Bioscience in brief
Crop Improvement & Management
Companion cropping: Push-Pull technology
Stem borers and violet vampires
Maize is the most important staple crop in Africa – more than 300 million people in the continent depend on it as their main food source. While average yields of maize in industrialized countries are eight tons per hectare, in developing countries they are often less than three tons. The most devastating maize pests are insects, in particular, maize stem borers (Busseola fusca). These moths lay their eggs in the leaves of maize plants. After hatching, the caterpillars eat the young growing leaves of maize and then burrow into the stems affecting the flow of water and nutrients in the plant. The attack is devastating: in heavily infested plants the stems weaken and the plants topple over and up to half of the crop can be lost. Insect damage is worse in plots with poor soil fertility and during periods of drought – situations when maize plants are already struggling to obtain sufficient nutrients and water for their growth and typical in Africa.
Controlling maize stem borers with insecticides is very difficult: the caterpillars are inside the plant and so well protected from contact with any chemicals that are applied. Insecticides are also expensive and out of reach for many smallholder farmers. They are toxic and also kill beneficial insects and harm the environment.
The parasitic weed Striga (Witchweed) is another devastating enemy. A single plant can produce thousands of tiny seeds which have the ability to remain dormant in the soil for up to twenty years. When the seeds germinate, they attach themselves to the roots of other plants – maize, sorghum and rice being favourites. Instead of deriving the water and nutrients they need from the soil, they steal them from the attacked plant. By the time Striga plants germinate and produce their beautiful purple flowers, it is too late for the farmer to do anything: the damage to the crops has been done and the entire harvest may be lost. In Africa Striga accounts for yearly loses of US$ 1.2 billion, and it affects the livelihoods of more than 25 million smallholder farmers. Striga has been called the “violet vampire”, the “cereal killer” and the “sleeping enemy” (and probably much worse). Hand weeding and crop rotation remain the most important control methods in smallholder African farms.
When plants arm themselves and fight back
Plants are directly or indirectly the source of food and energy for all animals. Animals either eat plants or eat other animals/insects that ate plants. This is problematic for plants – they can’t run away. And insects are the worst. A range of physical adaptations provides some protection against herbivores, such as sharp spines and thick hairs.
But largely, this is a chemical war. Plants produce an array of compounds to repel or damage attackers. They also make chemicals that attract the presence of beneficial insects that feed on attacking pests, or that mimic the smell of heavily infested plants so approaching insects may be fooled into believing there is nothing left to eat. Some of these chemical weapons may be made by a plant all the time but the production of other chemicals occurs only after the plant senses it has been damaged.
Since there is huge pressure on herbivores to obtain their food in spite of these defences, a struggle to break down the barriers ensues. This is often referred as co-evolution since plants then need to find new ways of fighting back and herbivores in their turn cannot give up… The situation is always dynamic and continually evolving, and agricultural systems are no exception – plant breeders will never be out of work!
With a helping hand from your friends
Inter-cropping, growing more two or more crops in close proximity, is a practice that can allow farmers to take advantage of the chemicals plants make to defend themselves.
Here’s the challenge: to devise a cropping system that intersperses maize plants with plants that repel the stem borers but attract their natural enemies, and identify another plant that attracts the pests – but does not allow them to multiply – to plant around the plots. This system would “push” pests away from the maize plants, and “pull” them into the borders of the plot. All the plants need to be locally available. And there is one more condition: all the companion plants have to be useful to the farmer either as food or feed for animals, and they also have to improve soil fertility. Maybe even help with Striga. An ambitious project: it took researchers in Mbita, Kenya, and Rothamsted, United Kingdom, years to develop this so-called Push-Pull technology. Thousands of farmers have also participated in this project and benefited from it.
