Distribution. Green peach aphid is found all over the world, including the tropics and temperate latitudes. In the United States, it is considered to be a pest in all states. In Canada, it occurs from Nova Scotia to British Columbia, where it is viewed as a pest principally due to its ability to transmit plant viruses (Beirne, 1972). In addition to attacking plants in the field, green peach aphid readily infests vegetables and ornamental plants grown in greenhouses. This allows high levels of survival in areas with inclement weather, and favors ready transport on plant material. When young plants are infested in the greenhouse and then transplanted into the field, fields are not only inoculated with aphids, but insecticide resistance may be introduced. The aphids are also reported to be transported long distances by wind and storms. The origin is Asia. (See color figure 153.)
Host Plants. Green peach aphid feeds on hundreds of host plants in over 40 plant families. However, it is only the viviparous summer stages that feed so widely; the oviparous winter stages are much more restrictive in their diet choice. In temperate latitudes the primary or overwintering hosts are trees of the genus Prunus, particularly peach and peach hybrids, but also apricot and plum. During the summer months the aphids abandon their woody hosts for secondary or herbaceous hosts, including vegetable crops in the families Solanaceae, Chenopodiaceae, Compositae, Cruciferae, and Cucurbitaceae. Vegetables that are reported to support green peach aphid include artichoke, asparagus, bean, beets, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, cantaloupe, celery, corn, cucumber, fennel, kale, kohlrabi, turnip, eggplant, lettuce, mustard, okra, parsley, parsnip, pea, pepper, potato, radish, spinach, squash, tomato, turnip, watercress, and watermelon. Crops differ in their susceptibility to green peach aphid, but it is actively growing plants, or the youngest plant tissue, that most often harbors large aphid populations (Heathcote, 1962). In warmer climates the aphids do not seek out overwintering hosts, but they persist as active nymphs and adults on hardy crops and weeds.
Broadleaf weeds can be very suitable host plants for green peach aphid, thereby creating pest problems in nearby crops. Tamaki (1975), for example, estimated that about 3-16 million aphids per acre were produced on weeds growing on the floor of peach orchards. Up to one-third of the aphids feeding on weed species carried beet western yellows virus (BWYV) (Tamaki and Fox, 1982). Peach trees are not a host of BWYV, so weeds are obviously good reservoirs for plant virus. Common and widespread weeds such as field bindweed, Convolvulus arvensis; lambsquarters, Chenopo-dium album; and redroot pigweed, Amaranthus retroflexus, are often cited as important aphid hosts (Annis et al, 1981).
Natural Enemies. Hundreds of natural enemies have been recorded, principally lady beetles (Coleoptera: Coccinellidae), flower flies (Diptera: Syrphidae), lacewings (Neuroptera: mainly Chrysopidae), parasitic wasps (Hymenoptera: Braconidae), and entomo-pathogenic fungi (mainly Entomophthorales). van Emden et al. (1969) provided a long list of beneficial organisms. Most are general predators, and move freely among green peach aphid, other aphids, and even other insects. In some cases the natural enemies are influenced by the host plant, crop cultural practices, and environmental conditions (Tamaki et al., 1981). Green peach aphid, and many other aphids, may occasionally be cannibalistic or predatory (Banks et al., 1968); however, this behavior occurs only under stressful condition and is not known to be an important factor in population regulation. Quantitative data generally are lacking for the influence of most natural enemies. Weather also reportedly contributes a significant change in aphid numbers, including direct mortality (Beirne, 1972), but this also is poorly documented.
