Distribution. Cabbage aphid is found throughout the United States and Canada, and in most other countries with a temperate climate. This aphid was probably introduced accidentally to North America from Europe by early colonists, as records of its damage to crops extend back to the late 1700s. (See color figure 157.)
Host Plants. Virtually all crops in the family Cru-ciferae are suitable hosts for cabbage aphid, but feeding is restricted to this family. The vegetable crops most severely affected are broccoli, Brussels sprouts, cabbage, and cauliflower, but collards, kale, kolhrabi, mustard, rape, and turnip are occasionally attacked. Beirne (1972) reported that in western Canada the principal injury of cabbage aphid was caused to early cabbage and cauliflower, whereas in eastern Canada turnip was the principal crop injured. Seed heads of radish are suitable hosts, as are numerous cruciferous weeds, especially Brassica spp. and shepherdspurse, Capsella bursa-pastoris. The mustard-oil glycoside, sini-grin, functions as a feeding stimulant for cabbage aphid, as it does for other insects that specialize in feeding on crucifers (Moon, 1967). (See color figure 20.)
Natural Enemies. Aphids normally are attacked by numerous predators and parasites, but cabbage aphid seems less susceptible to natural control than most species. Diaretiella rapae (MacIntosh) (Hymenop-tera: Braconidae) is invariably associated with this aphid, and often builds up to high levels late in the season, when aphids are most numerous. However, its ability to attain densities sufficient to suppress cabbage aphid populations is hampered by the presence of several species of hyperparasites (Pimentel, 1961; Oatman and Platner, 1973). Flower fly larvae (Diptera: Syrphidae), principally Allograpta, Syrphus and Scaeva spp., and predatory cecidomyiid Aphidoletes aphidi-myza (Rondani) (Diptera: Cecidomyiidae) larvae are generally reported to be the most important natural control agents (Oatman and Platner, 1973; Raworth et al., 1984). Lady beetles (Coleoptera: Coccinellidae), while sometimes found feeding on cabbage aphids, are unusually ineffective (George, 1957). Some authors have suggested that predators and parasitoids decreased cabbage aphid population size only after aphid population growth rate is reduced by host plant deterioration or other factors. Fungi sometimes affect cabbage aphids, but causes epizootics only at very high aphid densities.
Life Cycle and Description. Cabbage aphid may have numerous generations per year, depending on climate; 20 are reported from southern California. In the north, this species produces sexual forms and overwinters in the egg stage, whereas in the south sexual forms and eggs are not observed. Trumble et al. (1982b) reported that eggs were the overwintering stage in northern California, but that crop debris and weeds served as the overwintering site for females in the southern portion of the state. Unlike many aphids, cabbage aphid does not disperse to an alternate host to overwinter.
The winged (alate) parthenogenetic females are similarly dull green with a dark head, but the legs are dark-brown. The dorsal surface of the abdomen is marked with a single row of dark bars, and the veins of the transparent wings are dusky to black. The cornicles are dusky-green to black. The entire body of this insect is dusted with a fine white powder. The winged parthenogenetic female measures 1.6-2.8 mm long, averaging about 1.9 mm. The winged forms, though not strong fliers, quickly disperse over crop fields and to adjacent fields. The winged females have a shorter life span than the wingless form; the former persists only about 10 days. They also produce relatively few offspring, about 6-8.
In the autumn months, especially September and October, egg-laying forms may be produced. Apparently the oviparous forms can be produced both by winged and wingless summer forms. The oviparous female has a pale green or greenish-yellow body, with a row of dark bars located centrally on the back. This form is wingless and measures about 2.2 mm long. Males are also produced at this time, but unlike the wingless oviparous females, the males have transparent wings. Males are greenish-brown or yellowish, and have dark legs and a double row of dark bars on the back. The body measures about 1.3 mm long.
