The sweet smell of success
Many stemborers (insect larvae that burrow into the stem of the plant) are native to Africa and have always been a problem to farmers. But, one particularly damaging species, Chilo partellus, has invaded Africa from the Indian sub-continent and is now found throughout East and southern Africa. In the warm, humid lowlands of this region, it is often this foreign stemborer that causes the most extensive damage to maize and sorghums (up to 80% yield loss).
Chemical control of stemborers is difficult: pesticides are expensive, often toxic and relatively ineffective to these insects, because they burrow inside the plants. Classical biological control using native parasitoids has also had limited success in the past as they do not attack this foreign stemborer at sufficient levels to control it. However, greater success has been achieved with a small parasitic wasp, Cotesia flavipes, which has been imported from the Indian sub-continent - the original home of Chilo partellus. Native to India and Pakistan, C. flavipes has also successfully controlled stemborers in other regions of the world.
In collaboration with the UK-based International Institute of Biological Control (IIBC), parasitic wasps from maize crops in the Karachi area of Pakistan were collected by ICIPE and about 100,000 insects were released over a period of a few months in the coastal area of Kenya during 1993. Recovery surveys in 1994 revealed only one stemborer parasitized by the wasp but, undiscoveraged, sampling has continued in in 1998 the number of recoveries has "skyrocketed", according to Bill Overholt of ICIPE. All over Southern Kenya, below 1500 metres, levels of parasitism have reached on average 15% to 20% although at some sites they are as high as 60%. Successful releases have now also been made in Mozambique and Somalia, where stemborers are a serious problem, and releases are also being planned for parts of Zambia, Uganda and Zanzibar. The next stage is to introduce a second natural enemy which, it is hoped, will further suppress the stemborer population.
But the introduction of natural enemies or parasitoids to help control stemborers is only one approach to tackling the problem. ICIPE is also currently working alongside KARI, (Kenya Agricultural Research Institute) and IACR-Rothamsted (UK) to select grasses such as napier grass and sudan grass (Sorghum sudanesi), which are more attractive to the stemborers than the cereal crops, to act as 'trap' crops around the perimeter of crop fields. Napier grass, is particularly useful: due to its deep-rooting system, it is able to survive periods of drought to provide forage throughout the dry season. And, because of its height, napier grass acts as an excellent windbreak and prevents wind-lodging, which is common when maize and sorghum stems have been damaged by stemborers.
Exciting progress was also made last year with the discovery of a 'repellent' grass which was found to drive stemborers away. Melinis minutiflora, or molasses grass, has a distinctive, sweet smell and is particularly good at driving the stemborers away from maize and sorghum and into the trap crops around the edge of the field. But, although the grass may smell sugary sweet to humans, to parasitic wasps, it smells like a plant attacked by stemborer larvae. Molasses grass, grown as an intercrop between every two-three rows of maize or sorghum, not only acts to repel stemborers out of the crop, but it also attracts the wasps in to parasitize and kill any stemborers that may be left in the cereal crop. In initial field trials, if molasses grass is planted early enough, it has been found that damage to the crops can be reduced to as little as 5%.
It was previously thought that only damaged cereal plants could attract natural enemies to parasitise the stemborers but this research has shown quite clearly that the same response can be initiated in an intact plant. As Professor Pickett of IACR-Rothamsted, says, "These findings are not just of value in this project but for subsistence and high input agriculture all over the world."
So far, the on-farm trials for repellent crops and traps crops have been tested separately but the experimental evidence has demonstrated the combined 'push-pull' effect working very well. Farmers participating in the trials have also responded very enthusiastically to the results and have played an important role in passing the good news on to farmers in neighbouring areas.
