Push Pull in Africa
Harnessing the power of ecology to manage pests in maize plants in East Africa.
Globally, people depend on maize and other cereals to provide the bulk of their energy intake. In East Africa, maize is a major food crop which is farmed on a relatively small scale, and sometimes on a subsistence basis. Yield can be directly related to survival. Like other cereals, maize in East Africa is often attacked by agricultural pests, which can devastate the crop and result in yield losses of up to 100%. The ramifications of yield losses are serious, which is why Rothamsted teamed up with the International Centre of Insect Physiology and Ecology (ICIPE) to develop a solution to these pest problems. In this case, an ecological approach was the most viable, as pesticides are impractical for small scale farmers in this region.
In these areas, farmers have often employed the use of intercropping, where different plants from the main crop are planted within the main crop (often between rows of the main crop). We wanted to develop an ecological pest control regime using the concept of push-pull pest management. This strategy involves planting an intercrop which deters pests, typically by releasing volatiles which are repellent, in conjunction the ‘pull’ crop which draws in pests and is planted on the edges surrounding the crop. By trapping pests, the 'pull' crop breaks the cycle of infestation by disrupting the lifecycle of the pest.
To understand the story of Rothamsted’s push-pull system, it’s important to understand the biology of the pests affecting maize in East Africa.
In East Africa, maize is attacked by two species of stem borer (Busseola fusca & Chilo partellus) as well as two species of parasitic weed called striga (Striga hermonthica & Striga asiatica).
Busseola fusca & Chilo partellus
During the early life stages, stem borers briefly feed on leaves. After a few days, they bore into the plant in order to feed and develop. They tend to feed on several types of cereal plants, but their impact is particularly significant in maize crops, because of the importance of maize as a food crop in these regions. The cycle of infestation begins when adult moths lay eggs on the stems of maize.
Moths find suitable areas to lay eggs mainly by using their keen sense of smell. Juvenile moths feed and develop for about 2-3 weeks inside the plant, weakening the stem, and stunting growth. Unsurprisingly, this results in short, sparse plants, and low crop yields. As these moths mature and reproduce, the cycle is repeated again and again.
Striga hermonthica & Striga asiatica
This parasitic weed fastens onto maize roots, resulting in severely stunted growth of maize (usually they only grow to 1 foot tall when infested). This attachment forms from root to root - this is called ‘haustorial development’. By the time above-ground growth appears, the damage to the maize plant is significant, making hand weeding ineffective. One plant produces 20-80,000 seeds which lay dormant for up to 15 years. In just a few generations, this creates a dense seedbank – making strategies like crop rotation ineffective. Striga’s parasitic status means they only live when there is a host present so leaving a field fallow won’t reduce the vast seedbank already there. If a farm has striga, yield losses range from 30 – 100%.
Implications for farmers
For subsistence farmers, yield losses like this means there are 30-100% fewer kernels on the plate. When you factor in damage caused by stem borers, which typically accounts for a loss of yield of 20-40%, the threat of starvation becomes very real. To cater to the farming systems in East Africa, we knew we had to come up with a solution that was cheap, effective and suited to the size/scale of farm, so we did field trials in Kenya, using test plots of similar size to typical farm plots, with the same conditions, including striga infestations.
To start, we tested grasses and legumes in the field. For grasses, we looked to find varieties which are better than others at protecting themselves against stem borers. By splitting open the stems of many different species, and counting stem borers inside, we could determine how good the plant is at naturally preventing infestation. We also looked at legumes which are not attacked by stem borers because of their repellent attributes. Clues from these field trials lead to further testing of plants in the field and the lab.
What we found: the ‘push’
Originally, we chose to try desmodium (Desmodium uncinatum) as the intercrop for maize, as we thought its strong aromatic qualities would confuse moths and deter them from laying eggs. Although we found them to be effective this way, we had no idea that desmodium wasn’t just deterring moths above ground with their volatiles; underneath the soil, desmodium was also working to repress striga growth as well. At first, we didn’t know the mode of action for this, so we tested a few potential causes. One contributor to striga repression is the increased soil nitrogen level that is present in soil where desmodium is growing. Desmodium is a legume which fixes nitrogen in soil, making it more available to plants growing in that soil. This had an effect, but didn’t tell the whole story.
