Homoptera Aphididae

Natural History

Distribution. The bean aphid, which is probably a complex of closely related species, apparently is native to Europe, but it has been spread to most temperate areas of the world except for Australia and New Zealand. Although it occurs in tropical areas of Africa, it is not a very serious pest in warm environments. Bean aphid occurs throughout the United States and southernmost Canada. (See color figures 19 and 151.)

Host Plants. This aphid feeds on a wide range of hosts, though it seems to favor plants in the family Chenopodiaceae as summer hosts. Vegetables attacked include asparagus, beet, carrot, celery, corn, fava bean, leek, lettuce, lima bean, onion, parsnip, pea, spinach, pea, rhubarb, and squash. It also attacks sugarbeet, and in Europe it is considered to be a very serious pest because it transfers viruses to this crop. Flowers such as nasturtium and dahlia commonly support this insect, as do many weeds, including curly dock, Rumex crispus; lambsquarters, Chenopodium album; and shepherdspurse, Capsella bursa-pastoris. The taxonomy of this insect is confused, and some host records may prove to be due to other closely related species.

The winter, or primary, hosts of bean aphid are Euonymus spp. and Viburnum spp. In England, abundance of bean aphid is positively correlated with abundance of spindle tree, Euonymus europaeus (Way and Cammell, 1982). Similarly, in Colorado, bean aphid primarily affects crops grown adjacent to urban areas, where Euonymous and Viburnum spp. are cultivated as ornamental shrubs.

Natural Enemies. Fungi are an important mortality factor, often suddenly sweeping through high density populations. Several species of fungi are involved, but Neozygites fresenii apparently is most effective in suppressing bean aphid populations (Dedryver, 1978; Wilding and Perry, 1980; Rabasse et al., 1982). Parasitism by native parasitoids, especially by Lysiphlebus testaceipes Cresson (Hymenoptera: Braconidae), is usually apparent when aphids are abundant. General predators such as green lacewings (Neuroptera: Chry-sopidae), lady beetles (Coleoptera: Coccinellidae), and flower fly larvae (Diptera: Syrphidae) often are found feeding in bean aphid colonies. Ants commonly attend bean aphid, harvest honeydew, and apparently interfere with predators and enhance aphid survival (Capinera and Roltsch, 1981).

Natural enemies are believed to play an important role in the population cycles of bean aphid. When aphids are numerous on early season hosts they provide abundant food for natural enemies, which then reach high levels of abundance in late season populations, and greatly reduce the number of aphids that overwinter. Therefore, the aphid population in the following year is quite low. However, low early season populations fail to support natural enemies, so the aphids are free to reproduce and attain high late season and overwintering population densities. This, of course, means that the early season population will again be high. Because weather interacts with the aphids and their natural enemies, the cycling is not very predictable, so population-monitoring systems have been developed in England to aid in the prediction of aphid outbreaks.

Life Cycle and Description. The eggs hatch in the spring, often as early as February, and produce 1-2 generation of apterous (wingless) parthenogenetic females. This is followed by a generation of alate (winged) females that fly from the primary host to secondary (summer) host plants, where females reproduce parthenogenetically all summer. Alate and apterous forms are produced according to density and host-plant conditions, with high densities and depleted plants tending to produce alatae. In the autumn, in response to short-day conditions, alatae are produced that colonize the primary hosts. On the overwintering host, the females mate and lay eggs among cracks in bark and on bud axils.

  1. In temperate areas, bean aphid overwinters as an egg on one of its primary hosts. Initially green, the eggs soon turn shiny black. In warmer areas, the aphids reproduce continuously without producing eggs.
  2. Nymphs are dark, but bear four pairs of transverse white bars on the dorsal surface of the abdomen. Most authors report four instars. Tsitsipis and Mittler (1976), for example, indicated durations of 2, 2, 1.5, and 2.5 days for instars 1-4, respectively, when reared at about 20°C. Ogenga-Latigo and Khaemba (1985), however, reported only three instars, with durations of about 2.3, 3.0, and 2.5 days. Total nymphal development time requires 5-10 days at 28-17° C, respectively.
  3. Bean aphid is dark olive-green to dull-black in color. The body length is 1.8-2.4 mm in females, with males only slightly smaller. The appendages tend to be black, but the tibiae may be pale, in part. The wings are transparent. On some crops bean aphid may be confused with another black species, cowpea aphid, Aphis craccivora Koch; adults of this latter species are shiny black with whitish legs, and they average smaller in size.

Reproduction commences soon after attainment of the adult stage, usually a period of about 3-6 days. Both alate and apterous females reproduce. The adult produces about 85-90 nymphs during her reproductive period, which is estimated at 20-25 days. Most offspring are produced in the first 5-10 days of the reproductive period. Reproduction increases with temperatures, up to a threshold of about 24°C, and then decreases. The reproductive period is followed by a post-reproductive period of about seven days (Frazer, 1972). Apterous females give birth to more, and larger, nymphs than alatae (Dixon and Wratten, 1971).