Cabbage aphid is easily confused with turnip aphid, Lipaphis erysimi (Kaltenback), though turnip aphid generally can be distinguished by the very sparse occurrence of waxy exudate on the body and longer cornicles. In humid climates, however, there tends to be greater accumulation of waxy secretions on the turnip aphid's 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 it is long. In contrast, the cauda of turnip aphid is slender, about twice as long as it is wide.
A comprehensive treatment of cabbage aphid was provided by Herrick and Hungate (1911). Brief description of these aphids and keys for their identification are found in Palmer (1952) and Blackman and Eastop (1984). Cottier (1953) also provided a description of cabbage aphids.
Cabbage aphid populations, if not controlled, often build to very high densities. Heavily infested plants acquire a grayish appearance due to the mass of aphid bodies on the foliage. Honeydew and sooty mold are often evident. High densities also cause the leaves to wrinkle and curl, usually cupping downward. Under dry conditions, aphids cause the plants to wilt, and leaf tissue that has been fed upon may turn yellow. Cabbage aphid prefers the youngest tissue and highest portions of the plant, but may occur on both the upper and lower surface of foliage. Flower heads of seed crops may be attacked, reducing the setting of seed. Contamination of the plants with honeydew and
aphids can cause considerable loss. In California, cabbage aphid is the most serious aphid pest of Brussels sprouts and broccoli because they are hard to remove from the head of the plant and thereby contaminate the produce (Trumble, 1982b; Pickel et al, 1983).
Wolfenbarger (1967) estimated the impact of aphid densities on cabbage yield. He projected that when aphid densities averaged one per square inch (0.16 per sq cm) on the most heavily infested leaf of each plant, the proportion of plants in a field that was aphid-free would be 64%. Further, he estimated that when there was an average aphid density as low as one per plant, the proportion of unmarketable heads would be 12%.
In addition to the direct effects of feeding by aphids on plant growth, and the damage caused by aphid contamination of foliage, cabbage aphid can also be a vector of plant viruses. Over 30 viruses are known to be transmitted by B. brassica. Cauliflower mosaic and cabbage ring spot virus transmission were studied by Broadbent (1954); cabbage aphid transmits cauliflower mosaic more effectively because this virus concentrates in the young tissue of the plant, which is the preferred feeding site of the aphid. Host-plant resistance offers considerable potential, but currently it is difficult to put into practice. Both very hairy and hairless cultivars are likely to be less suitable for parasitoid activity than plants with intermediate densities of plant hairs.
Cultural Practices. As the eggs and other forms of aphids often overwinter on crop residue, destruction of crop remains after harvest is strongly recommended. This recommendation is often disregarded by home gardeners because the crucifers are quite cold tolerant, and it is sometimes possible to leave plants in the field and to harvest them throughout the winter months. If this is done, however, efforts should be made to bury plant debris deeply before new crops are put into the field.
The presence of weed hosts, or cruciferous oilseed crops such as rape, may exacerbate problems with cabbage aphid. In contrast, interplanting crucifers with nonhost crops may decrease aphid problems (Gliess-man and Altieri, 1982). In studies conducted in New Zealand, aphid populations were decreased when cabbage was interplanted with pollen-rich flowering plants due to the increased predatory activities of flower flies (Diptera: Syrphidae) (White et al., 1995).
Crop condition influences cabbage aphid abundance. Wearing (1972) reported that intermittent water stress was favorable and continuous water stress was detrimental to aphid reproduction and survival. van Emden (1966) indicated that high nitrogen and low potassium levels in foliage favored aphids.
Host-Plant Resistance. Several strains of crucifer crops have been observed to exhibit measurable levels of host-plant resistance. However, there are also numerous biotypes of the aphid, and no plant varieties have yet been developed that were permanently resistant to aphid attack. Also, varieties that are resistant to attack by cabbage aphid may be more suitable for other insects. For example, Way and Murdie (1965) reported that "glossy" Brussels sprouts, strains lacking the superficial waxy bloom commonly found on cabbages, supported fewer cabbage aphids than did the normal waxy strains. However, green peach aphids, Myzus persicae (Sulzer), were more abundant on glossy than waxy strains.
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