Callosobruchus maculatus

Callosobruchus maculatus is a species of beetles known commonly as the cowpea weevil or cowpea seed beetle. It is a member of the leaf beetle family, Chrysomelidae, and not a true weevil. This common pest of stored legumes has a cosmopolitan distribution, occurring on every continent except Antarctica. The beetle most likely originated in West Africa and moved around the globe with the trade of legumes and other crops. As only a small number of individuals were likely present in legumes carried by people to distant places, the populations that have invaded various parts of the globe have likely gone through multiple bottlenecks. Despite these bottlenecks and the subsequent rounds of inbreeding, these populations persist. This ability to withstand a high degree of inbreeding has likely contributed to this species’ prevalence as a pest. It is used as a model organism for both research and education due to its quick generation time, sexual dimorphism, and ease of maintenance. The cowpea weevil lacks the 'snout' of a true weevil. It is more elongated in shape than other members of the leaf beetle family. It is reddish-brown overall, with black and gray elytra marked with two central black spots. The last segment of the abdomen extends out from under the short elytra, and also has two black spots. The beetle is sexually dimorphic and males are easily distinguished from females. The females are sometimes larger than males, but this is not true of all strains. Females are darker overall, while males are brown. The plate covering the end of the abdomen is large and dark in color along the sides in females, and smaller without the dark areas in males. There are two morphs of C. maculatus, a flightless form and a flying form. The flying form is more common in beetles that developed in conditions of high larval density and high temperatures. The flying form has a longer lifespan and lower fecundity, and the sexes are less dimorphic and can be more difficult to tell apart. The egg is clear, shiny, oval to spindle-shaped, and about 0.75 millimeters long. The larva is whitish in color. A female adult can lay over a hundred eggs, and most of them will hatch. She lays an egg on the surface of a bean, and when the larva emerges about 4 to 8 days later, it burrows into the bean. During development, the larva feeds on the interior of the bean, eating the tissue just under the surface, leaving a very thin layer through which it will exit when it matures. It emerges after a larval period of 3 to 7 weeks, depending on conditions. In colder climates the gestation period is typically longer taking anywhere from 4–13 weeks to emerge. Larval crowding can occur when up to 8 or 10 larvae feed and grow within one bean. Crowding limits resources for each individual, leading to longer development time, higher mortality, smaller adult size, and lower fecundity.