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Cowpea weevil

Callosobruchus maculatus (Fabricius)

Southern Cowpea Weevil

Callosobruchus chinensis (Linnaeus) (Coleoptera: Bruchidae)

Natural History

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, cowpea, faba bean, lentil, pea, and soybean. However, cowpea 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), Choetospila 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 Rein-hard, 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.

Egg. The eggs are oval, and broader at one end. The female glues the egg to the seed, and the side that attaches to the seed is flattened. The egg is 0.4-0.8 mm long and 0.3-0.5 mm wide. The duration of this stage is only 3-4 days under ideal conditions, but may extend to over 30 days during cold weather. The young larva chews a hole through the egg chorion directly into the seed, or through the pod wall and then into the seed. Females of cowpea weevil commonly produce 100 or more eggs during their life span, and southern cowpea weevil about 50, although some strains produce fewer eggs. Access by adults to water or sugar water increases fecundity. Females also produce more eggs when they are not constrained by availability of suitable oviposition sites.

Larva. There are four instars. First instars are whitish, short and thick in appearance, with small thoracic legs. Long bristles are present dorsally, and shorter bristles near the legs. Latter instars are similar, but lack the long bristles, and the thoracic legs are even less apparent. They attain a length of about 3.5-4.5 mm at maturity. The body of larvae of southern cowpea weevil is expanded behind the head, giving a humpbacked appearance when viewed in profile. Head capsule widths are about 0.14, 0.24, 0.33, and 0.48 mm for instars 1-4, respectively. Duration of the instars is normally three, three, four, and five days, respectively, for southern cowpea weevil at 30?C, but it varies greatly with temperature and host suitability. For cowpea weevil, the length of each instar is about a day longer. During the summer in California, the larval development period is reported as 17-22 days. Optimal temperature for development of both species is 30-32?C, with little or no development at temperatures below 20?C. Low humidities ( < 70%) also are limiting, especially for eggs and young larvae (Howe and Currie, 1964).

Pupa. The pupa is white and stout in shape, resembling the adult in appearance. The length is 3.2-5.0 mm. The duration of the pupal stage of cowpea weevil was reported to average 6.4 days at 25?C by Howe and Currie (1964). The adult remains at the pupation site for sometime before emerging, however, so the period from transformation to a pupa to emergence of the adult from the seed totals about 10.2 days at 25?C.

Adult. The adults are oval in general shape, with cowpea weevil somewhat more elongate than southern cowpea weevil. In cowpea weevil, the general color is whitish to light brown below and brownish marked with dark brown or black above. The color pattern is rather variable, however. The head and antennae are dark brown or black, with a dark spot on the thorax behind the head. Elytra are broadly rounded at their tips. Each elytra bears a large black, quadrate spot along the leading edge; the tip is also darkened. The legs are reddish to blackish. The tip of the abdomen is exposed, and is readily visible from above. It is blackish, but is usually marked with a longitudinal gray line dorsally.

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 oviposition sites. Females of cowpea weevil produce a sex pheromone (Lextrait et ah, 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).

Damage

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.

Management

Sampling. Blacklight traps can be used to sample adults (Keever and Cline, 1983).

Insecticides. A great deal of effort has been made in less-developed countries in evaluating the natural materials that have insecticidal or repellent properties for protection of cowpea under primitive storage conditions. A variety of effective techniques can protect this vital protein resource. Natural materials such as ash, sand, vegetable oil, and fruit, cashew, ginger, and neem extracts provide protection (Su, 1976, 1978 and 1991; Singh et al, 1978; Echendu, 1991; Cockfield, 1992). The mode of action of oil, one of the most effective suppression techniques, seems to be in reducing egg respiration and ability of the egg to release toxic metabolites (Don-Pedro, 1989).

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.