The Larger Grain Borer (Prostephanus truncates)
The Larger Grain Borer is a highly destructive primary pest of farm-stored maize and dried cassava roots.
The region of origin of the Larger Grain Borer (LGB) is Central America between Mexico and Panama. Since the end of the 1970s, this Bostrichidae beetle has been introduced and spread with the import of maize to Africa. It has been reported in Benin, Burkina Faso, Burundi, Ghana, Guinea, Kenya, Malawi, Mozambique, Namibia, Niger, Nigeria, Rwanda, South Africa, Tanzania, Togo, Uganda, Zambia and other countries. This tropical insect will attack the whole grain on the cob, both before and after harvest. The adults bore into the grain, making holes by their tunneling action, and generating large quantities of flour and dust. Without proper control, losses of up to 35% may occur in 5-6 months in maize storage.
Recognition & Biology
The LGB is a small, 3-4 mm long insect with a dark brown color. The thorax has rows of teeth on its upper front edge and the head is tucked down underneath the thorax so that it cannot be seen from above. It has a cylindrical shape and teeth on the thorax like other insects in this family. The larvae have three pairs of legs and are white, fleshy, and sparsely covered with hairs. The females lay 30-50 eggs into the bored tunnels and chambers in the product (e.g. in maize and dried cassava). A life cycle can be completed within 44 and 61 days, depending on food and climatic conditions.
Monitoring & Control
Without natural enemies in Africa, it was very easy for the beetle to build large populations in a short period of time. So the fast spread/proliferation over the whole continent was possible. To stop this “wreak-havoc-insect,” the GTZ-company from Germany (Deutsche Gesellschaft für Technische Zusammenarbeit) started a project in the year 1987, to find natural enemies. Natural enemies like parasitic wasps and insect diseases were identified and tested for use in the field.
The most important finding was the predator Teretriosoma nigrescens, which is able to find the LGB and attack its larva, pupae and eggs. The predator cannot stop the survival of the beetle, but it is able to suppress the population a considerable level. To discover this beetle, the scientists fabricate traps, baited with chemical attractant (Kairomone), which is produced by the male LGB. The pheromone is arranged in plastic capsules, which is placed in qualified traps. These traps are attractive for the LGB as well as for the T. nigrescens, so it is possible to attract and to monitor both with the same trap.
To control the pest in the grain storages, the application of insecticides in several formulations is performed (powder or liquid application). Abio-products, e.g. Bacillus thuringiensis and a plant repellent called Neem was also quite successful. Maize in bigger stores (silos, special warehouses made of bricks) was treated with gas in order to kill the stored product pest. Suitable containers (e.g. metal drums) and regular inspections are helpful.
The biological agents are still in place in Africa and do a great job of managing this very destructive post-harvest insect pest.
The LGB has been found in the United States in Arizona, California, Montana, Missouri, New Jersey, New York and Texas. This pest can build up a population in maize, stored on the cob in sheath. In the U.S., the corn is mainly stored in bulk in silos, so that this insect has difficulty attacking the kernel.