The winged aphids disperse freely, but their eventual disposition depends largely on wind and windbreaks, because they do not have strong powers of flight. Thus, leeward sides of hills and wind-breaks are the areas where aphids accumulate. They also are deposited more heavily on the edges of crops. Because small fields have proportionally more "edge," aphid mean density also tends to be higher in small plantings. Dispersal of up to 30 km sometimes occurs.

An excellent summary of bean aphid ecology was given by Cammell (1981). Tsitsipis and Mittler (1976) provided information on rearing aphids on both

Adult female bean aphid, wingless form.
Adult female bean aphid, winged form.

plants and artificial media. Keys for identification of bean aphid, and most other common aphids, are 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 bean aphid from most other common vegetable-infesting aphids.

Damage

Aphids tend to reproduce rapidly and build to high numbers on plants that are actively growing. They concentrate their feeding on young tissue and may deplete nutrients needed for plant growth. In sugar-beet, for example, bean aphid infestation decreased foliage and root weights, and sucrose yield (Capinera, 1981). Symptoms of infestation include curling of leaves and stunted plants. Studies on bean suggest that aphid feeding during the preflowering stage is more damaging than later in development (Khaemba and Ogenga-Latigo, 1985). Research from England indicates that aphid suppression is warranted when 5% of faba bean plants are infested (Bardner et al., 1978). In leafy vegetables such as celery, however, contamination of foliage with aphid bodies or honeydew is also a very important factor, as these conditions will cause crops to be rejected (Godfrey and Chaney, 1995).

Estimates of honeydew production indicate that aphids may produce up to 30-40 drops per day (Banks and Macaulay, 1964). Significant damage is, however, infrequent in North America.

Bean aphid is capable of transmitting numerous plant viruses, including many common viruses of vegetable crops. Kennedy et al. (1962) listed over 60 diseases, mostly stylet-borne viruses, transmitted by bean aphid. Although not usually considered as important a vector as many other species (Fereres et al., 1993), some work indicated that this insect was quite effective at transmitting beet yellows virus (Kirk et al., 1991).

Management

  1. This species often develops very high densities, perhaps 5000 aphids per plant. Because of the difficulties in dealing with such abundance, procedures for estimating aphid density tend to avoid counting actual numbers per plant, and focus instead on counts of subsamples, determination of number of leaves infested (Banks, 1954), or length of infestation (Capinera, 1981). In England, forecasts of aphid populations are made based on densities of overwintering eggs on spindle tree. Considerable savings in insecticide use are possible in years when aphid densities are low (Way et al., 1977; Gould and Graham, 1977). Suction traps may also be used in forecasting, particularly in determining time of migration (Way et al., 1981). There does not seem to be need for such a forecasting system in North America owing to the relatively minor pest status of this insect. To monitor invasion of celery, and most likely other vegetable crops, yellow water pan traps and yellow sticky traps can be used (Godfrey and Chaney, 1995), though sticky traps should be avoided because aphids are often damaged by the adhesive making identification difficult.
  2. Insecticides are applied as foliar sprays or, for systemic insecticides, also as granules at planting time. However, use of granular materials in short-season crops such as beans is sometimes impossible, because the insecticide residue is too high at harvest time. In some crops, disruption of pollinators is a concern.

Bean aphid possesses both an alarm pheromone and a sex pheromone (Dawson et al., 1990). The alarm pheromone causes the aphid to become disturbed, and may induce it to leave the host plant. This may prove useful in inhibiting virus transmission, increasing contact with insecticide residues, or by increasing aphid exposure to predators or unsuitable environmental conditions. The sex pheromone is active only at short distances, and its use is yet to be developed.

Biological Control. Research in England has demonstrated that introduction of entomopathogenic fungi could suppress aphid abundance on beans, and increase yield (Wilding, 1981). However, Erynia neoaphidis and Neozygites fresenii fungi are effective only at high aphid densities and under cool, moist weather conditions.

Cultural Practices. Faba beans have been examined for their resistance to bean aphid, and some cultivar differ in susceptibility. This species is more susceptible, however, than more primitive Vicia species (Holt and Birch, 1984).

Border plantings of scorpion weed, Phacelia tanaceti-folia (Hydrophyllaceae), have been shown in England to enhance predation of bean aphid, and other aphid species, by flower flies (Diptera: Syrphidae). Adult flower flies require nectar for energy, and pollen for sexual maturation and egg production, with this flowering annual easily manipulated to provide these resources (Hickman and Wratten, 1996).

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