Rhopalosiphum padi Linnaeus Homoptera Aphididae

Natural History

Distribution. This species probably originated in North America, but is now found in Europe, Asia, and New Zealand as well as throughout the United States and southern Canada. (See color figure 150.)

Host Plants. Bird cherry-oat aphid may alternate between winter and summer hosts, or in mild-winter climates it can persist throughout the year on summer hosts. The winter, or primary, host is Prunus spp. The summer, or secondary, hosts are numerous species of grasses, including all the major cereals and pasture grasses. Sedges, iris, and a few other plants are occa sionally reported as hosts, but these records are suspect. It is found principally on wheat, barley, oat, and rye, but in some locales it is a major pest of corn, including sweet corn.

Susceptibility of hosts to infestation and suitability for aphid reproduction have been assessed by many investigators, including Villanueva and Strong (1964), Dean (1973), Leather and Dixon (1982), and Weibull (1993). Among the best hosts are such wild or forage grasses as wheatgrass, Agropyron spp.; brome, Bromus spp.; ryegrass, Lolium and Lilium spp.; and wild oat, Avena fatua. Among crop plants barley seems to be optimal in most areas, but sweet corn is very suitable in the Pacific Northwest.

Natural Enemies. Bird cherry-oat aphid is susceptible to attack by many of the predators, parasitoids, and pathogens affecting other aphids. Lady beetles (Coleoptera: Coccinellidae), green lacewings (Neurop-tera: Chrysopidae), brown lacewings (Neuroptera: Hemerobiidae), and flower flies (Diptera: Syrphidae) are the most common predators. In Europe, and presumably in North America, eggs suffer high mortality rates during the winter months owing to cold weather in most northern locations and predators in most mild locations. In small grain crops grown in Idaho, Aphelinus varipes (Forester) (Hymenoptera: Aphelinidae); Aphidius ervi Haliday, Diaeretiella rapae (M'Intosh), Lysiphlebus testaceipes (Cresson) and Praon sp. (all Hymenoptera: Aphidiidae) were recovered from bird cherry-oat aphid, with A. varipes most abundant (Feng et al., 1992). Fungi have some significant effects on aphid populations, but this occurs mostly in irrigated fields. Precipitation, particularly heavy rainfall, also has direct negative effects on aphid survival during the summer months.

Life Cycle and Description. The biology of this aphid varies, depending on availability of primary hosts and weather conditions. A generation can be completed in about five days when reared at 26°C, but it requires about 22 days at 13°C. Longevity of the individual aphid is often 18-20 days. Aphids developing on primary hosts undergo 2-3 generations before overcrowding induces formation of winged (alate) forms which disperse to grasses. Aphids may reproduce continuously during the summer months, but reproduction is disrupted if temperature exceeds 30°C for even a few hours daily. Aphids feeding on wild grasses and grains may migrate back to Prunus and produce eggs in the autumn, or they may remain on Graminae through the winter months. In Manitoba, winged aphids are found dispersing in late May and June, and then again in October (Robinson and Hsu, 1963). In the northern Great Plains region of the United States, however, little overwintering occurs, this area instead becomes re-invaded annually by winged migrants from the south. Some individuals crawl down the plant stems and survive below-ground during cold weather (Kieckhefer and Gustin, 1967), and though this does not appear an important method of overwintering under the cold winter climate conditions of Idaho (Feng et al., 1992), it likely is effective in warmer climates.

  1. The egg is green initially, but turns black with age, and is deposited on the bark of Prunus trees. The egg measures about 0.57mm long and 0.26mm wide. In England, there is a steady but low rate of attrition in eggs during the winter months due primarily to predation, followed by a fairly sharp increase in egg loss in the spring, resulting in loss of 70-80% (Leather, 1981).
  2. There are four instars. The mean body length of nymphs is 0.6, 0.9,1.2, and 1.4 mm for instars 1-4, respectively. Development time of nymphs reared at 26°C, which is about optimal for development, is 1.4, 1.1, 1.1, and 1.4 days for instars 1-4, respectively. Nymphs are green, but there is pronounced orange or rust color posteriorly near the base of the cornicles. First instar nymphs have only four antennal segments, instars two and three have five segments, and last instar and adults bear six segments. Aphids reared under cool conditions are greenish black and those reared under warmer conditions are light-green. Aphids feeding on Prunus live on bud and young leaf tissue; the latter may become deformed and form a pseudo-gall partly enclosing and protecting the aphid colony. Older leaf tissue also is attacked, and eventually some of the adults are winged. The shift to winged, dispersing forms coincides with a decrease in the nutritional quality of Prunus foliage (Dixon, 1971; Dixon and Glen, 1971).
  3. The body of the wingless adults (apterae) is greenish or olive brown, with the head and prothorax yellowish-brown. The legs are green or darker, the cornicles green but with the tips dusky. A rusty colored patch is found around the base of the cornicles. The body length is 1-2 mm, averaging 1.7 mm. In the winged forms (alatae), the abdomen usually is light-green but darker medially, though sometimes it is entirely dark green. A rusty brown patch may be found around the base of the cornicles. The head and thorax are black, but the prothorax is green. The appendages are dusky to blackish. The body length is 1.8-2.0 mm. The prereproductive period of young adults is normally less than a day. Viviparous (parthe-nogenetic) adults produce 10-50 nymphs during their life span, with host plant, plant age, and temperature affecting reproduction. However, normal fecundity is 40-50 at optimal temperature during a reproductive period of about 15 days. Young plants and cool weather enhance reproduction.

This species is easily confused with several grain-infesting species, but fortunately is not too readily confused with the other common corn-infesting aphid, corn leaf aphid, Rhopalosiphum maidis (Fitch). The body of bird cherry-oat aphid is ovate, whereas the body of corn leaf aphid is elongate. Also, the rust-orange color about the base of the spiracles distinguishes bird cherry-oat aphid from the solid blue-green color of corn leaf aphid.

This species was included in the aphid keys of Palmer (1952), Blackman and Eastop (1984), and Olsen et al. (1993). The biology of this insect in Europe was reviewed by Leather et al. (1989). Developmental biology was given by Villanueva and Strong (1964), Elliott and Kieckhefer (1989), and Michels and Behle (1989).

Damage

This species is known principally for its damage to small grains. It builds to high densities and damages grain plants directly, and it is also an important vector

Adult female bird cherry-oat aphid, wingless form.
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Adult female bird cherry-oat aphid, winged form.

of several plant viruses, most notably barley yellow dwarf. Both persistent and nonpersistent viruses are vectored (Kennedy et al., 1962). However, in some areas it is the most abundant aphid on corn. Corn also serves as an important link or bridge for aphids and barley yellow dwarf virus between spring and autumn small grain crops (Blackmer and Bishop, 1991).

Management

This aphid is principally a pest of small grains, and sampling protocols have been developed for such crops, but not for corn. For example, Ekbom (1987) and Elliott et al. (1990) presented binomial or presence-absence protocols for R. padi sampling on small grains. Nevertheless, aphids may attain high numbers on sweet corn and stimulate application of foliar insecticides. There is evidence of resistance to certain insecticides in Europe.

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