Distribution. Cowpea weevil and southern cowpea weevil species are found throughout the United States, Central America, South America, Africa, southern Asia, and Australia. Their origin is uncertain, but their distribution is closely linked with that of their principal host plant, cowpea. This plant is reported to have originated in central or western Africa, and so the beetles may have a similar origin. In North America, they are a serious pest only in the southern states. Although both species occasionally are detected in the central provinces of Canada, they are usually associated with imported legume seed, and are not considered to be damaging.
Host Plants. These species develop successfully on a number of legume seeds including chickpea, cow-pea, faba bean, lentil, pea, and soybean. However, cow-pea is most suitable, and faba bean and soybean are relatively poor hosts. These insects are primarily pests of stored seed, with little damage occurring in the field.
Natural Enemies. Several parasitoids are known, including Anisopteromalus calandrae (Howard), Choetos-
pila elegans Westwood, Dinarmus laticeps (Ashmead), and Lariophagus texanus Crawford (all Hymenoptera: Pteromalidae), but their incidence generally is low. An egg parasitoid, Uscana semifumipennis Girault (Hymenoptera: Trichogrammatidae) is an important mortality factor in some locations (Paddock and Reinhard, 1919).
Life Cycle and Description. These species complete several generations per year. An entire generation can be completed in 30-40 days. In the southern United States, the number of generations is commonly reported to be 5-8. The life cycles of these two species of Callosobruchus are very similar, and here they are treated as one except for where significant differences are known.
Cowpea weevil exhibits density-related polymorphic morphology, behavior, and physiology. Adults developing from insects reared under high density conditions have less pubescence on their body, allowing more black color to be visible, resulting in an overall darker color. High density beetles are also more oval in body shape, with the tip of the abdomen extending far beyond the elytra. More important, high-density beetles tend not to fly, but produce more eggs and deposit them early in adult life. Presumably, this polymorphism is an adaptation to varying resource abundance (Utida, 1972). (See color figure 134.)
In southern cowpea weevil, general body color is reddish brown, marked with black. The thorax is reddish, with a white spot at the center of the hind margin. The elytra are marked with a dark spot at the tip, and another at the leading edge, a pattern very similar to that of cowpea weevil. However, the tip of the abdomen does not markedly protrude, as in cowpea weevil. Also, the reddish antennae are much more serrated, particularly in the male. As in the case with cowpea weevil, the color patttern can be variable.
The mature beetle chews a hole in the seed coat and escapes from the seed. The exit hole may be a complete circle or a portion may remain attached and function as a door hinge. It is not necessary for the adults to feed before they mate and commence oviposition. In fact, there is little evidence that consumption of more than nectar occurs and even this is infrequent. However, the adults seemingly benefit from access to water and sugar water, as those provided with such nutrients live longer and produce more eggs. Oviposition may begin within one day of the emergence of adult. Duration of the adult stage is estimated as 7-18 days, with an average longevity of about 15 days in warm weather, but these values are lengthened considerably during cool weather. In the field, oviposition generally occurs on over-ripened pods that have split along the suture. They prefer to deposit directly on bean seed, but if this is not available, they will deposit eggs on pods. They will also oviposit on immature pods. Messina (1984) estimated that 20% of eggs deposited on green pods successfully develop into adults. In storage, they deposit eggs freely on all surfaces of dry beans. They avoid damaged seeds when selecting ovi-position sites. Females of cowpea weevil produce a sex pheromone (Lextrait et al., 1995) and oviposition-deterring pheromone (Wasserman, 1981; Credland and Wright, 1990).
A good general description of these insects was given by Garman (1917) and Back (1922). Paddock and Reinhard (1919) and Larson and Fisher (1938) provided detailed information on cowpea weevil, and Chittenden (1912f) described southern cowpea weevil. The developmental biology of both species was given by Howe and Currie (1964).
Larvae feed within the seed. A single larva often consumes only a small portion of the embryo, and may not disrupt seedling germination. However, attack by more than one insect per seed is possible, and is more damaging. Seeds may become infested either in the field or in storage, with most damage resulting from the latter. Thus, these insects are not usually considered pests of fresh-market crops. These bruchids, and particularly cowpea weevil, are major pests of cowpea and some other legumes where seed is not treated with insecticide or is stored in a manner that allows invasion of insects. The presence of even a single insect in a seed causes significant weight loss in seeds (Ofuya and Bamigbola, 1991), and even low levels of contamination are unacceptable in American commerce. The problem is severe enough that American dried-bean production is centered in northern climates not because these legumes cannot be grown in the southern states, but because northern locations largely avoid problems with damaging populations of bruchids.
Chemical fumigation is the standard practice to protect stored seeds. However, it is also possible to disinfest seed through modification of storage atmosphere; displacement of oxygen by nitrogen is effective (Storey, 1978).
Host-Plant Resistance. Although some legumes are quite resistant to these insects, some others seem uniformly susceptible. Cowpea, probably the most important host, displays measurable but low-level resistance (Fitzner et al., 1985). Sources of pod and seed resistance have been identified (e.g., Schalk 1973; Tale-kar and Lin, 1981 and 1992; Rusoke and Fatunla, 1987), but commercially acceptable cultivars with high levels of resistance are not generally available. Rough-
seeded varieties are less preferred for oviposition by adults (Nwanze et al., 1975).
Cultural Practices. Disinfestation of seed is critical both for storage of food and preservation of viable seed stock. Seeds can be disinfested with solar heaters, which take advantage of solar radiation to produce high temperatures that are lethal to larvae, pupae, and adults. Solar heaters can be constructed with some clear plastic, and achieve temperatures of 57°C for 1 h or 65°C for 5 min, which provides effective disinfestation (Kitch et al., 1992).
The spatial relationship of seed storage to production areas is also significant. In addition to planting insect-free seed, it is immensely valuable to have crop-producing areas distant from storage. This is important, because dispersal of beetles from storage is a major source of field infestation. If on-farm storage is desired, storage bins should be cleaned of all old seed and the walls treated with insecticide.
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