Gabriele Stoll
Natural Crop Protection in the Tropics
Letting Information Come to Life
Insect pests in field and Storage
Larger grain borer
Prostephanus truncatus (Coleoptera: Bostrichidae)
Larger grain borer: Adult
Damaged maize grain
Host plants
Maize, cassava, sweet potatoes.
It is reported to breed only in maize and cassava. The adults can, however, live in and damage many stored products as well as structural wood and wooden utensils.

P. truncatus is an indigenous pest of stored maize in Central America and has since the 1980s become established in Tanzania, from where it spread to the neighbouring countries. Today this pest is also present in West Africa.

Life cycle
The female lays 30-50 eggs into the produce (maize, cassava). The larvae develop inside the grain dust produced by the feeding action of the adults. A life cycle can be completed within 44 days for males and 61 days for females. On emergence, the adults seek out grain in either shelled crop or on cobs.

Damage pattern
The adults bore into the maize grains, making holes by their tunnelling action and generating large quantities of dust. The adults prefer grain on cobs more than shelled grain, probably due to the fact that the loose grain is more difficult to penetrate.

Control measures
Natural enemies
Parasitoid (larva):
Ascogregarina bostrichidorum

• Regular inspection

Storage facilities
• Shelling of maize and storing it in suitable containers.
• Store produce in airtight containers, e.g. metal drums.
• Underground pits may be tested because the temperature is cool and oxygen levels are reduced, thus restricting the development of storage pests.
Insect-controlling plants
Castor beans
Velvet leaf*
10% ethanolic extract
5-10%  slurry
1.5% (vol/vol)
0.5% powder (w/w)
2.5-10% slurry

< 10% damage
< 16% damage
Highly effective
< 10% damage
Leaf, root
* the slurry of the velvet leaf is extremely bitter, therefore this treatment is suggested for application on planting seed.

TIERTO found out that using plant material in the form of a slurry produces better results than plant powders. In his research the slurry was made by weighing out powder into 150 ml containers and adding sufficient water to give a 10% concentration (w/w), and stirring until a smooth paste was obtained. The treatment was performed by pouring the grain into prepared slurries and stirring with a rod until all grains were coated.