The ephemeral nature of aphid infestation in many crops is believed to prevent the beneficial organisms from consistently locating the aphids and reproducing in a timely manner. Nevertheless, anyone who has frequently observed green peach aphid at high densities probably has observed sudden population decreases following the appearance of lady beetles, wasp parasi-toids, or entomopathogenic fungi. For example, green peach aphid infesting spring-harvested spinach crops in Arkansas and Oklahoma is suppressed late in the growing season by Erynia neoaphidis fungus. Unfortunately, the disease epizootic often occurs too late to keep aphids from attaining high numbers, and fungus-infected aphids remain attached to foliage, providing a serious contaminant of spinach foliage (McLeod et al., 1998). Various studies that selectively excluded or killed beneficial organisms have demonstrated the explosive reproductive potential of these aphids in the absence of biological control agents, thus demonstrating their value in reducing damage potential. In greenhouse crops, where environmental conditions and predator, parasitoid, and pathogen densities can be manipulated, biological suppression is more effective and consistent.
Integration of chemicals with natural enemies offers promise for enhanced protection from aphid damage. Shean and Cranshaw (1991) demonstrated that Apheli-nus semiflavus Howard (Hymenoptera: Encyrtidae) and Diaeretiella rapae (Mcintosh) (Hymenoptera: Bra-conidae) differed significantly in their relative susceptibility to insecticides, depending on the chemical evaluated. Also, these parasitoids, while in the mummy stage, were less susceptible to insecticide toxicity than green peach aphid. In general, however, insecticide use in crops is more disruptive to parasi-toids than to aphids, leading to larger aphid populations. Sublethal doses of some insecticides also increase aphid reproduction (Lowery and Sears, 1986).
Despite the beneficial nature of these biotic agents, virus diseases can be effectively transmitted by very low aphid densities. In crops susceptible to aphid-borne virus disease, natural enemies alone are probably destined to be relatively ineffective in preventing damage.
Life Cycle and Description. The life cycle varies considerably, depending on the presence of cold winters. van Emden et al. (1969) provided a useful review of the life cycle. Development can be rapid, often 10-12 days for a complete generation, and with over 20 annual generations reported in mild climates. Where suitable host plants cannot persist, the aphid overwinters in the egg stage on Prunus spp. In the spring, soon after the plant breaks dormancy and begins to grow, the eggs hatch and the nymphs feed on flowers, young foliage, and stems. After several generation on Prunus spp., dispersants from overwintering hosts deposit nymphs on summer hosts. In cold climates, adults return to Prunus spp. in the autumn, where mating occurs and eggs are deposited. All generations except the autumn generation culminating in egg production are parthenogenetic.
The offspring of the dispersants from the overwintering hosts are wingless, and each produces 30-80 young. The wingless (apterous) aphids are yellowish or greenish. They measure about 1.7-2.0 mm long. A medial and lateral green stripes may be present. The cornicles are moderately long, unevenly swollen along their length, and match the body in color. The appendages are pale. The rate of reproduction is positively correlated with temperature, with the developmental threshold estimated to be about 4.3°C. As aphid densities increase or plant condition deteriorates, winged forms are again produced to aid dispersal. The nymphs that give rise to winged females may be pinkish. The dispersants typically produce about 20 offspring, which are always wingless. This cycle is repeated throughout the period of favorable weather.
In the autumn, in response to change in day length or temperature, winged male and female aphids are produced which disperse in search of Prunus. Timing is an important factor, as foliage on the Prunus hosts is physiologically optimal as leaves begin to senesce. Females arrive first and give birth to wingless (apterous) egg-laying forms (oviparae). Males are attracted to oviparae by a pheromone, capable of mating with several females, and the eggs are produced. The ovi parous female deposits 4-13 eggs, usually in crevices in and near buds of Prunus spp. The oviparous female is 1.5-2.0 mm long, and pinkish in color.
Parthenogenic reproduction is favored in the many parts of the world, where continuous production of crops provides suitable host plants throughout the year, or where weather allows survival on natural (noncrop) hosts. The average temperature necessary for survival of active forms of green peach aphid is estimated at 4-10°C. Plants that readily support aphids during the winter months include beet, Brussels sprout, cabbage, kale, potato, and many winter weeds.
