Distribution. This species may be of Asiatic origin, but now has a world-wide distribution. Corn leaf aphid does not persist in areas of severe winters, so it is absent from most of Canada, and northern Europe and Asia, and re-invades large areas of temperate regions annually. In North America, it is normally found on corn through most of the United States, but is relatively uncommon on this host in the northwestern states, preferring instead to feed on barley. In Canada, it is found principally in southern areas of the eastern and prairie provinces. (See color figure 154.)
Host Plants. Corn leaf aphid feeds on numerous grasses in addition to corn, and is considered to be a serious pest of cereal grains due to its ability to transmit virus diseases. Although corn is the only vegetable crop affected, several field crops are fed upon, including barley, chufa, oat, rye, millet, sorghum, Sudan grass, sugarcane, and wheat. Barley is the most important early season host. While the leaf blades are rolled, aphid colonies develop within the furled-barley leaves. However, once the corn is about 30-days old it also becomes very suitable for aphids; the tassels forming within the furled leaves are particularly suitable for aphid population growth. As the tassel extends, aphids disperse over the entire plant to feed.
Among weeds and prairie grasses known to serve as hosts are barnyardgrass, Echinochloa crusgalli; buffalo grass, Buchloe dactyloides; crabgrass, Digitaria san-guinalis; foxtail, Setaria spp.; gramagrass, Bouteloua spp.; and Johnsongrass, Sorghum halepense (Robinson and Hsu, 1963; Kieckhefer, 1984).
Natural Enemies. These aphids feed on exposed areas of the plant and often are subject to significant levels of predation and parasitism. Numerous species of lady beetles attack corn leaf aphid. In Ontario, for example, Foott (1973) reported that Hippodamia conver-gens Guerin-Meneville, H. tredecimpunctata (Say), and Coleomegilla maculata Timberlake were the dominant lady beetle predators, but Adalia bipunctata (Linnaeus), Cycloneda sanguinea Linnaeus, Hippodamia parenthesis (Say), and Coccinella transversoguttata Brown (all Coleoptera: Coccinellidae) also were present. Despite their abundance, however, the lady beetles were numerous and effective predators only after the aphid populations attained high and damaging densities.
Other common predators include various flower flies (Diptera: Syrphidae), predatory midges (Diptera: Ceci-domyiidae), minute pirate bugs (Hemiptera: Antho-coridae), and lacewings (Neuroptera: Chrysopidae).
Several wasps parasitize corn leaf aphid. Lysiphlebus testaceipes (Cresson) (Hymenoptera: Braconidae) is reportedly the most widespread and common parasi-toid, but Aphelinus varipes (Foerster) (Hymenoptera: Aphelinidae) was an important parasitoid in Texas (Gilstrap et al., 1984). Other parasitoids include Diaer-etiella rapae (M'Intosh) and Ephedrus persicae (Froggatt) (both Homoptera: Aphidiidae) and Aphelinus asychis Walker (Hymenoptera: Aphelinidae).
Life Cycle and Description. This species routinely overwinters as viviparous females (displaying parthenogenetic reproduction) in the southern states, north to about northern Texas. During mild winters, there is evidence that overwintering may occur at least as far north as southern Illinois. Overwintering does not occur in northern states and in Canada. Oviparous (egg producing) forms and males are rare, though in Pakistan eggs overwinter on Prunus sp. The northern areas of North America are assumed to be invaded annually (Rose et al., 1975), with the timing of invasion and the number of subsequent generations in an area a function of weather. The short life cycle of this aphid, normally 6-12 days, allows production of 20-40 generations per year in such southern locations as Texas and about nine generations in Illinois, but fewer further north.
Elements of corn leaf aphid biology were found in Wildermuth and Walter (1932), Foott (1977), and Steiner et al. (1985), and descriptive material in Davis (1909), Wildermuth and Walter (1932), and Cottier (1953). Kring (1985) described the instars. Corn leaf aphid was included in keys to small grain aphids published by Pike et al. (1990) and Olsen et al. (1993).
Corn leaf aphid is commonly found feeding on the tassel and silk of the corn plant in addition to the leaves. It interferes with pollen production and fertilization, resulting in poor kernel fill of the ears. Infestation also can cause a delay in plant maturity and reduced-plant size (Bing et al., 1991). Honeydew secreted by the aphids supports growth of sooty mold fungus, causing an unsightly appearance of the ears.
In general, corn plants are very tolerant of aphids, and only extremely dense infestations cause injury. If soil moisture conditions are inadequate, however, then the damage by the aphids is increased, and yield reductions are more likely.
The ability to transmit plant viruses greatly exacerbates the damage potential of this aphid. Several diseases, including barley yellow dwarf, beet yellows, cucumber mosaic, lettuce mosaic, maize streak, and maize stripe are transmitted. Of particular concern is transmission of maize dwarf mosaic—a serious disease of sweet corn (Straub, 1984). Although failing to colonize most vegetable crops, corn leaf aphid is one of the most abundant aphids collected in crops, and is implicated in the transmission of numerous stylet-borne viruses (Hander et al., 1993).
Cultural Practices. Early plantings can escape injury, especially in northern areas where aphids do not overwinter. In New York, for example, corn planted before June 10 escaped injury by corn leaf aphid and maize dwarf virus, but incidence of aphids and disease increased thereafter (Straub and Bothroyd, 1980). Little work on the management of corn leaf aphid as a virus vector has been reported, other than the assessment of corn varieties for resistance, but approaches discussed in the sections on green peach aphid, Myzus persicae (Sulzer), and melon aphid, Aphis gossypii (Glover), are applicable.
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