The sweet smell of molasses…
Zeyaur Khan, an entomologist at icipe, Kenya, noticed that maize plants growing next to molasses grass (Melinis minutiflora) had little stem borer damage, while plants in nearby fields were heavily damaged. Molasses grass is a fodder crop with a very strong, sweet smell. Researchers established that the molasses grass all the time produces a smell that other plants (including maize) only make when they are heavily infested. While the stem borers stay away because the smell signal usually indicates that the plant has little left to be eaten, the smell attracts the natural enemy of maize stem borers: the parasitic wasp (Cotesia sesamiae). The wasps inject their eggs inside the stem borer caterpillars, and when they eggs hatch they eat the caterpillars. Just one row of molasses grass planted every ten rows of maize works well as a repellent. In addition to effectively protecting their maize crops without having to spend any money on insecticides, farmers could also harvest the molasses grass to feed their milk cows and goats.
On the search for a multi-tasking legume
But the researchers wanted a companion plant that provided, in addition to natural insect repellents, also a natural fertiliser. Legumes have the ability to fix atmospheric nitrogen (the largest component of air) into a form than can be incorporated into proteins. So instead of depleting the soil of nutrients, they replenish it. Cowpea (Vigna unguiculata), grown for food and fodder, and the silverleaf desmodium (Desmodium uncinatum), a fodder legume that tolerates shade well, looked like promising candidates for repellent crops.
Khan and his colleagues started to test desmodium at icipe’s Mbita Point Station at Lake Victoria and the trials confirmed the usefulness of this legume as a maize stem borer repellent. The trials also led to an amazing discovery: plots planted with desmodium as an intercrop in areas heavily infested with Striga became virtually free of the parasitic plant in a single growing season. A star find.
Researchers characterised the compounds that desmodium releases in the root to understand the effect on Striga. They discovered that it first causes the germination of Striga seeds but then stops them from growing further (it causes suicidal germination). For many farmers the control of Striga is the main motivation for adopting the Push-Pull system.
Setting the trap
After studying more than 400 grasses, researchers at icipe selected Napier grass (Pennisetum purpureum) and Sudan grass (Sorghum sudanese) as trap plants to be planted in the borders of plots. Both species attract stem borers but both also have a way to defence themselves from the pests. Sudan grass also attracts parasitic wasps and Napier grass exudes a sticky gum when the borers enter the stem that kills the caterpillars. Both grasses make nutritious fodder for cows and goats.
Spreading the word
Khan and his colleagues began to disseminate the Push-Pull technology in 1997 and since then nearly 50,000 smallholder farmers have adopted it. Most of them had struggled before to harvest enough to feed their families but now can sell the surplus in the market. Companion cropping also grows well with sorghum and rice which is extremely susceptible to Striga. Some farmers have adapted the system to better suit their needs, for example by incorporating edible beans in the cropping mix. The availability of fodder has also encouraged farmers to invest in cows and goats, and to start small dairy operations. This means better nutrition for the family and perhaps a small business on the side. Farmers are actively involved in spreading the information to other farmers.
Difficulties remain. Setting up companion cropping requires hard work at the beginning: farmers have to establish desmodium and trap grasses before they can plant their crop. And sourcing enough desmodium seeds can be a problem too although several farmers have established how to propagate desmodium vegetatively.
Companion cropping can boast that once established it is fully sustainable. Plants in this system provide everything needed: fertilisers, insect and weed control, and sources of protein for a more nutritious diet. The soil quality improves, and since no toxic chemicals are sprayed, the level of biodiversity increases.
Soon we will have to produce enough food for 10 billion people with increasing fuel and input prices and with minimal impact to our natural resources. Smart agriculture is likely to be the way forward everywhere.
John Pickett who leads the collaborative work in the UK (Rothamsted Research) believes that the dramatic demonstration on farm in Africa that companion cropping can robustly control pests and weeds paves the way for new GM based approaches in which the crop plants themselves provide the necessary crop protection chemistry for Africa and the rest of the world.
We thank John Pickett, Toby Bruce and Tony Hooper for information and comments.