Callosobruchus maculatus (Fabricius, 1775)
Cowpea Seed Beetle
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POLYPHAGA Emery, 1886
CHRYSOMELOIDEA Latreille, 1802
Callosobruchus Pic, 1902
Native to warmer parts of Africa where it remains widespread and abundant, this notorious pest of stored peas and beans is now established and generally common in tropical and sub-tropical regions worldwide, it rarely becomes established in cooler temperate areas but is regularly recorded with imported foods worldwide including, occasionally, Europe and the UK. Beyond this the species is bred in huge numbers and sold commercially as live exotic pet food; these are readily available and even a small sample will provide a constant abundance of adults if kept warm and moderately dry. Natural hosts include a wide range of Fabaceae and even on wild hosts the species may become abundant, but during dispersal the adults will readily transfer to cultivated varieties and, as they may be continuously breeding, are among the most destructive of tropical legume pests and a huge amount of research goes into their control and eradication. One aspect of this is their ability to adapt to hosts plants less suitable for larval development and for this selection to be inherited, hence although it is a major pest of e.g. cowpeas green gram and lentils, it is very difficult as reservoir populations are always available to re-infest crops and stored foods. Adults are short-lived, even under ideal conditions, generally between one and two weeks, and they do not feed but after mating females oviposit for most of their lives, each producing about one hundred eggs. Ideal temperatures for oviposition are high, between 30 and 35°C, and females may be reluctant laying on previously-infested host material, they attach one or two eggs to the outside of each food item and move through stored food as they do so. Larvae emerge within a week or so, they bore through the base of the egg and enter the seed where they will feed on sugars and the developing cotyledons, here they may be detected as they push any frass produced into the empty egg which turns a conspicuous white or creamy colour. If too many eggs are laid onto a single seed the larvae compete and some will either die-off or try to move to other seeds, each seed usually hosts a few larvae which will excavate small chambers as they feeds and in which they will eventually pupate. Larval development varies on conditions but at 32°C and 90% RH it may
be complete within 21 days with pupation occurring 26 days after oviposition. In warmer regions infection usually begins in the field where eggs are laid on developing seed pods, these are very attractive to the beetles but become less so as they dry out, and seeds that have been dried before storage are very resistant to attack, conversely, freshly harvested seeds can soon develop serious infestations as breeding colonies rapidly grow. For this reason stored peas etc where often fumigated before modern legislation prohibited this, but now they are often kept at low temperatures or in controlled atmospheres. Under artificial conditions they behave differently and will attack any dried peas or beans, and only a few adults will soon produce a population that will consume all available food. Unfavourable breeding conditions prevent the species becoming established in temperate regions but the coleopterist really should obtain a culture because when kept inside in a container they reproduce like hell on any suitable substrate and are quite fascinating to observe, they need to be kept confined though as they can fly well and will readily disperse in search of more host material; two forms are known, a flightless form and a flying form, the flying form is more prevalent in conditions of high larval density, it has a longer lifespan and the sexes are less distinct and may be difficult to distinguish.
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2.5-3.5 mm. Easily distinguished among our bruchid fauna by the overall appearance, broadly-oval and more or less continuous in outline, forebody dull grey or black (more rarely the head and/or pronotum may be red) with two densely pubescent calli at the base of the pronotum, elytra very variable but usually black with a symmetrical pattern of orange or red, often substantially or entirely reddish-brown, dorsal surface with dense recumbent pubescence throughout, appendages vary from entirely red to substantially darkened, the front and middle legs usually at least in part red. Head produced in front of transverse and weakly convex eyes, evenly convex and finely and densely punctured throughout, antennae serrate, although often only weakly so in the male, with the terminal segment distinctly elongate. Pronotum transverse, broadest across the base and evenly narrowed to a rounded anterior margin, lateral margins smooth, basal margin widely and strongly produced medially, surface densely punctured and finely pubescent throughout. Elytra parallel-sided or slightly narrowed from rounded shoulders to separately curved apical margins, striae narrow, finely punctured and complete to the apex, interstices broad and flat. Abdomen with fine pale pubescence which may be denser towards the apical margins of the ventrites but mostly does not extend to the lateral margins. Legs long and robust, hind femora with two subapical ventral teeth that are equal in size in both sexes.