Distribution. The origin of pea aphid is likely Europe or Asia, though it is now found throughout the world in regions with temperate climates. In North America, it was first noted about 1878, and became first serious pest problem about 1900 when it caused extensive damage in the mid-Atlantic states from New Jersey to Virginia, and in eastern Canada from Nova Scotia to Quebec. By the 1950s it had spread throughout the United States and Canada. (See color figure 148.)
Blue alfalfa aphid is native to Asia, but it has spread to Australia, New Zealand, and South America in addition to North America. First observed in California in 1974, it now is widely distributed—blue alfalfa aphid reached Nebraska in 1979, Kentucky and Georgia in 1983, and Maryland in 1992. This species is not yet known as a pest in Canada.
Host Plants. Pea aphid and blue alfalfa aphid are known principally as pests of Leguminosae. As they are efficient vectors of plant viruses, however, they also cause loss in crops on which they normally do not feed, such as cucurbits.
Pea aphid is prone to develop races or subspecies with slightly different host ranges, so populations may differ somewhat in their damage potential to specific legumes (Auclair and Srivastava, 1977). Pea is the most suitable vegetable host for this species, and faba bean and lentil are sometimes damaged. Other hosts that are important in pea aphid biology are alfalfa, sweet pea, vetch, and such clovers as alsike clover, red clover, white clover, and sweet clover.
Blue alfalfa aphid is not widely recognized as a vegetable pest, but Ellsbury and Nielson (1981) demonstrated that pea, lentil, and cowpea were suitable hosts. For the pea aphid, many legume forage crops are suitable hosts.
Natural Enemies. Many natural enemies of pea aphid have been documented (Fluke, 1929), and most also will attack blue alfalfa aphid. Flower fly (Diptera: Syrphidae) and lady beetle larvae (Coleoptera: Cocci-nellidae) species are most numerous among the natural enemies, and sometimes important in regulating aphid population densities. However, fungi (Wilding, 1975; Pickering and Gutierrez, 1991) and parasitic wasps (Hymenoptera: principally Braconidae) also are very important, with introduced species of Aphi-dius normally dominant. The braconid Aphidius ervi Haliday is often reported to be the most important parasitoid, and was observed in North America soon after the introduction of pea aphid, probably being introduced simultaneously. Initially it did not provide effective suppression, and other introductions of beneficial species were made, apparently including additional subspecies or races of A. ervi. Among the other species introduced were A. smithi Sharma and Subba Rao; A. eadyi Stary, Gonzalez, and Hall; A. urticae Hali-day; and A. staryi Chen, Gonzalez, and Luhman. Apparently there has been displacement among species and strains of parasitoids, with the native A. pisi-vorus Smith displaced by imported A. smithi, and then the latter species displaced by A. ervi (Gonzalez et al., 1995b). Blue alfalfa aphid has proved especially susceptible to parasitism by A. ervi, but damage by pea aphid also has been reduced in the western states. Good sources of information on pea aphid parasites are Mackauer and Finlayson (1967), Angalet and Fue-ster (1977), and Marsh (1977).
A rather predictable cycle of events is observed in many pea aphid populations. After building to high densities on either forage legumes or peas in the spring months, predators, parasites, and fungi take a toll. Both the natural enemy densities and weather are unfavorable at mid-summer, and aphid populations decline. Warm, humid weather, in particular, favors development of fungal epizootics among pea aphids. With the subsequent collapse of natural enemy populations and the return of cooler, more favorable weather for aphids in the autumn, pea aphid populations typically increase again. In Canada and other northern latitudes the aphid populations may not attain high densities until late summer, so there is only one population peak.
Blue alfalfa aphid displays similar cycles, but tends to be favored by cooler weather; thus, it is most common in the early spring and is displaced by pea aphid as the season progresses. Because it is present early in the season, predator and parasitoid populations often are low, and have little suppressive effect. The same biological control agents usually affect both species of aphids, and the presence of blue alfalfa aphid early in the year seems to favor the increase of some agents such as the fungus Pandora neoaphidis, which then has its greatest impact on the later-developing pea aphid population (Pickering and Gutierrez, 1991).
