Distribution. This species is known from all continents where agriculture occurs. It can be a serious pest in both temperate and tropical areas, especially the latter in Africa and Asia. Cowpea aphid appears to have been introduced from Europe to the western hemisphere, though the date is unknown. It now occurs throughout the United States and southern Canada. (See color figure 152.)
Host Plants. Cowpea aphid feeds on numerous plants, but tends to be most abundant on plants in the family Leguminosae. Summer hosts include such vegetables as asparagus, carrot, cowpea, kidney bean, lettuce, and lima bean. Other crops affected include alfalfa, apple, citrus, hairy indigo, hairy vetch, pinto bean, red clover, rye, sesbania, sweet clover, and wheat. Common weeds serving as hosts include dandelion, Taraxacum officinale; lambquarters, Cheno-podium album; pigweed, Amaranthus spp.; shepherds-purse, Capsella bursa-pastoris; dock, Rumex spp.; goldenrod, Solidago spp.; kochia, Kochia scoparia; pep-perweed, Lepidium spp.; and Russian thistle, Salsola kali. Palmer (1952) reported the overwintering host in Colorado as locust trees, Robinia sp., but overwintering aphids usually persist on succulent plants in warmer areas.
Natural Enemies. The natural enemies of cowpea aphid include many of the common aphid predators such as lady beetles (Coleoptera: Coccinellidae), lacewings (Neuroptera: Chrysomelidae and Hemero-biidae), and flower flies (Diptera: Syrphidae). (See the section on green peach aphid, Myzus persicae (Sulzer) for a more complete discussion on aphid predators.) Parasitoids are also known, including Ephedrus persicae Froggatt, Praon abjectum (Haliday), and Diaere-tiella rapae (M'Intosh). The latter, a cosmopolitan species attacking numerous aphids, is well known. Some biological information on this parasitoid is available in Wheeler (1923) and Schlinger and Hall (1960).
Life Cycle and Description. Sexual reproduction is absent or unproven from cowpea aphid populations in North America; overwintering normally occurs in greenhouses and in warm regions. As is the case with most aphids, their development time is very short. Kaakeh and Dutcher (1993) reported mean generation times of as few as 8.4 days on favored hosts such as cowpea, to 12.2 days on hairy vetch.
Developmental biology was given by Radke et al. (1973), Elliott and McDonald (1976), and Abdel-Malek et al. (1982). A brief description of this aphid, and keys for its identification, are available in Palmer (1952)
and Blackman and Eastop (1984). Cottier (1953) also provided a good description. Stoetzel et al. (1996) published a key for cotton aphids that is also useful to distinguish cowpea aphid from most other common vegetable-infesting aphids.
Cowpea aphid colonizes various plant tissues, including stems, pods, young leaves and old leaves, with the stem tissue most preferred (Srikanth and Lak-kundi, 1988). The aphid removes plant sap, disrupting the normal plant growth pattern, including reduction in root growth and nodulation. At high aphid densities, plants are deformed and stunted, and seed set is reduced. Cowpea aphid does not produce toxins or other persistent effects, so that once the aphids are removed, normal plant growth rates are resumed. In fact, a degree of compensatory growth occurs, with plants exposed to short-term aphid infestations displaying an increased rate of growth once aphids are removed. This allows plants that support aphids for short periods of time to recover and produce crop yields equivalent to uninfested plants (Hawkins et al., 1986).
The principal damage by this species is disease transmission. Over 30 stylet-borne and persistent plant viruses, including diseases of bean, beet, cow-pea, onion, pea, cucurbits, and crucifers are transmitted (Kennedy et al., 1962). For example, cowpea aphid was reported to be one of the most common potyvirus vectors in squash cultivated in Arkansas, despite failing to colonize and establish large colonies on the crop (Hander et al., 1993). Exposure of this aphid to alarm pheromone does not result in decreased virus transmission (Yang and Zettler, 1975).
Cowpea aphid can also be beneficial in some agricultural systems. In the southeastern United States, leguminous cover crops such as sesbania, hairy indigo, hairy vetch, and red clover are planted in pecan orchards to support high densities of cowpea aphid. In pecan production systems cowpea aphids do no harm, but serve as valuable sources of food for general insect predators such as lady beetles (Coleoptera: Coccinelli-dae), which then also prey on the pecan aphid complex (Kaakeh and Dutcher, 1993).
Host-Plant Resistance. Some cultivars of cowpea are resistant to cowpea aphid (Pathak, 1988). The basis for resistance seems to be related to interference with feeding, and the inability of aphids to colonize and to produce progeny (Givovich et al., 1988). In greenhouse studies comparing resistant and susceptible varieties, aphid survivorship and reproduction were decreased when fed on foliage of a resistant cultivar (Annan et al., 1997).
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