In many countries in sub-Saharan Africa, farmers regularly face the risk of serious losses to their stored grain due to insect pests. Practices used to reduce this risk include both traditional ones, such as mixing grain with ash, and synthetic insecticides. The former have limited effect and the latter are often unobtainable or unaffordable, and farmers rightly question the mixing of pesticides with their food.

 

In the Buhera district of Zimbabwe, maize eaten within two months of harvest suffers little damage from insects, but levels of damage then start to rise, until after eight months about 35% of the grain is affected. Yet, with only a single maize crop a year, long-term storage is essential for household food security, and for improving income generation by allowing more flexibility in when the farmer sells grain to the market.

 

Diatomaceous earths (DEs) are formed from the fossils of phytoplankton (diatoms). When insects come into contact with the DEs, the waxy layer is absorbed from their exoskeleton resulting in water loss, dehydration and death. DEs have extremely low mammalian toxicity.

 

It is believed that observation of birds and mammals taking dust-baths to rid themselves of parasites led the Chinese to start using DEs in pest control more than 4000 years ago. Although DEs are beginning to be used in large-scale grain treatment in Australia and other countries, there was no knowledge of their efficacy under tropical small-scale farming conditions.

 

Research with farmers by NRI, the University of Zimbabwe and the Institute of Agricultural Engineering has now shown that DEs offer a real alternative to conventional synthetic insecticides and to traditional practices for maize, sorghum and cowpeas. Farmers themselves conducted the evaluation, using their own criteria for home consumption and sale, and they are now keen to buy DEs. A sustainable low-cost source may exist in Zimbabwe. Preliminary tests have shown a raw local DE sample to be effective against storage pests, and the use of local DE sources will be the central theme of future work.

 

Further work