
Order: Coleoptera (‘sheath-wings’)
Family: Bostrichidae
Lesser Grain Borer:
3mm long
CHARACTERISTICS:
Forewings hard and leathery, meeting along mid-line of dorsal surface; hindwings membranous, sometimes lacking; biting mouthparts; well developed thorax; complete metamorphosis with egg, larval, pupal and adult stages. Antennae usually less than 11 segments with loose 3-segmented antennal club; prothorax more or less covering downward-turned head; hind coxae touching.
SPECIES CHARACTERISTICS:
Lesser Grain Borer (‘Root destroyer from Dominica’) (Rhyzopertha dominica)
Adult, 2.3 – 3.0 mm long: colour dark redbrown / black; cylindrical body with prothorax tuberculate, especially anteriorly; elytra with well defined rows of punctures; 5-segmented tarsi.
AREAS WHERE FOUND:
The Lesser grain borer originated in South America but is now a cosmopolitan pest especially in warm countries. It is a thermophilic pest which is particularly successful where temperatures are elevated; it is not cold hardy and there is only limited development at temperatures less than 23°C. It is associated with a wide variety of vegetable materials including wheat, barley, maize, rice, millet (dari seeds), sorghum, dried potatoes, dried herbs and biscuits. Infestations have also been recorded in wood and books. In Australia and India it is a serious pest of grains. Infestations are encountered in grain stores including ships holds, flour mills and animal feed mills.
IMPORTANCE AS A PEST:
Lesser grain borers are primary pests of grain and will therefore attack undamaged grain rendering it susceptible to attack by secondary pests. Both the adults and larvae feed on the grain creating floury dust and potentially leaving little but empty husks. The adults are active and may infest a large number of kernels whilst the larvae penetrate kernels and develop within the grain. Infestations in wheat can lead to reduced flour yields and will affect the quality of dough. Both volume and loaf characteristics can be adversely affected. Commodities may be tainted by insect excreta and secretions. Heavily infested wheat is reputed to have an honey-like odour.
LIFE-CYCLE:
The female Lesser grain borer lays between 300- 500 eggs over a period of ca three weeks. They are laid singly or in clusters from 2-30 and are often attached to the grain. Depending upon temperature the eggs hatch in 7-18 days to give white larvae with brown heads and relatively small legs. These bore into the grains where they feed and develop into fleshy forms with a typical C-shape. There are up to five moults leading to pupation in the grain. The pupal stage lasts about one week. The total life cycle lasts from 24-133 days depending upon temperature. At 26°C and 70% RH (14% MC in commodities) the life cycle lasts 45 days. Adults can live for 10 months.
CONTROL:
Assessment of infestations
A variety of trapping techniques are available for measuring stored product beetle infestations. These include pit fall traps, bait bags, insect probe traps and adhesive traps. Whatever system is employed adequate records must be kept.
GLOSSARY OF TERMS:
Prothorax: First thoracic segment, Tarsus (Tarsi): Apical section of leg (the foot).