Cowpea weevil

Callosobruchus maculatus (F.)

Coleoptera: Bruchidae

The cowpea weevil is worldwide in the tropics and subtropics distributed. It is the dominant pest of pulses in Africa. In colder climates the pest often occurs in warehouses and stored legume seeds.

Host plants and damage

The cowpea weevil is a major pest of legumes. Preferred host plants are Vigna unguiculata (cowpea), Lens culinaris ssp. culinaris (lentil), Vigna radiata (mung bean), Vigna mungo (black gram), Cajanus cajan (pigeon pea), Pisum sativum (pea) and Phaseolus (beans).

Cowpea weevil larvae infest the seeds of legumes during fruiting stage and post-harvest. Usually, infestations start in the pods before harvest and larvae are carried into storage where they feed on seed interiors.

Morphology & biology

Female cowpea weevils are gluing up to 115 oval eggs on seed surfaces. The eggs are translucent grey in colour and have a flat base. Within 5-6 days the eggs turn white and the larvae hatch. First instar larvae bore through the testa and cotyledons. Developing larvae feed only in a single seed whereby excavate a chamber as they grow. Pupation occurs inside the seeds. Adults bore tunnels to seed surfaces and leave the kernels.

Under optimum environmental conditions at 32 °C and 90 % relative humidity, the life cycle requires about 21 days. However, at 25° C and 70 % relative humidity, development takes about 36 days.

Cowpea weevil adults are pale brown in colour and measure 2.0-3.5 mm in length. Females have two large lateral dark patches mid-way along the elytra and smaller patches at the anterior and posterior ends. Males are less distinctly marked. Adults do not feed on stored products and live usually not longer than 12 days.

An active- or flight-form of adult C. maculatus has been reported. The adults of the flight-form have a white pygidium. They are apparently more active than the common beetle form. In addition, marks along elytra of the active form are more visible than are marks of the common beetle form.

Monitoring

The presence of eggs cemented to the surface of pods is the first symptom of infestation. Substantial losses occur by larval feeding. Symptoms of heavy infestation are hollowed out seeds with round holes.

Advanced infestations of cowpea weevil can be detected by the presence of hollowed seed cases with visible bore-holes and mobile adults. By cracking the kernels or by using x-rays the interior of seeds can be inspected for larvae.

Pest management

Biological control

Important larval parasitiods on cowpea weevil are: Lariophagus distinguendus Förster, Anisopteromalus calandrae Howard, and Dinarmus basalis (Rondani). D. basalis is present in all different regions of West Africa and can be used for biological control in stores, particulary in humid zones.

Cultural control

Physical control
One alternative to the use of insecticides is the use of cold storage. Exposure to -18ºC for more than 50 minutes kills 100% of the adults. Immature stages have proved to be much more cold tolerant. After 180 minutes of exposure, adult emergence is reduced by more than 99% for all treated ages.

Chemical Control

Fumigation treatments with methyl bromide or phosphine as well as the use of organophosphates can control cowpea weevil populations.

When small lots of pulses are stored the admixture of vegetable oil in stored pulses can give good protection. Peanut or neem oils are also used for coating seeds. These oils prevent the attachement of cowpea weevil eggs to the seeds. However, large quantities of inert material or oils are required for effective pest control.

Terpenes emitted from oil extracts of the Graminaea Cymbopogon schoenanthus L. were toxic to cowpea weevils and its parasitid Dinarmus basalis. However, the accumulation of hogh terpene concentrations in the seds could affect quality of these seeds and their use in human food.