Exploiting phytochemicals for developing a 'push-pull' crop protection strategy for cereal farmers in Africa

Abstract

Lepidopteran stemborers and parasitic weeds in the genus Striga are major constraints to efficient production of cereals, the most important staple food crops in Africa. Smallholder farmers are resource constrained and unable to afford expensive chemicals for crop protection. Development of a push-pull approach for integrated pest and weed management is reviewed here. Appropriate plants were discovered that naturally emit signalling chemicals (semiochemicals). Plants highly attractive for egg laying by stemborer pests were selected and employed as trap crops (pull), to draw pests away from the main crop. Of these, Napier grass, Pennisetum purpureum (Schumach), despite its attractiveness, supported minimal survival of the pests' immature stages. Plants that repelled stemborer pests, notably molasses grass, Melinis minutiflora P. Beauv., and forage legumes in the genus Desmodium, were selected as intercrops (push). Desmodium intercrops suppress Striga hermonthica (Del.) Benth. through an allelopathic mechanism. Their root exudates contain novel flavonoid compounds, which stimulate suicidal germination of S. hermonthica seeds and dramatically inhibit its attachment to host roots. The companion crops provide valuable forage for farm animals while the leguminous intercrops also improve soil fertility and moisture retention. The system is appropriate as it is based on locally available plants, not expensive external inputs, and fits well with traditional mixed cropping systems in Africa. To date it has been adopted by more than 30,000 smallholder farmers in East Africa where maize yields have increased from ∼1 t ha(-1) to 3.5 t ha(-1). Future directions for semiochemical delivery by plants including biotechnological opportunities are discussed.

Crop Diversification: A Potential Strategy To Mitigate Food Insecurity by Smallholders in sub-Saharan Africa

“…The push-pull strategy, also referred to as stimulo-deterrent diversionary strategy, entails the control of pests by repelling them from the main crop (push) using stimuli that masks a host or is repellent or a deterrent while simultaneously attracting (pull) the pest, using highly attractive stimuli, to other areas such as traps or trap crops where they are concentrated and eventually eliminated (Cook, Khan, & Pickett, 2006;Miller & Cowles, 1990). Recent studies have reported successful control of lepidopteran stem borers on maize crop in Kenya, by employing crop mixtures in push-pull strategies (Hassanali et al, 2008), a technique so far adopted by more than 30,000 farmers across East Africa (Khan, Midega, Bruce, Hooper, & Pickett, 2010).…”

Volatile Semiochemical Mediated Plant Defense in Cereals: A Novel Strategy for Crop Protection

“…This represents a sophisticated defence response which provides timely and effective biological control early on, before much damage is caused by the hatching larvae [11,28]. Some plant species constitutively release volatile semiochemicals that repel insect pests and attract natural enemies of the herbivore [29,30]. More examples on ecological functions of plant-derived volatile semiochemicals have been well documented [9,23,31,32].…”

“…One of the most successful examples of a stimulo-deterrent diversionary strategy is a 'push-pull' cropping system developed to control cereal stemborers for resource-poor farmers in Africa [30]. The system involves intercropping cereal crops maize or sorghum, with the fodder legumes desmodium spp.…”

“…(silverleaf Desmodium uncinatum or greenleaf D. intortum) and molasses grass (Melinis minutiflora) and planting Napier grass (Pennisetum purpureum) or Sudan grass (Sorghum vulgare sudanense) around this intercrop (Figure 2). Volatile semiochemicals such as (E)-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene produced by the desmodium and molasses repel (push) push away the stemborer pests while those produced by Napier and Sudan grasses attract (pull) stemborer moths to lay eggs in the grass instead of maize [30]. In addition to producing repellent volatile semiochemicals that push away the stemborer moths, the root exudates of Desmodium spp have been shown to effectively suppress the parasitic weed, Striga spp.…”

What works in conservation? Using expert assessment of summarised evidence to identify practices that enhance natural pest control in agriculture

“…This practice, the 'push-pull system', has been carefully studied in maize and bean crops, in Africa. Well-designed, replicated trials have demonstrated positive effects on natural enemies, reduced pest number, pest damage and increased yields in response to this practice (for example, Khan et al 2010; see http://www.conservationevidence.com/actions/ 753, or Wright et al (2013) for links to all 10 relevant studies). It clearly has strong potential to enhance natural pest regulation.…”