As striga seeds germinated, as they do in the presence of maize roots, desmodium also secreted chemicals that interrupted root attachment. We took the roots into the lab and performed chemical analysis. We found that desmodium was releasing a compound called isochaftoside. This finding confirmed our suspicions that desmondium is alleopathic, meaning it can influence the biology of other organisms by producing and releasing chemicals into its environment. In this case, the desmodium caused seeds to germinate but then prevented them from growing. This finding is of vital importance, because the seedbank of striga already in the soil could finally be reduced, despite their dormancy and abundance.
Next: the pull
After the moths are repelled, or pushed away from the maize, we found that planting Napier grass (Pennisetum purpurem) around the crop attracted or pulled in the moths, enticing them to lay their eggs there. However, when the larvae tried to bore into Napier stems, they were stopped in their tracks by a milky glue-like substance. The life cycle of the larvae is interrupted, preventing them from becoming mature moths.
Does it work?
As is often the case in agriculture, we see a variety of yields occur because of complex environmental factors. Fortunately for farmers, the worst off farms saw drastic yield increases, while less infested farms saw less of a spike, but benefited from the system. This served not just to help farmers survive in the short term, but allowed for an increase to their earning potential.
The push-pull system certainly works to control pests, but we wanted these extra plants to serve multiple purposes on the farm to make best use of resources and space. Desmodium, for example, is a legume, meaning it fixes nitrogen, making this essential nutrient available to the maize crop. Desmodium also acts as a protective covering on the soil, retaining soil moisture, reducing soil erosion and improving water efficiency. Napier grass and desmodium are nutritious feed for livestock, benefiting the farm more broadly, so resources are maximised in this system.
Push-pull – where is it now?
Rothamsted’s main role has been to work on the science of the push-pull strategy. Our ICIPE colleagues have worked tirelessly to educate farmers using all sorts of approaches. The graph below shows the number of farmers that have adopted this strategy. There are currently 68, 689 farmers practicing push-pull pest management. It is our hope that this system will be useful to an even greater number of farmers in the future. Public education work is on-going, thanks to a Bill & Melinda Gates Foundation Grant (covering Western Kenya and Nigeria) as well as an EU ADOPT project grant (covering Tanzania, arid Kenya and Ethiopia).
- Khan, Z.R., Hassanali, A., Overholt, W., Khamis, TM., Hooper, A.M., Pickett, J.A., Wadhams, L.J., and Woodcock, C.M. 2002. Control of witchweed Striga hermonthica by intercropping with Desmodium spp., and the mechanism defined as allelopathic. Journal of Chemical Ecology, 28, 1871-1885.
- Khan, ZR (Khan, ZR); AmpongNyarko, K., Chiliswa, P., Hassanali, A., Kimani, S., Lwande, W., Overholt, W.A., Pickett, J.A., Smart, L.E., Wadhams, L.J., Woodcock, C.M. 1997. Intercropping increases parasitism of pests. Nature. 388. 631-632 DOI: 10.1038/41681
- Khan, Z.R., Pickett, J.A., van den Berg, J., Wadhams, L.J., Woodcock, C.M., Exploiting chemical ecology and species diversity: stem borer and striga control for maize and sorghum in Africa. 2000. Pest Management Science. 56. 957-962
- For more information on ICIPE and push-pull: http://www.push-pull.net/
- Hooper, AM.,Tsanuo, M.K., Chamberlain, K., Tittcomb., Scholes, J., Hassanali, A., Khan, Z.R., Pickett, A. Isoschaftoside, a C-glycosylflavonoid from Desmodium uncinatum root exudate, is an allelochemical against the development of Striga. 2010. Phytochemistry. 71. 904-908