Distribution. Potato aphid occurs throughout the United States and southern Canada. As a serious pest, however, its range is mostly the northeastern and northcentral United States, and eastern Canada west to Ontario. Potato aphid is a highly variable species, and may eventually be shown to be a species complex. The origin of potato aphid is thought to be North America, though it is now found widely around the world. (See color figure 147.)
Host Plants. Potato aphid is polyphagous, though its predominant hosts are potato, tomato, and sometimes corn in the summer, and wild or cultivated rose in the winter and spring. Sometimes spinach and lettuce are heavily infested during the autumn months. Potato aphid also are found to feed on such diverse vegetable crops as asparagus, beet, celery, chicory, corn, cucumber, eggplant, horseradish, kale, lettuce, mustard, pea, parsnip, pepper, potato, pumpkin, rhubarb, spinach, sweet potato, tomato, turnip, and watercress, but many of these plants are suitable for brief period of times, usually during the seedling stage. Potato aphid may be found in association with other crops such as clover, field corn, hops, peach, pawpaw, soybean, strawberry, sugarbeet, sunflower, and tobacco. It also infests such flowers as canna, geranium, gladiolus, hollyhock, iris, lily, poppy, rose, rud-beckia, and tulip. Among the weed hosts are black nightshade, Solanum nigrum; groundcherry, Physalis spp.; hairy nightshade, Solanum villosum; hoary cress, Lepidium draba; jimsonweed, Datura stramonium; lambsquarters, Chenopodium album; matrimony vine, Lycium sp.; morningglory, Ipomoea purpurea; pepper vine, Solanum jasminoides; pigweed, Amaranthus spp.; pepperweed, Lepidium spp.; plantain, Plantago spp.; ragweed, Ambrosia spp.; round-leaved mallow, Malva rotundifolia; shepherdspurse, Capsella bursa-pastoris; sow thistle, Sonchus oleraceus; smartweed, Polygonum spp.; wild lettuce, Lactuca sp.; and winter cress, Barbarea vulgaris. These aphids commonly move from host to host as the quality of the plants deteriorates due to seasonal changes. They are capable, however, of feeding indefinitely on the same hosts if the plants remain nutritionally suitable. Host associations were given by Patch (1915), Smith (1919), and Landis et al. (1972). Although rose is normally reported to be the overwintering host in cold climates, there are occasional reports that either apterae (wingless forms) or eggs are found during the winter months in cold climates on such diverse plants as asparagus, raspberry, and various weeds.
Natural Enemies. Potato aphid has many natural enemies, as is commonly the case with aphids. Most of the common lady beetles (Coleoptera: Coccinellidae), some lacewings (Neuroptera: Chrysopidae), flower flies (Diptera: Syrphidae), and the predatory midge Aphidoletes aphidimyza (Rondani) are reported to prey on potato aphid (Walker et al., 1984b; Perring et al., 1988; Dean and Schuster, 1995). Ground beetles (Coleoptera: Carabidae) affect aphid populations, but not as greatly as canopy-level predators (Boiteau, 1986).
Among the parasitoids of potato aphid are Aphidius, Diaeretiella, Ephedrus, and Praon spp. (all Hymenop-
tera: Aphidiidae) and Aphelinus and Dahlbominus spp. (both Hymenoptera: Eulophidae). Only Aphidius nigripes Ashmead is regularly abundant, accounting for over 90% of the parasitism of potato aphid in Maine over an 11-year period (Shands et al., 1965). In studies of potato aphid attacking tomato in North Carolina, Walgenbach (1994) reported that parasitism rates were inversely density dependent, with parasitism higher when aphid densities were low. Aphidius nigripes also was the dominant parasitoid in North Carolina, California (Sullivan and van den Bosch, 1971), Ohio (Walker et al., 1984b), and Quebec (Brodeur and McNeil, 1994).
Several closely related species of fungi affect potato aphid, and are considered suppressive (Shands et al., 1962). Frequency of fungal epizootics is positively correlated with aphid density (Boiteau, 1986). High levels of rainfall, though perhaps aiding in development of epizootics, are not a necessary prerequisite (Shands, 1962). Soper (1981) discussed the significance of fungi for aphid control, emphasizing the importance of not interfering with natural disease outbreaks by applying fungicides or other suppressive chemicals.
Life Cycle and Description. In northern areas, potato aphid has a sexual component to its life cycle and overwinters in the egg stage. In the spring, potato aphid feeds on rose, where 2-6 generations are completed. The aphids abandon the rose in the summer months and fly or walk to other suitable hosts, where several additional generations occur and high densities are attained. In the autumn, the winged forms disperse, usually back to rose. On rose, autumn migrant females, oviparous (egg-laying) females and males are found, mating occurs, and overwintering eggs are produced by oviparous females. In mild-winter, southern regions at least as far north as Virginia, the sexual forms are not produced, or the egg is not the only stage of overwintering, as the aphid can reproduce parthe-nogenetically (viviparously) throughout the year in such climates.