In the Pacific Northwest, both yellow and green strains coexist (Tamaki et al., 1982). The yellow strain is holocyclic—a sexual generation in the autumn produces overwintering eggs. The green strain is anholo-cyclic—no sexual generation is produced. The egg-producing populations are less tolerant of cold weather, and deposit eggs on Prunus. The anholocyclic populations remain active throughout the year by feeding during the winter on weeds growing adjacent to warm springs, drainage ditches, and slopes exposed to solar radiation.
There are several synthetic diets suitable for aphid culture, such as the one described by Mittler et al. (1970). Cottier (1953) provided a good description of green peach aphid. Keys for identification of green peach aphid, and many other common aphids, were found in Palmer (1952), and Blackman and Eastop (1984). Stoetzel et al. (1996) published a key for cotton aphids that is also useful to distinguish green peach aphid from most other common vegetable-infesting aphids.
Adult female green peach aphid, winged form.
Green peach aphids can attain very high densities on young plant tissue, causing water stress, wilting, and reduced-growth rate of the plant. Prolonged aphid infestation can cause appreciable reduction in yield of root crops and foliage crops. Early season infestation is particularly damaging to potato, even if the aphids are subsequently removed (Petitt and Smi-lowitz, 1982). Contamination of harvestable plant material with aphids, or with aphid honeydew, also causes loss. In Arkansas, mild winters allow good overwintering survival of green peach aphid on spinach, thereby leading to contamination problems (McLeod, 1987). However, green peach aphid does not seem to produce the high volume of honeydew observed with some other species of aphids. Blemishes to the plant tissue, usually in the form of yellow spots, may result from aphid feeding. Leaf distortions are not common except on the primary host. Contamination of vegetables by aphids sometimes presents quarantine problems (Stewart et al., 1980), and fumigation techniques have been developed that kill the insects without causing harm to the vegetables.
The major damage caused by green peach aphid is through transmission of plant viruses. Indeed, this aphid is considered by many to be the most important vector of plant viruses all over the world. Nymphs and adults are equally capable of virus transmission (Namba and Sylvester, 1981), but adults, by virtue of being so mobile, probably have greater opportunity for transmission. Both persistent viruses, which move through the feeding secretions of the aphid, and nonpersistent viruses, which are only temporary contaminants of aphid mouthparts, are effectively transmitted. Kennedy et al. (1962) listed over 100 viruses transmitted by this species. Some of the particularly damaging diseases include potato leafroll virus and potato virus Y to Solanaceae, beet western yellows and beet yellows viruses to Chenopodiaceae, lettuce mosaic virus to Compositae, cauliflower mosaic and turnip mosaic viruses to Cruciferae, and cucumber mosaic and watermelon mosaic viruses to Cucurbitaceae. A discoloration in potato tubers, called net necrosis, occurs in some potato varieties following transmission of potato leafroll.
Management options for green peach aphid are very similar to procedures discussed for melon aphid, Aphis gossypii Glover.
Green peach aphid is often a pest of cold-weather crops like spinach. Aphids are inherently difficult to kill with contact insecticides because they are often found under the leaves or on new, sheltered growth. Cold weather (less than about 20° C) exacerbates the problem because of less volatilization (fumigation) by the insecticide (Wolfenbarger, 1972). Even systemic insecticides, which kill aphids feeding under the leaf when applied to the upper surface, are much less effective at cool temperatures (McLeod, 1991).
Biological Control. Mackauer (1968) noted that because green peach aphid was able to develop at lower temperatures than its parasitoids, the wasps were very beneficial only in benign climates or where temperature could be controlled, as in some greenhouses. Indeed, there has been considerable success using parasitoids, the entomopathogenic fungus Ver-ticillium lecanii, and the predatory midge Aphidoletes aphidimyza (Diptera: Cecidomyiidae) for greenhouse-grown vegetables, especially in Europe (Gilkeson and Hill, 1987; Milner and Lutton, 1986).
