Push-Pull: Win-Win

Food security and sustainable agriculture seem to be current buzzwords. In reality, the drive towards sustainability in cropping practices began at least 20 years ago.

That's when Zeyaur Khan, head of the Grass Ecosystems Programme of icipe (the International Centre of Insect Physiology and Ecology) in Nairobi, Kenya, began his research and conceived the Push-Pull idea. The continuous development and application of his research since then have earned him the TWAS Prize in Agriculture 2011.

During the TWAS Prize lecture in Tianjin, delivered alongside other TWAS Prize winners and dedicated to the founder of icipe and also a TWAS founding Fellow, Thomas Odhiambo, Khan explained that some 13 species of maize stem-borers were present in Africa and that in any one place there were likely to be 5 or 6 species present. Associated with these are a number of parasitoid wasps that can help keep the stem-borer populations under control. The over-use of pesticides, however, can destroy this natural pest-control, often leading to stem-borer outbreaks – and hungry farmers.

On arriving in Africa from his native India, Khan noticed not only that many African farmers practise intercropping, but also that there were many wild grass species growing around farmers' fields. These, he reasoned, were responsible for providing a steady supply of adult moths able to fly into the farmers' maize crops and lay their eggs.

The diversity of the grasses, however, gave Khan an idea. What if he could identify a type that the moths liked to lay their eggs on, but that the hatching caterpillars did not like to eat?

He began by screening dozens of species and eventually hit on napier grass (Pennisetum purpureum), which is 2-3 times more attractive to the stem-borer moths for egg-laying, but inhibits the development of the larvae. It was also chosen for its alternative use as a fodder crop.

In 1996, Khan planted his first trials of maize plots surrounded by rows of napier grass to 'pull' the stem-borers away from the maize. It proved a success. But Khan wanted to develop the technique further. He wondered if there were other species that would actually repel the moths. If so, and if these were intercropped with the maize (a practice the farmers were already familiar with), then the results could be improved further.

After various trials, he eventually hit on the legume Desmodium – for four good reasons. Not only did it both repel the stem-borers and attract their natural enemies, and not only did it (like all legumes) fix nitrogen in the soil, and not only can be used as fodder, but it also prevented the growth of Striga, a parasitic weed that can be an additional scourge to resource-poor African farmers.

Armed with this success – the use of Desmodium to 'push' pests out of the crop – Khan put the two technologies together in 1999. Working with 420 farmers over a 5-year period, he validated the 'Push-Pull' technology by showing that it could almost double farmers' yields. Indeed, they obtained higher milk yields from the supply of fodder crops, too.

"Benefits included the protection of biodiversity (as no pesticides were being used), increased household income, and, linked to this, the empowerment of women," added Khan.

The full impact of Khan's work can only be appreciated, however, when it is considered that some 53,000 farmers in eastern Kenya are now using the Push-Pull technology.

But Khan is not content with these figures. By developing more drought-tolerant Push-Pull varieties, he is now expanding the system for use in sorghum and millet crops. With such 'climate-smart push-pull' in place, he believes he can reach one million farming households by 2020.