Cowpea Bruchid
(Coleoptera: Bruchidae)
Synonyms: Bruchus chinesis; Pachymerus chinensis.
Hosts: A pest of pulses, cowpea, soybean, gram, pigeon pea, lablab etc.
Damage: Both larvae and adults cause damage to the grains. They bite holes in the grains to enter inside and feed on kernel, damaging several grains in the process. As the beetles can actively fly, the infestation can start in the fields, where the beetles deposit their eggs on the pods.
Life cycle: Adult beetle is 3-4 mm long, female being larger, brownish in colour, broader at shoulders and rounded posteriorly. There are dark patches on elytra and thorax. Adults show sexual dimorphism. Males possess deeply emarginated or indented eyes and prominently serrate antennae, while in female these characters are not distinctly marked.
In females tip of abdomen is exposed while in males it is covered by elytra. They are active beetles and readily fly when disturbed. Fecundity is about 100 eggs per female. Eggs are whitish, elongated and stuck on the grains or on pods and sometimes on the surface of the container. Incubation period is 3-6 days. Grubs are scarabeiform or eruciform, plump and with short legs and yellowish in colour.
First instar larvae bear functional legs and a pair of thoracic plates to facilitate boring into the seeds. They feed on the inner contents of the grain and may damage several grains during development. Larval period may vary between 12 and 20 days. Pupation takes place inside the grain and pupa is dark brown in colour. Occasionally pupation may take place outside the grain in a cocoon made of excretory matter. Completion of life cycle takes 4-5 weeks and there may be 6-7 overlapping generations in a year.
Distribution: Cosmopolitan in the tropics and subtropics of the world. A closely related species, Callosobruchus maculatus is found existing along with chinensis. Adults of the former species are elongated and darker and pesterior part of the abdomen is exposed.