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Script 72.10

Notes to broadcasters

The stem borer is one of the major insect pests of maize in eastern and central Africa. Stem borers cause maize plants to become brown and stunted, and the damage can seriously reduce yields.

Scientists from the International Centre for Insect Physiology and Ecology discovered two different plants that affect the stem borer behaviour – one plant that can ‘push‘ stem borers away from the maize and one plant that can ‘pull‘ them away from the maize. Together with staff from Kenya’s Ministry of Agriculture and the Kenya Agricultural Research Institute (KARI), they developed what they call a ‘push-pull’ strategy to control stem borers, which they are introducing to farmers.

This ‘push-pull’ strategy is described in the following radio program. It requires that farmers plant Napier grass and Desmodium in their fields to help them increase maize yields and improve soil fertility. Before broadcasting this program you may want to find out if and where farmers in your audience can get seed for these plants. The following organizations, companies and individuals are possible sources for further information about the availability of Desmodium and Napier grass in your region.


Script

Host:
Farmers are probably some of the most observant people you will ever meet. They have to be. And some of the things they observe are the pests in their fields. By knowing the life cycle and behaviour of insect pests, they can see how their crops are affected – and make decisions about how to minimize their crop losses.

But scientists are also interested in the behaviour of insect pests. And clever observation by scientists in Kenya has lead to some important discoveries. Scientists developed a ‘push-pull’ system to control stem borers in maize. They found one plant that ‘pulls’ stem borers and one plant that ‘pushes’ them away from maize.

Stay tuned to hear more about this discovery.

MUSIC (5 seconds).

Host:
If you are familiar with stem borers, you probably know that they like to lay their eggs in the stem of maize plants. What you might not know is that they also lay their eggs on wild grass. In fact, it turns out that they prefer some wild grass. So, if they have the choice between maize and wild grass, they will lay their eggs on the tastier grass.

Knowing this, the scientists who were looking at the problem of stem borers started an experiment. The experiment involved a simple cropping strategy using both Napier grass and Desmodium legumes.

First, they planted a grass called Napier grass or elephant grass [Pennisetum purpureum] in a border around the maize fields. They wanted to see if the grass would attract the stem borer moths and lead them away from the maize. They planted the grass and waited to see what would happen.

And their plan worked! The stem borer moths left the maize and moved to the Napier grass that was growing at the edge of the field. In other words, the tastier Napier grass ‘

pulled
‘ the harmful insects away from the maize. That’s where they laid their eggs. When the eggs hatched, the larvae bored into the Napier grass instead of the maize.

So the scientists discovered that Napier grass can ‘pull’ stem borers away from maize by providing a more attractive place for them to lay their eggs. But that’s not all. Napier grass also produces a sticky substance like gum that traps the stem borers and eventually kills them.

MUSICAL BREAK.

Host:
At the beginning of the program I said that scientists also discovered a plant that could ‘
push
‘ the stem borers
out
of the field. They did experiments with a species of plant called Desmodium. This plant gives off chemicals that drive the stem borers away. So when Desmodium is planted in between the rows of maize, the smell repels the insects and pushes them away from the maize. And that’s the ‘push’ part of the strategy. So now you know about the ‘push-pull’ method of controlling stem borers in maize!

MUSICAL BREAK.

Host:
You might also be interested to know that there are other benefits from using the ‘push-pull’ system. For one thing, both Napier grass and Desmodium make good livestock fodder. For another, Desmodium species planted in alternate rows with maize can really help the soil. It adds nitrogen and improves soil fertility. And last, but not least, researchers found that the Desmodium plant puts a chemical in the ground that suppresses striga weed – so there are fewer weeds growing in the field.

Host:
As we heard in previous programs, striga is a serious pest weed that grows into the roots of maize and kills it. So a plant that also helps to control striga weed is good news for farmers.

Since the scientists carried out those experiments, many farmers in Kenya also tried the ‘push-pull’ method to see if it would control insect pests in their own fields. Some farmers just used the ‘push’ method – they alternated rows of Desmodium species with their maize; others used the ‘pull’ method – planting a border of Napier grass around their field; but most used both ‘push-pull’ together, as recommended by the scientists. The good news is that many farmers have had good results and have been able to increase their yields. In addition farmers now have a good source of fodder and keep healthier cows which produce more milk than before.

But of course no method is right for every farmer. For example, if you live in place where there is a lot of rain and you want to try planting Napier grass, you have to be careful because Napier grass can grow out of control with a lot of water. You can also use other grasses – like Sudan grass – instead of Napier grass. Find out which grasses work best in your area.

Whatever method you choose to control insect pests in your crops, it’s best to gather as much information as you can – about both the pest and the method before you proceed.

Acknowledgements

  • Contributed by Jennifer Pittet, researcher/writer, Thornbury, Canada.
  • Reviewed by Dr. Annalee Mengech, Head of Information and Publications Unit, International Centre of Insect Physiology (ICIPE), Nairobi, Kenya.

Information Sources