation in some cases reduced D. noxia population growth (Archer et al., 1995; Farias et al., 1995), but in others did not (Farias et al., 1995). A phenotypic study showed delayed development and less severity of symptoms with decreased numbers of aphids feeding on wheat plants treated with potassium phosphate in comparison with untreated plants for both resistant (Tugela DN) and susceptible (Scheepers) cultivars (Venter et al., 2014).
Although spring grazing can increase subsequent D. noxia population growth on the regrown plants (Messina et al., 1993a), it can also delay D. noxia growth and thus insecticide applications (Walker et al., 1990). Because D. noxia populations must survive the period between the harvest of one year's crop and the planting of the next, management of crop residues and alternative host plants appear promising. The current practice in much of the western USA is to plough under all crop residues and strictly control non-crop plants with herbicides. Although this does reduce reservoirs for D. noxia and other pests, it also reduces sources of natural enemies of these pests. Replacement of wild grasses currently used for forage and for conservation reserve programmes with grasses less suitable for D. noxia growth appears promising. Grass species vary in suitability for D. noxia development (Kindler et al., 1991a, 1992; Mowry et al., 1995). Furthermore, Kindler et al. (1991c, 1992) have found resistance to D. noxia within species of forage grasses. Fungal endophytes may provide some of this resistance (Kindler et al., 1991b; Clement et al., 1992).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
D. noxia has a great economic impact on cereal crops (Brooks et al., 1994). It is a phloem feeder like other aphids and the symptoms evident on plants are a result of this feeding mechanism. By feeding on the phloem, the aphid damages the plants through nutrient drainage (Dixon, 1985) which results in chlorosis, necrosis, wilting, stunting, and curling of the leaves, misshapen or nonappearance of new growth, and localised cell death at the site of aphid feeding. D. noxia further elicits an increase in essential amino acids in the phloem sap by triggering the breakdown of proteins in infested wheat leaves (Burd and Burton, 1992; du Toit, 1986; Ma et al., 1998; Miller et al., 2001). The damage to the foliar tissue is thought to play a role in the pest’s ability to increase nutritional quality of the host plant (Botha et al., 2006).
Barley and wheat are the most important cultivated hosts of D. noxia and considerable yield losses can occur in these crops. It is particularly injurious to late-sown barley in continental climates. Spring wheat suffers greatest yield loss when attacked during tillering to boot stage; winter wheat suffers greatest loss after vernalization (Gray et al., 1990). Early, heavy infestations on barley can cause total crop loss (Adisu et al., 2003) and significantly affect grain quality (Bregitzer et al., 2003). Aphid feeding on susceptible genotypes causes chlorosis and longitudinal streaking of leaves, and emerging leaves remain tightly rolled, which traps spikes and prevents their normal development (Mornhinweg, 2011). Infestation of wheat seedlings by D. noxia in the autumn can reduce the ability of seedlings to survive the winter (Thomas and Butts, 1990; Storlie et al., 1993). In general, damage is greatest when crop ripening coincides with peak aphid numbers.
D. noxia has been a pest of small grains in Russia since 1912, when the first serious outbreaks on barley were recorded. In the Crimea, D. noxia has reduced harvests by up to 75% in some years. In Eurasia generally, where it is thought to have originated, it is only occasionally a serious pest; with short-lived outbreaks being reported (Grossheim, 1914; Tuatay and Remaudière, 1964; Fernández et al., 1992).
Since its appearance in Texas in 1986 (Stoetzel, 1987), D. noxia has become a major pest of wheat and barley in the USA, causing over US$850 million in direct and indirect losses from 1987 to 1992 (Brooks et al., 1994). During the 1992/93 cropping season, over 7 million acres (20%) of dryland winter wheat and 1 million acres (33%) of barley were infested throughout the western USA (Webster and Amosson, 1994). In Canada, yield losses ranging from 25 to 37% without insecticide treatments in field trials (Butts et al., 1997).
Since its introduction in 1978, it has also become the major pest of wheat in South Africa (Walters et al., 1980), where it can cause over 80% yield loss if not controlled in the summer rainfall region (Tolmay and Prinsloo, 1994). In Ethiopia, it has been a major pest of barley for over two decades, with 20-30% yield loss in some areas (Haile and Megenasa, 1987). D. noxia is also a pest in Yemen (Erdelen, 1981).
Macedo et al. (2009) showed that D. noxia affected net photosynthesis of wheat. Wheat plants tolerant to D. noxia often exhibit increased photosynthetic rates, increased growth rates, increased stored root carbon and/or an enhanced ability to shunt stored carbon from roots to shoots (Burd and Elliott, 1996; Haile et al., 1999; Reidel and Blackmer, 1999; Smith and Chuang, 2013). In addition to direct feeding damage, major indirect losses in wheat and barley can be caused as a result of D. noxia transmitting Barley yellow dwarf virus (BYDV-PAV). However, D. noxia is a less effective vector of BYDV than Rhopalosiphum padi, R. maidis, Schizaphis graminum or Sitobion avenae (El-Yamani and Bencharki, 1997).
There have been conflicting reports on the effect of D. noxia on yield and yield components of host plants. For instance, in Canada, Ba-Angood and Stewart (1980) found that aphid feeding significantly reduced the number of grains per ear, whereas Milne and Delves (1999) in Australia found that aphid feeding did not affect grain numbers per ear. However, aphid feeding usually significantly reduces 1000-seed weight and grain yield per ha (e.g., Ba-Angood and Stewart, 1980; Milne and Delves, 1999; Damate, 2015). The percentage yield loss from D. noxia on irrigated wheat in Ethiopia ranged from 15 to 93%, depending on variety and season (Damate, 2015).
Damage ratings based on leaf chlorosis have frequently been used to evaluate resistance to D. noxia. However, some workers (e.g., Frank et al., 1989; Smith et al., 1991; Assad et al., 2004) suggest that leaf chlorosis may not be effective in separating resistant entries. To increase the accuracy of evaluation, leaf damage other than chlorosis, i.e. leaf rolling and leaf trapping, should also be considered (Nematollahy et al., 1998).