In the field, biological control agents may be differentially affected by the cropping system. For example, Tamaki et al. (1981) found that the wasp Diaeretiella rapae (Hymenoptera: Braconidae) was more effective in broccoli, whereas lady beetles (Coleoptera: Cocci-nellidae) and bigeyed bug (Hemiptera: Lygaeidae) predators were more effective on radish.
Cultural manipulations may benefit predators and parasitoids. In Washington, bands placed around the trunks of peach trees provided good harborage for predators that may suppress the aphids in the spring, thereby reducing the number dispersing to vegetables (Tamaki and Halfhill, 1968). In California asparagus, a brown lacewing (Neuroptera: Hemerobiidae) consistently reduces green peach aphid populations, but benefits from application of supplemental food sprays (Neuenschwander and Hagen, 1980). In New Zealand, pollen levels were supplemented by interplanting flowering plants with cabbage, increasing predation of aphids by flower flies (Diptera: Syrphidae) (White et al, 1995).
Cultural Practices. The overwintering behavior of green peach aphid, which in many areas is restricted to Prunus or other relatively restricted sites, has fostered research on techniques to reduce aphid abundance and disease transmission to vegetables by either removing the overwintering site or by eliminating the aphids before they disperse to vegetables. Destruction of peach and apricot trees (often found in association with houses), and treatment of trees with dormant oil and insecticide, have been used in western states to disrupt aphid population increase and disease transmission (Powell and Mondor, 1976). Similarly, vegetable and flower plants grown in greenhouses during the winter months have been shown to be an excellent source of infestation during the following spring (Bishop and Guthrie, 1964), and incidence of leafroll in potatoes grown in Idaho is directly related to the abundance of aphids in home gardens. In Colorado, inspection of garden centers and treatment of seedlings found infested with aphids are important elements of the overall potato leafroll reduction effort. As is usually the case with aphids, green peach aphid populations tend to be higher when plants are fertilized liberally with nitrogen fertilizers (Jansson and Smilowitz, 1986).
Small arthropods such as green peach aphid are susceptible to injury by secretions of glandular leaf hairs found on some plants, including wild potato (Tingey and Laubengayer, 1981). Younger aphids are particularly susceptible. Although this is a promising source of plant resistance, this valuable trait has yet to be incorporated into popular commercial cultivars.
Disease Transmission. Because some of the virus diseases transmitted by green peach aphid are persistent viruses, which typically require considerable time for acquisition and transmission, insecticides can be effective in preventing disease spread in some crops. Research in Minnesota (Flanders et al., 1991) showed that potato leafroll virus was transmitted within the potato crop principally by wingless aphids moving from plant to plant. Infected seed potatoes are the principal source of leafroll in most potato crops, so planting disease-free seed is obviously an important step in minimizing the incidence of the disease. Growers commonly inspect fields for signs of disease, and remove and destroy infected and nearby plants— a process called "roguing." This procedure reduces the ability of aphids to spread disease from plant to plant. Insecticides may not keep winged aphids from alighting in a crop and quickly transmitting nonpersistent virus, but they can certainly prevent the secondary spread of virus within a crop by colonizing aphids. As is the case with melon aphid, however, insecticide resistance is a severe problem in many areas. Application of mineral oil (Ferro et al., 1980; Lowery et al., 1990) and use of aluminum or white plastic mulch (Wyman et al., 1979) decrease virus transmission. (See melon aphid for further discussion of disease transmission.) Aphids that are not effectively repelled by reflective mulch seem to thrive on mulched crops (Zalom, 1981a) and exhibit high rates of reproduction. Therefore, even in mulched crops some aphid control is necessary.
Green peach aphid is quite responsive to alarm pheromone, which is normally produced when aphids are disturbed (Phelan et al., 1976). Application of alarm pheromone has shown the potential to disrupt virus transmission (Gibson et al., 1984), but this has yet to become an operational technology. A sex pheromone is also known from this aphid, but it functions only at short distances, and has not yet proved useful in aphid management (Dawson et al., 1990).
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