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Dorsal and side view of adult Rhyzopertha dominica. Note position of head underneath thorax, a characteristic of the family Bostrichidae. Source: PaDIL Pests and Diseases Image Library http://www.padil.gov.au
Larvae of Rhyzopertha dominica. Photo: USDA, ARS (Public Domain)
Lifecycle of Rhyzopertha dominica. ©Paul Boles, Agricultural Research Service, United States Department of Agriculture
Rhyzopertha dominica adult. Image by Gary Alpert, Harvard University, Bugwood.org.
Dorso-lateral view of adult R. dominica (museum set specimen). ©Georg Goergen (source CABI CPC)
Rhyzopertha dominica adult. © Christoph Benisch - www.kerbtier.de. All rights reserved.
The lesser grain borer is found throughout the tropical and also in temperate countries. It is a pest of stored maize and a variety of stored products. Both adults and larvae feed internally in maize grains.
Lesser grain borer, American wheat weevil, Australian wheat weevil, grain weevil, stored grain eater
Phylum: Arthropoda; Class: Hexapoda (Insecta); Order: Coleoptera; Family: Bostrichidae (Bostrychidae is also a common spelling)
The origin of the lesser grain borer is uncertain but it now has a cosmopolitan distribution. It is a serious pest of dried stored products throughout the tropics and it is also found in temperate countries. It has most likely spread as a result of the international trade in food products combined with its strong flying ability.
The eggs are ovoid in shape, 0.6 mm in length, 0.2 mm in diameter, laid loosely in grains. They are white when first laid, and turn rose to brown before hatching.
The larvae are white to cream coloured, with biting mouthparts and three pairs of legs. The young larvae are mobile in grain bulks but become immobile and gradually more C-shaped as they complete their development concealed within grain or flour.
The adult is 2-3mm long, reddish-brown in colour with a slim cylindrical shape. The elytra (wing cases), which cover membranous hind wings, have regular rows of coarse punctures (finer at sides) covered with curved setae (hairs). The front edge of the pronotum (plate-like covering of front segment of the thorax) has a saw-toothed appearance. The head is not visible when viewed from above and its antennae end in a distinctive 3-segmented club-shape.
It is possible to confuse the lesser grain borer with other storage insect pests such as the Large Grain Borer – LGB (Prostephanus truncatus) and the maize weevil (Sitophilus zeamais). However, the saw-toothed front edge of the pronotum in the lesser grain borer family (Bostrichidae) is very distinctive. In contrast, the LGB is larger (3-4.5 mm long), dark brown in colour, its body surface has many small wart-like outgrowths (tubercles) and the end of its body terminates in a straight edge. The end of the body of the maize weevil is more rounded than that of the lesser grain borer, its head is visible from above and mouthparts are 'beak-like' and the antennae elbowed.
Lesser grain borer females lay between 200 and 500 eggs in their lifetime. The eggs are ovoid in shape, 0.6 mm in length, 0.2 mm in diameter, laid outside grains either singly or in clusters of up to about 20. They are typically white when first laid, turning rose to brown before hatching. The larvae, which are white to cream in colour with three pairs of legs and biting mouthparts, undergo four instars. The larvae are quite mobile and linear in shape at first but become more immobile and C-shaped as they develop. The larvae eat into cereal grains or another suitable hard substance (e.g. wood) where they complete their development. Adults, which can live up to 240 days, emerge by chewing through the outer grain layers.
Both adult and larva damage the grain by boring/eating.
The lesser grain borer feeds mainly on maize, oats, barley, rice, millet, sorghum, wheat, durum wheat, chilli, coriander, turmeric, cassava, beans, ginger, wheat flour, and a variety of dried stored products. The lesser grain borer can also be found mainly in cereal stores, and food and animal feed processing facilities. They have been trapped using pheromone -baited flight traps several kilometres from any food storage or processing facility.
Post-harvest and storage
Seeds and grains
The pest hollows previously undamaged grains, eating the starch. The holes are characteristic with an even edge. Adults and larvae feed on the entire grain apart from the bran leaving behind empty husks and flour.
Damaged grain is distinctive (as detailed above). Infested grain may give off a sweet smell. Sieving a sample of grain is the simplest detection method but it will only detect adults and larvae that are outside the grain.
Removing spilt grain from around storage facilities can reduce lesser grain borer populations. So far the use of resistant varieties has not been utilised as a control method.
The removal of adult insects from the grain by sieving can reduce populations but this is very labour-intensive. The addition of inert dusts such as ash and clay to the grain can reduce insect numbers by causing the insects to die from desiccation.Biological control
There are a few parasites and predators that attack the lesser grain borer but their effectiveness in the field is uncertain. The fungus Beauveria bassiana can be used as a biological insecticide.
Controlled atmosphereWhere suitable infrastructure exists, low oxygen and carbon dioxide-enriched atmospheres can be used to control stored product pests.
Freezing and HeatingWhere the infrastructure exists, freezing for several days and heating for 24 hours have proved to be effective control methods for stored product pests.
Chemical controlThe insecticides Chlorpyrifos-methyl and pirimiphos-methyl, while effective against most stored grain insect pests, are not very effective against the lesser grain borer. The fumigant phosphine can be used in sealed storage facilities. Pesticides are poisons so it is essential to follow all safety precautions on labels.
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Anne M. Akol, Makerere University; Maneno Y. Chidege, Tropical Pesticides Research Institute; Herbert A.L. Talwana, Makerere University; John R. Mauremootoo, BioNET-INTERNATIONAL Secretariat.
We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).
BioNET-EAFRINET Regional Coordinator: email@example.com