Weather. In addition to the effect of weather on disease incidence, there also can be direct effects of weather on pea aphids. In cold climates, which are normally characterized by abundant snowfall, egg survival rates are high and overwintering success is not highly variable. In mild-winter areas, such as Oregon and Washington, overwintering success is more variable. Mild winters favor the survival of overwintering females and eggs. During cold winters, however, when temperatures fall below freezing, neither adults nor eggs survive well. In addition to the cold temperatures, the lack of snow cover is thought to reduce the overwintering success rate. Also, aphids are especially susceptible to destruction by such adverse weather owing to heavy rainfall in the early part of the season, soon after egg hatch (Cooke, 1963). The lower developmental threshold for pea aphid is about 5.5°C, whereas its principal parasitoids is 6-7°C (Campbell and Mackauer, 1975). The higher developmental threshold of the parasitoids allows the aphid populations to increase to higher densities during cool spring weather.
Life Cycle and Description. Acyrthosiphon aphids complete their life cycle quickly. These aphids can reach maturity and begin reproduction 10-12 days after birth. The number of generations completed annually by pea aphid is estimated at about 13 in Indiana, 14-15 in Wisconsin, 15 or more in Washington and Oregon, and 20-22 in Virginia. The overwintering stage of pea aphid varies with climate; in cold regions the eggs overwinter, in warm areas females persist, and in temperate climates both eggs and females can be found during winter months. In blue alfalfa aphid, overwintering is similar. Unlike many species of aphids, these species do not migrate to a woody host for overwintering. However, they do commonly disperse from annual legumes in the summer to perennial legumes such as alfalfa and clover in the autumn, so the difference in behavior is not great (Evans and Gyrisco, 1956).
Blue alfalfa aphid is similar in appearance to pea aphid. However, the blue alfalfa aphid averages slightly smaller in size, apterae measuring about 2.52.7 mm long and alatae measuring about 2.5 mm.
Although this species tends to have a slightly blue-green appearance, this is not a useful diagnostic character. An important diagnostic character is antennal color. In pea aphid the distal portion of each antennal segment is dark, giving the appearance of black bands spaced widely on the antennae. In contrast, the antennae of blue alfalfa aphid are uniformly dark.
Sexual forms are produced by pea aphid in the autumn in response to shorter day length and cooler temperature, and males and females may be alate or apterous. The females greatly resemble the apterous viviparous females, differing principally in size. Oviparous females measure only about 2.0-2.2 mm long. The greenish males also bear brown coloration on the thorax, and usually brown or yellowish-brown on the head and abdomen. The males measure less than 2.0 mm long. Sexual forms are rare at the latitude of northern California, Tennessee, Virginia, and southward, but in Indiana and Oregon both eggs and viviparous forms are found during the winter months. Production of sexual forms is less predictable than in many other species of aphids. For example, oviparous females may be produced by either alate or apterous females, and the same female may produce both viviparous and oviparous forms alternately. Fecundity of oviparous forms seems unknown, but based on examination of ovaries, at least 25 eggs are produced by each female. Cooke (1963) suggested that this value may be the maximum, due to inclement weather late in the year. Blue alfalfa aphid seems to have a similar biology.
The aphids produce and respond to alarm phero-mone. Pheromone production normally results when aphids are disturbed, and the standard response by aphids is to walk away from the source of the phero-mone or to drop from the plant. Responsiveness to pheromone is increased when the substrate is vibrated (Clegg and Barlow, 1982). Young instars are less responsive (Roitberg and Myers, 1978).
Description of pea aphid development and biology were given by many authors; among the most complete were Davis (1915) and Cooke (1963). Developmental biology of pea aphid was provided by Hutchison and Hogg (1984), and of blue alfalfa aphid by Kodet et al. (1982). Culture of pea aphids on plants was described by Halfhill (1967), and on artificial diet by Akey and Beck (1971), and Cloutier and Mackauer (1975). A description of blue alfalfa aphid can be found in Takahashi (1965); life table studies of blue alfalfa aphid were given by Poswal et al. (1990).
Pea and blue alfalfa aphids cause direct injury by removal of plant sap from leaves, stems, blossoms, f -i ■ • <■; Yl ft
Adult female pea aphid, wingless form.
Adult female pea aphid, wingless form.
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