||Stemborers and striga are the major pests of cereal crops in eastern and southern Africa. Losses to stemborers can reach as high as 80% in some areas and an average of about 15-40% in others, while those due to striga range from 30-100% in most areas in the region. When the two occur together, farmers often lose their entire crop. Spraying with pesticides is not only expensive and harmful to the environment, but usually ineffective as the chemicals cannot reach deep inside the plant stems where stemborer larvae reside. Preventing crop losses from stemborers and striga could increase maize and sorghum harvests enough to feed an additional 27 million people in the region.
The International Centre of Insect Physiology and Ecology (ICIPE) and partners have developed a habitat management strategy which makes use of resilience of nature and the built-in checks and balances to operate in man-made environments such as maize fields by manipulating the agro-ecohabitat. The approach relies on a carefully selected combination of companion crops to be planted around and among maize or sorghum plants. Both domestic and wild grasses can help to protect the crops by attracting and trapping the stemborers. The grasses are planted in the border around the maize and sorghum fields where invading adult moths become attracted to chemicals emitted by the grasses themselves. Instead of landing on the maize or sorghum plants, the insects head for what appears to be a tastier meal. These grasses provide the "pull" in the "push-pull" strategy. They also serve as a haven for the borers' natural enemies. Good trap crops include well-known grasses such as Napier grass (Pennisetum purpureum) and Sudan grass (Sorghum vulgare sudanense). Napier grass has a particularly clever way of defending itself against the pest onslaught: once attacked by a borer larva, it secrets sticky substance that physically traps the pest and effectively limits its damage. The natural enemies lurking among the grasses go into action and dispatch the borers in both maize or sorghum and grass hosts plants.
The "push" in the intercropping scheme is provided by the plants that emit chemicals (kairomones) which repel stemborer moths and drive them away from the main crop (maize or sorghum). The best candidates discovered so far with the repellent properties are members of leguminous genus Desmodium spp.Desmodium is planted in between the rows of maize or sorghum. Being low-growing plant it does not interfere with the crops' growth and furthermore has the advantage of maintaining soil stability and improving soil fertility through nitrogen-fixation. It also serves as a highly nutritious animal feed and effectively suppresses striga. Another plant showing good repellent properties is molasses grass (Melinis minutiflora) , a nutritious animal feed with tick-repelling and stemborer larval parasitoid attractive properties.