Cowpea Weevil

Cowpea Weevil,(Callosobruchus maculatus (Fabricius)) is a pest of field peas growing in fields as well as dry seed in storage. The adult is 1/8 to 1/5 inch long; is dark colored and has four pale brown spots on the wing covers. Like the pea weevil, the damage is caused by the grub-like larva. Reproduction may be continuous in peas stored under warm temperatures. There are several generations annually. .

Cowpea weevil are insects that have a habit of feeding within leaves or needles, producing tunneling injuries. Several kinds of insects have developed this habit, including larvae of moths (Lepidoptera), beetles (Coleoptera), sawflies (Hymenoptera) and flies (Diptera). Most of these insects feed for their entire larval period within the leaf. Some will also pupate within the leaf mine, while others have larvae that cut their way out when full-grown to pupate in the soil.

Life Cycle: Cowpea bruchid adults are small (3-mm long) orange-brown beetles with a body that tapers towards thehead. Cowpea (and other) bruchids do not have the elongated snout of true weevils, which infest cereal grains. Cowpea bruchids are typically orange-brown with dark markings, but colour can vary and they are easily confused with other closely related bruchid species. Their wing covers (elytra) are slightly shorter than their abdomen.

The eggs are small, flat and white and, despite being only 0.6 mm long, are readily visible on the surface of seeds. The C-shaped larvae look like short cream-coloured maggots, and are rarely seen as they feed within seeds. However, prior to pupation larvae excavate a pupal chamber just below the seed surface, visible as a small transparent window in the seed coat.

Ovipositing females discriminate against seeds that already bear eggs and also distinguish between seeds bearing few eggs vs. many eggs. Such behaviour leads to a non-random, uniform dispersal of eggs among seeds and reduces the frequency and intensity of larval competition. Ovipositing females glue each egg to the seed coat, laying eggs on the largest beans first. Upon hatching, the larva chews its way into the seed directly beneath the oviposition site whilst the upper surface of the egg chorion remains intact on the seed. Frass is pushed into the empty egg during feeding, causing the latter to appear white. The life cycle is completed inside the seed with the mature larva excavating underlying seed tissue, leaving a visible ‘window’ just below the seed testa, within which it pupates and ecloses. The adult emerges through a circular hole in the ‘window’ and can mate within an hour.

Damage: Bruchid damage is important because of overall weight loss; altered nutritional quality and the presence of insect frass, excrement and dead insects in and on the seed; and loss of seed viability. Bruchids in stored seed are a major problem because of their ability to re-infest stored seed.

Thorough pre-harvest cleaning of storage, transport and harvesting equipment is critical for the management of bruchids. Grain residues in silos, harvesters, augers etc. must be eliminated, as they can harbour bruchids which then infest newly-harvested grain. A single beetle is able to cause 3.5% weight loss in cowpea seeds. Infestations can cause up to 60% loss in seed weight and up to 66% loss in protein content of pulses. Even slight bruchid feeding damage to the embryo impairs germination. Feeding on the cotyledon will not affect germination but the vigour of the young plant will be reduced.

Seed weevils occasionally become pests of stored beans, cowpeas, and peas. Damage consists of complete or partial destruction of infested seeds by numerous round holes or destruction of all but the outer shell. Many bean weevils may develop from a single seed and later be discovered on windows and doors. They do not bite or sting humans or pets, spread disease, or feed on or damage the house or furniture.

Controls:The homeowner frequently sees seed weevils for the first time on windows and doors as they emerge from stored seeds, are attracted to light, and attempt to escape. Usually, there is little concern for their presence until a sack of dried beans or peas, especially homegrown, is emptied and found full of holes.

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