Push and Pull: Biological control of stemborer and Striga

[img_assist|nid=181|title=|desc=|link=none|align=right|width=100|height=43]In issue No. 41, the Biotechnology and Development Monitor published an article by Verkleij & Kuiper about various approaches to controlling Striga (witchweed) and Orobranche (broomrape). The root parasites are a major biotic constraint to food production particularly in Africa. The authors showed that conventional methods of controlling these weeds have only a limited impact and that transgenic plants do not offer a workable solution either. Now the International Centre for Insect Physiology and Ecology (ICIPE) in Kenya has developed a push-pull system against stemborer in maize that also controls Striga.

Cereal stemborers, the larval stage of certain moths (Busseola fusca, Sesamia calmistis, Eldana saccharina, Chilo archalociliellus and Chilo partellus), can cause the loss of about 20 to 40 per cent of the potential yield of a maize or sorghum crop. They are difficult to control because the eggs and the larvae are hidden deep inside the stems. In a push-pull system different plants are sown with the crop. The pests are repelled from the field by one plant and, at the same time, attracted by others planted outside the field. In this way they do not feed on the actual crop plant itself.
In the combined cropping system developed by the ICIPE for South and East Africa, the stemborer is attracted to napier grass (Pennisetum purpureum) or Sudan grass (Sorghum vulgare sudanense) by their smell. The grass is grown in several rows outside the maize field and produces a gummy substance that traps the larvae’s. In the ICIPE pilot project only about 10 per cent of the stemborer larvae’s eventually survived. The project started with the knowledge that stemborers were indigenous to East Africa long before maize was introduced and that the insect must have feed on another type of grass in the past. Farmers in the neighbourhood of the ICIPE research station were invited to choose which grass they thought might be most suitable. They preferred napier and Sudan grass because both make good fodder. Varieties of wild grass that looked like ‘weed’ were rejected.
From the inside of the field, the stemborer is repelled by molasses grass (Melinis minutiflora) or by silver leaf desmodium (Desmodium uncinatum). In tests, the use of molasses grass reduced the maize crop losses from 40 per cent to 4.6 per cent. Desmodium seems to be even better equipped for inter-cropping. As a leguminose it binds nitrogen and thus enriches the soil. It also keeps the soil moist, reduces erosion and can be used as fodder. But most important, desmodium intercropped with maize suppresses the growth of Striga by a factor of 40 in comparison to monocropping of maize. The scientific reason for this is unknown but ICIPE has set up a research project, funded by the Rockefeller Foundation (USA) to investigate the phenomena.