Losses attributed to parasitic weeds in particular Striga hermonthica, Benth (African witchweed) on the other hand range between 30 and 100% in most areas and are often exacerbated by the low soil fertility prevalent in the region. The soils are highly degraded due to continuous cropping with limited or no external inputs to improve soil fertility. When the two pests occur together, farmers often lose their entire crop
Many systems for integrating pest control techniques that rely on improving cultural practices to minimize fertilizers and pesticide inputs (Glen et al. 1995; Van Emden&Peakall 1996; Waterlow et al. 1998; Brooks&Roberts 1999; Gurr et al. 2004) by the use also of hybrid seeds (seed produced by cross-pollinated plants in which the crosses are specific and controlled; hybrids are bred to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved color and disease resistance) have been developed but they are often unavailable for subsistence farmers in Africa because economically inappropriate (Gurr et al. 2004; Hester & Harrison 2005).
‘Push-pull’ is a novel approach in pest management which uses a repellent intercrop and an attractive trap plant. Insect pests are repelled (‘push’) from the food crop and are simultaneously attracted to a trap crop (‘pull’). In resource-poor maize farming systems this technology controls both stem borers and striga and improves soil fertility. Maize is intercropped with a silverleaf forage legume (Desmodium uncinatum, Jacq) and Napier grass (Pennisetum purpureum, Schumach.), a cattle forage grass, is planted around the intercrop. The silverleaf legume produces volatile chemicals which repel the stem borer moths from the maize while those released by Napier grass attract female moths to lay eggs. In addition, the roots of silverleaf legume produce chemicals which stimulate the germination of striga’s seeds and others which inhibit their attachment to maize roots causing suicidal germination. The legume also improves soil fertility through nitrogen fixation.
The main target of this technique are a series of lepidopterous pests attacking maize and other cereals in subsistence Africa. The main feature of the ‘push-pull’ approach is to have two or three rows of the trap crop, Napier grass, growing around the maize plot at a distance of 1 m. This bare patch of ground is required between the maize and the Napier grass so that the traps crops would not take water or soil nutrients from the main crop (A. Hassanali 2007). The maize itself is intercropped with silverleaf on a one-to-one basis. After harvesting the maize, silverleaf plants, growing perennially, are allowed to set seed which the farmer can harvest or sell. Silverleaf is then cut down to the ground, fed to cattle or ensilaged (the process of storing and fermenting green fodder for livestock) with Napier grass and for the next season the maize is sown into a drill cut by a hoe. Silverleaf produces volatile chemicals, such as (E)-ß-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene, which repel the stem borer moths from the maize ('push'), while those released by Napier grass, such as octanal, nonanal, naphthalene, 4-allylanisole, eugenol and linalool, attract female moths ('pull') to lay eggs. A sticky exudation from the grass then kills the stem borer larvae in response to penetration by first and second instar (developmental stage of arthropods) stem borer larvae. The roots of silverleaf also produce chemicals which stimulate the germination of striga’s seeds, such as 4'',5''-dihydro-5,2',4'-trihydroxy-5''-isopropenylfurano-(2'',3'';7,6)-isoflavanone, and others which inhibit their attachment to maize roots (suicidal germination) such as 4'',5''-dihydro-2'-methoxy-5,4'-dihydroxy-5''-isopropenylfurano-(2'',3'';7,6)-isoflavanone thereby reducing the bank of striga’s seeds. The legume can also improve soil fertility through nitrogen fixation in association with Rhizobium spp (Franck). Peak nodulation occurs 1 month before flowering (Whiteman, 1969).
- Brooks, G. T. & Roberts, T. R. (eds) 1999 Pesticide chemistry and bioscience, the food-environment challenge. Cambridge,
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- Glen, D. M., Greaves, M. P. & Anderson, H. M. (eds) 1995 Ecology and integrated farming systems. Chichester, UK:Wiley.
- Gurr, G. M., Wratten, S. D. & Altieri, A. (eds) 2004 Ecological engineering for pest management, advances in habitat manipulation for arthropods. Wallingford, UK: CAB International.
- Hassanali A. et al. 2007 Integrated pest management: the push-pull approach for controlling insects pests and weeds of cereals, and its potential for other agricultural systems including animal husbandry Phil. Trans. R. Soc. B (2008)
- Hester, R. E. & Harrison, R. M. (eds) 2005 Sustainability in agriculture. Cambridge, UK: Royal Society of Chemistry.
- Van Emden, H. F. & Peakall, D. B. 1996 Beyond silent spring, integrated pest management and chemical safety. London, UK: Chapman and Hall.Waterlow et al.,1998
- Waterlow, J. C., Armstrong, D. G., Fowden, L. & Riley, R. (eds) 1998 Feeding a world population of more than eight billion people — a challenge to science. Oxford, UK: Oxford University Press.
- Whiteman, 1969 P.C. Whiteman The effects of close grazing and cutting on the yield, persistence and nitrogen content of four tropical legumes with Rhodes grass at Samford, southeastern Queensland Aust. J. Exp. Agric. Anim. Husb., 9 (1969), pp. 287–294
- HYPERLINK "http://www.fao.org/ag/AGP/AGPC/doc/Gbase/data/pf000030.htm" http://www.fao.org/ag/AGP/AGPC/doc/Gbase/data/pf000030.htm (retrived on March 2012)