In Virginia, winged viviparous females disperse from winter hosts such as kale and spinach during March and April to young warm-season plants such as weeds and potato. They often remain on these hosts until the plants deteriorate, often from overcrowding by aphids. The hot weather does not favor the aphids, and their numbers greatly diminish, but begin to increase again in the autumn. They are abundant in the winter on cool-season plants such as spinach, cru-cifers, and weeds. Viviparous females live for about 30 days, with 10 days in the nymphal stage and the remainder as adults. Occasionally they will survive up to 50 days. They produce, on average, about 50 young aphids, but sometimes up to 80 offspring, at about 2.5 per day.
The sexual forms also differ in appearance, both between sexes and in comparison with the partheno-genetic forms. In the oviparous female, wings are absent. The head and thorax are whitish, the abdomen pinkish or greenish, and the tibiae are dark. This form measures only about 2.15 mm long. The male has wings. The male also has a dark head and thorax, with a brownish green abdomen and dark appendages. The male measures less than 1.6 mm long.
The biology of potato aphid was described by Patch (1915), Houser et al. (1917), and Smith (1919). A good description appeared in Cottier (1953). Keys that included this aphid were found in Palmer (1952) and Blackman and Eastop (1984).
Young tissue, usually the growing tip of the plant, is attacked first by potato aphids. As the aphids multiply they spread over the entire plant, removing plant sap
Adult female potato aphid, winged form.
throughout. The leaves take on a distorted appearance, with the leaf edges turned downward. This form of injury is typical of many aphids on a variety of plants, but is especially evident on potato infested with potato aphid. Potato plants may die back from the tip downward, and heavy infestation can kill potato plants. Leaves may be covered with honeydew, which fosters growth of sooty mold. At times, potato aphid can be the most important pest of potato.
On tomato, pepper, and eggplant the leaf deformities are less apparent though stunting of the growing tips is evident. Another difference is that the blossoms are especially preferred in the latter hosts, and blossom drop or fruit deformities may occur. Walgenbach (1997) reported that potato aphid infestation resulted in higher sunscald damage to tomato fruit, presumably owing to decreased shading by foliage. Also, damage by stink bugs (Hemiptera: Pentatomidae) and leaffooted bugs (Hemiptera: Coreidae) was increased in the presence of potato aphids, perhaps because these hemipterous insects also feed on aphids. Despite the many ways in which potato aphids damage tomato, Walgenbach (1997) determined that greater than 50% of the leaves had to be infested before tomato yields were significantly depressed.
In the northwestern United States, potato aphid is very abundant on corn (Landis et al., 1972), often covering the lower surface of leaves. Despite its great abundance, it does not cause plant deformities and is not considered to be a serious pest on this crop.
In lettuce, potato aphid may be a contaminant that reduces marketability, particularly the ability to ship the crop to other nations (Hinsch et al., 1991).
Potato aphid can transmit numerous plant viruses (Kennedy et al., 1962), though it is not considered to be an especially effective vector. Among the viruses transmitted to potato and tomato are cucumber mosaic virus and potato virus Y.
Cultural Practices. Studies conducted in Maine demonstrated that planting practices could influence damage to potato. Delay of planting from early May to late May or early June, resulted in up to 90% reduction in aphid infestation. Early hilling operations, wherein row ridges are heightened with soil from the row middles, similarly deprived dispersing aphids of young plant tissue, resulting in lower aphid densities (Shands et al., 1972). Delayed planting was also shown to be beneficial in New Brunswick (Boiteau, 1984).
Cultural practices can also interfere with aphid host selection behavior. Reflective mulches, particularly aluminum mulch, are sometimes recommended for disruption of aphid invasion of crops. Evaluation of aluminum mulch in potato, however, showed that the beneficial effect of the reflective mulch was slight or of short duration, and therefore impractical (Shands and Simpson, 1972). The undersowing of potato with rye grass also affects aphids, resulting in decreased aphid densities in fields with grass, and no loss in crop yield (McKinlay, 1985).
In northern climates, aphids deposit overwintering eggs upon wild and cultivated rose, and develop on these plants in the spring. Therefore, it is desirable to destroy, or to treat with an insecticide, such overwintering sites before the aphids disperse to crops. Increasingly, aphids are shipped northward from southern climates on young plants, or overwinter in northern areas in greenhouses. Care should be taken to keep from introducing aphids into fields on transplants.
Host-Plant Resistance. There are significant differences in susceptibility to aphid infestation among tomato (Gentile and Stoner, 1968c), lettuce (Reinink and Dieleman, 1989), and to a lesser extent potato, varieties. Obrycki et al. (1983) suggested that though high densities of glandular trichomes are detrimental to aphids, plant varieties with moderately high tri-chome densities might be preferable because such varieties are more compatible with the action of predators and parasitoids. The basis for resistance in lettuce is uncertain, but butterhead varieties are less susceptible to infestation.
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