butional records and economic impact of both species are sometimes incorrectly reported. Turnip aphid is found throughout the United States, where it sometimes is more important than cabbage aphid, Brevicor-yne brassicae (Linnaeus), as a vegetable pest. In Canada, it is found in the southernmost regions of the country from the east coast to the west coast. Its origin is Europe. (See color figure 156.)
Host Plants. This aphid has been found associated with all Brassica crops, including such vegetables as broccoli, Brussels sprouts, cabbage, cauliflower, col-lards, kale, kohlrabi, mustard, and turnip. Other cruciferous plants such as the vegetables radish and watercress, and the weed shepherdspurse, Capsella bursa-pastoris, also are known as suitable hosts. Records from lettuce, bean, and onion are likely due to incorrect identifications.
Natural Enemies. Turnip aphid is host to virtually the same parasitoids and predators found attacking cabbage aphid. Diaeretiella rapae (MacIntosh) (Hyme-noptera: Braconidae) is frequently reported to be an important biological suppressant, but in rapidly maturing crops such as radish apparently there is insufficient time for the parasitoids to attain high levels of abundance. Various lady beetles (Coleoptera: Coccinellidae), flower flies (Diptera: Syrphidae), lace-wings (Neuroptera: Chrysopidae), and the fly Aphido-letes aphidimyza Rondani (Diptera: Cecidomyiidae) prey on the aphids (Davis and Satterthwait, 1916a; Paddock, 1915b; Pimentel, 1961). Fungus epizootics sometime occur when aphid population densities are high, especially during the autumn months.
Life Cycle and Description. The turnip aphid is very prolific. Under the temperate conditions of Indiana, Davis and Satterthwait (1916a) reported that 1125 generations could be produced annually. Under the warmer conditions of Texas, Paddock (1915b) observed a maximum of 35 generations. Aphid longevity is usually 20-40 days, and reproduction begins about six days after aphids reach maturity. Wingless parthenogenetic females typically produce 80-100 young, often at 4-6 per day. Winged parthenogenetic females produce fewer offspring.
The winged (alate) adult females are pale green, with a black head and thorax. The last three abdominal segments are marked dorsally by narrow black bands. The sides of the abdomen also bear black patches. The legs are brownish to blackish, and the antennae are black. The cornicles are dusky yellowish, with a small black area around the base. The wings are transparent, but marked with conspicuous black veins. Winged females measure about 1.4-2.2 mm long.
Males have also been observed occasionally, but they are too infrequent for their biology to be known. The wingless male is quite small, measuring 1.2-1.3 mm long. It is olive-green to brown.
Turnip aphid generally can be differentiated from cabbage aphid by the very sparse occurrence of waxy exudate on the body, and by the slightly longer cornicles, 0.23 mm, of the former. In humid climates, however, there tends to be greater accumulation of waxy secretions on the aphid body. Therefore, Blackman and Eastop (1984) recommended using the shape of the cauda—a structure found at the tip of the abdomen, to differentiate these two species. When viewed from above, the cauda of cabbage aphid is triangular, about as wide as long. In contrast, the cauda of turnip aphid is slender, about twice as long as wide.
The biology of turnip aphid was described by Paddock (1915b) and Davis and Satterthwait (1916a). Keys for identification of turnip aphid were provided by
Adult female turnip aphid, wingless form.
Palmer (1952) and Blackman and Eastop (1984). A good description was provided by Cottier (1953).
Turnip aphids feed mainly on the undersides of leaves, but under high aphid densities or during the winter months the aphids may move to the center of the plant and feed on both the upper and lower surfaces of tender young foliage. They also feed readily on the stem tissue of the flowers. Heavy infestations can kill plants, and even light infestations can cause the leaves to cup, with yellowing of the foliage occurring where aphids are concentrated. Stunting of the plants is common, but contamination of foliage with aphid bodies, cast skins, and honeydew is the principal type of direct damage.
Turnip aphid may also serve as a vector of stylet-borne plant viruses. Kennedy et al. (1962) listed over 10 viruses transmitted, including cabbage black ring spot, cauliflower mosaic, and radish mosaic. Turnip mosaic virus also may be transmitted.
Host-Plant Resistance. Varietal differences occur among turnip cultivars in ability to support turnip aphid (and cabbage aphid) populations (Kennedy and Abou-Ghadir, 1979). As the aphid-resistant variety is only moderately resistant to aphids, and is more susceptible to attack by caterpillars, variety selection many not be a satisfactory management tactic.
Disease Transmission. The techniques used to prevent transmission of plant viruses by aphids has been discussed under melon aphid. Turnip aphid produces an alarm pheromone when disturbed (Phelan et al., 1976). Alarm pheromones have been shown to inhibit virus transmission experimentally (Gibson et al., 1984), but thus far there is no practical application of this technology.
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