Scientific Name: Callosobruchus maculatus (Fabricius, 1775)

Common Name: Cowpea weevil

Synonyms

Bruchus maculatus (Fabricius 1775)

Taxonomic Position

Class: Insecta

Order: Coleoptera

Family: Bruchidae

Genus: Callosobruchus

Species: Callosobruchus maculatus

Origin and Distribution

C. maculatus originated in Africa and distributed throughout the tropical and sub-tropical regions of India.

Pest Destructive Stage

Primary pest

Eggs –laid or stuck individually onto seed or pod

Larvae – immobile, concealed within seed

Adults – do not feed on commodity, fly readily

Host Range

Mainly Pulses

Acacia nilotica (gum arabic tree), Cajanus cajan (pigeon pea)     , Cicer arietinum (chickpea), Fabaceae (leguminous plants), Glycine max (soyabean), Lens culinaris subsp. culinaris (lentil), Phaseolus (beans), Vigna radiata (mung bean), Vigna unguiculata (cowpea), Voandzeia subterranea (bambara groundnut).

Damage symptoms

Damage is distinctive. As adult emerge from seeds they leave behind neat circular holes in the seed, behind which is a large cavity left by the larvae. Loss of seed material is considerable and each beetle would have consumed about 25% of the seed endosperm from which it emerged. Damaged seed often does not germinate due to inefficient amount of endosperm and damaged germ. Heavy infestation of bruchids can cause heating of the commodity which results in quality loss and mould growth.

Identification features

Eggs

The eggs are small, glued on the grain singly but many eggs could be seen on a single grain. Freshly laid eggs translucent smooth and shining, which later became yellowish white and very prominent against the contrasting background of the grain surface. Size of egg is 0.47 ± 0.08 mm in length and 0.12±0.21 mm in breadth.

Larvae

The larva is white, yellowish and somewhat C-shaped with a small head. The 4^th instar larvae are similar to 3^rd instar but differed in size and shape. In this instar the larva seemed to become larger and fed on the entire endosperm voraciously. It burrowed into a position just underneath the seed coat prior to pupation.

Pupae

During the time of pupation larval structures is broken down and adult structures developed; the rudiments of the wings appeared at the first day, at second day appendages such as legs, antenna and proboscis developed freely and at 3^rd day eyes, mouth part, forewing, hind wing and legs with cuticular hair developed. At 4^th day almost all the parts developed but intersegmental region of the abdomen remained colourless and forewing was light green in colour. At 5^th day forewings changed into dark brown with black patches. The male and female pupal period ranged from 6-7 days and 5-6 days respectively.

Adults

• C. maculatus adults are 2.0-3.5 mm long.
• The antennae of both sexes are slightly serrate.
• Females often have strong markings on the elytra consisting of two large lateral dark patches mid-way along the elytra and smaller patches at the anterior and posterior ends, leaving a paler brown cross-shaped area covering the rest.
• The males are much less distinctly marked. In common with other species of Callosobruchus, C. maculatus has a pair of distinct ridges (inner and outer) on the ventral side of each hind femur, and each ridge bears a tooth near the apical end. The inner tooth is triangular, and equal to (or slightly longer than) the outer tooth.
• Pygidium in female oblique either entirely testaceous or black interrupted by median testaceous stripe while in male pygidium is vertical, either entirely testaceous or testaceous with black posterolateral spots.

Detection methods

Infestation quickly become obvious as a result of the presence of eggs stuck to the outside of seed together with the presence of the active adults and emergence holes in seeds. Samples can be examined with naked eye or under magnifying glass or stereoscopic binocular microscope and by using soft X-ray and seed transparency method hidden infestation can be detected. Acoustic detectors are commercially available which detect the sounds that internal stages of these beetles make during feeding. 

Traditionally, prior to consumption many consumers have used flotation in water to separate infested grains from uninfected. Infested grains will float whereas intact grains mostly sink.