Subsistence farmers in Kenya are transforming their lives with a simple yet cutting-edge scheme dreamed up by two British scientists. The project has already multiplied maize yields, delivered fresh milk, seen off two major pests and brought cash to an economy on the edge of starvation. And it couldn't be more organic.
The lesson started in an English country garden. Scientists at the world's oldest agricultural laboratory at Rothamsted in Hertfordshire set out to learn precisely why volatile chemical signals from one plant species switched on the appetites of a predator insect, and another switched them off. Black bean aphids fall upon legumes because their tiny antennae are tuned precisely to pick up the "eat-me" signal from a legume, but stay away from brassicas because cabbages send out a chemical "don't eat me" call sign. Plant cabbages and beans side-by-side and the insect detects both signals and gets thoroughly confused. So canny gardeners intercrop.
The challenge for Lester Wadhams and John Pickett at Rothamsted Research was to make the same strategy work on the shores of Lake Victoria in Kenya, where some of the world's poorest farmers lose up to 80% of their maize crop to the stem borer moth. The moth drills into the maize stem to lay its eggs. The tiny caterpillars hatch, and wax fat on maize stalks, which then fall down before the ears ripen. If your income is less than a dollar a day, and your maize is all you have in the world, stem borer attack is a calamity.
They began the project in 1994, working with Kenyan partners at Icipe, the international centre of insect physiology and ecology in Nairobi, looking for a plant with volatile semiochemical signals to entice the stem borer away from the maize, and another to repel the little beast. They staked out identical plots 50 metres square. They planted one entirely with maize. They planted Pennisetum purpureum, known locally as napier grass, around the rim of the other plot, and then rows of molasses grass, or Melinis minutiflora between the rows of maize. In the experimental plot, there were fewer stem borers in the maize, but greater numbers of parasitic wasps. The molasses grass was pumping out two chemical signals, one of which discouraged stem borers, and one of which attracted a parasitic wasp that preyed on stem borers.
This was a bonus, but the napier grass around the plot - the "trap" that drew the stem borer away from the maize - also played a cruel trick on the moth. The larvae hatched, and began to bore into the grass stem, at which point the grass produced a sticky exudate that killed 90% or more of the invaders, leaving just enough to make it worthwhile for the parasitic wasps to hang around. So the grasses protected the maize in three ways, by pushing them away from the crop, by attracting them to a trap crop, and by killing them with wasps and sticky sap. Economically, it was a winner. The experimental crop - even though a far smaller area was planted - produced 30% more maize than the traditional plot.
But there was another bonus. Both napier and molasses grasses are edible forage for cattle. When the two scientists first went to one arid region on the shores of Lake Victoria, there were only four cows producing any milk in reasonable quantity. Thanks to the napier grass revolution, there are now 470 cows producing milk sold locally.
"And that is important to the economy, because it means they have ready cash all the time to pay school fees and things like that, so they don't have to sell their maize when it is harvested at low prices and buy it back later on at high prices," Dr Wadhams says. So far 1,500 farmers have begun planting the grasses.
But the grass is not the end of the story. Kenyan farmers knew about intercropping anyway: they would plant rows of beans between the maize for a bit of extra protein and to fix some nitrogen fertiliser into the soil. So the two scientists started looking at a South American legume called Desmodium uncinatum, or silverleaf. What they saw kept them looking for the next six years. The curse of poor farmers on poor land in Africa is witchweed, or striga. This germinates when it gets chemical signals from a potential host plant, and parasitises the roots. It feeds off the crop, kills it, flowers, and produces tens of thousands of tiny seeds which can lie dormant for up to 20 years. But when silverleaf was intercropped with maize, there was almost no striga, even though the field of maize next door was almost devastated by it.
So now the farmers, who once could expect one or two tonnes per hectare, plant silverleaf and grasses with their maize, and are harvesting up to five tonnes per hectare. They also have more milk, because the silverleaf provides a rich protein source of fodder for cattle. Money is a problem, because until a Kenyan seed company sets up, the silverleaf seed has to be imported, expensively, but the experiment is being extended into Tanzania and Uganda. For the moment, it is hard to see much of a downside. "We are sitting here saying this is a panacea, and this works fantastically well," Wadhams says. "We take our colleagues out to Kenya on other aspects of the work, and although they have heard John Pickett and myself talk ad nauseam about this, when they get there, they say: 'God, this works even better than you told us'."