Push-Pull Strategies for Insect Pest Management
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PUSH-PULL STRATEGY FOR INSECT PEST MANAGEMENT Zeyaur R. Khan International Centre of Insect Physiology and Ecology (ICIPE) Nairobi, Kenya and John A. Pickett Rothamsted Research Harpenden, Hertfordshire, United Kingdom The ‘push-pull’ strategy, a novel understanding of the pest’s biology and tool for integrated pest management the behavioral/chemical ecology of the programs, uses a combination of interactions with its hosts, conspecifics, behavior-modifying stimuli to manipulate and natural enemies. The specific the distribution and abundance of insect combination of components differs in pests and/or natural enemies. In this each strategy according to the pest to strategy, the pests are repelled or be controlled (its specificity, sensory deterred away from the main crop abilities, and mobility) and the resource (push) by using stimuli that mask host targeted for protection. apparency or are repellent or deterrent. Among several push-pull The pests are simultaneously attracted strategies under development or used in (pull), using highly apparent and practice for insect pest control, the most attractive stimuli, to other areas such as successful example of the push-pull traps or trap crops where they are strategy currently being used by farmers concentrated, facilitating their control. was developed in Africa for controlling The term ‘push-pull’ was first stemborers on cereal crops. This conceived as a strategy for insect pest strategy was developed using management by Pyke, Rice, Sabine and technologies appropriate to resource Zaluki in Australia in1987. They poor farmers and has shown a high investigated the use of repellent and adoption rate and spontaneous attractive stimuli, deployed in tandem, to technology transfer by farmers, resulting manipulate the distribution of in significant impact on food security by Heliocoverpa spp. in cotton to reduce increased farm production in the region. reliance on insecticides, to which the moths were becoming resistant. The Management of cereal stemborers in concept was later formalized and refined Africa through push-pull strategy by Miller and Cowles in the US in 1990, Maize and sorghum are the who termed the strategy ‘stimulo- principal food and cash crops for deterrent diversion’ while developing millions of the poorest people in the alternatives to insecticides for control of predominantly mixed crop-livestock the onion fly, Delia antiqua. farming systems of eastern Africa. The development of a reliable, Stemborers are one of the major robust, and sustainable push-pull constraints to increased maize strategy requires a clear scientific production. At least four species of stem borers (Chilo partellus, Eldana intercrops and trap crops, using plants saccharina, Busseola fusca and that are appropriate to the farmers. This Sesamia calamistis) infest maize and push-pull strategy does not use any sorghum crops in the region, causing chemical deterrents or toxins, but uses reported yield losses of 30-40% of repellent plants to deter the pest from potential output. Stemborers are difficult the main crop. The trap plants used in to control, largely because of the cryptic this push-pull strategy have the inherent and nocturnal habits of the adult moths ability of not allowing development of and the protection provided by the stem trapped stemborers, thus reducing the of the host crop for immature stages. number of trapped insects. The strategy The main method of stemborer control, also attempts to fully exploit the natural which is recommended to farmers by enemies in the cereal farming system. the Ministry of Agriculture in these The push-pull strategy for cereal countries, is use of chemical pesticides. stemborers involves trapping However, chemical control of stemborers on highly attractant trap stemborers is uneconomical and plants (pull) while driving them away impractical to many resource-poor, from the main crop using repellent small-scale farmers. Therefore, reducing intercrops (push). Plants that have been the losses caused by stemborers identified as effective in the push-pull through improved management tactics include Napier grass strategies is urgently needed which (Pennisetum purpureum), Sudan grass could significantly increase cereal (Sorghum vulgare sudanense), production, and result in better nutrition molasses grass (Melinis minutiflora), and purchasing power for many maize and desmodium (Desmodium uncinatum and sorghum producers. To put and Desmodium intortum). Napier grass stemborer control within the reach of and Sudan grass are used as trap African farmers, simple and relatively plants, whereas molasses grass and inexpensive measures need to be desmodium repel ovipositing developed and tailored to the diversity of stemborers. Molasses grass, when African farming systems. Several intercropped with maize, not only national and international agricultural reduced infestation of the maize by research centers continue to devote stemborers, but also increased increasingly scarce resources towards stemborer parasitism by a natural the development of technologies enemy, Cotesia sesamiae (Fig. 2). All intended to increase farm production four plants are of economic importance through stemborer management but to farmers in eastern Africa as livestock with little impact. fodder and have shown great potential A push-pull strategy for managing in stemborer and striga management in cereal stemborers in Africa was farmer participatory on-farm trials. developed by scientists of the These innovations have found ready International Centre of Insect Physiology acceptance among the resource-poor and Ecology (ICIPE) in Kenya and farmers in Africa. Although directed at Rothamsted Research in the United resource-poor farmers, lessons can be Kingdom, in collaboration with other learned and applied to organic or low- research organizations in eastern Africa. input agricultural systems. More than The strategy involves combined use of ten thousand farmers in eastern Africa are now using push-pull strategies to produced during damage to plants by protect their maize and sorghum from herbivorous insects. It is likely that these cereal stemborers. compounds, being associated with a high level of stemborer colonization and, How push-pull strategy works in some circumstances, acting as The push-pull strategy undertakes foraging cues for parasitoids, would be a holistic approach in exploiting repellent to ovipositing stemborers. This chemical ecology and agrobiodiversity. was subsequently demonstrated in The plant chemistry responsible for behavioral tests. Investigating the stemborer control involves release of legume volatiles, it was shown that D. attractive volatiles from the trap plants uncinatum also produced the ocimene and repellent volatiles from the and nonatriene, together with large intercrops. To understand the chemical amounts of other sesquiterpenes, ecology of the push-pull system, volatile including α-cedrene. chemicals from trap and repellent plants Six host volatiles were found to have been investigated using gas be attractive to gravid stemborers: chromatography (GC) coupled- nonanal, napthelene, 4-allylanisole, electroantennography on the antennae eugenol and linalool. Recent studies of stemborers and their natural enemies. have indicated that the differential GC peaks consistently associated with preference of moths between maize and EAG activity were tentatively identified sorghum and Napier grass trap crops is by GC coupled-mass spectrometry (GC- related to a large burst of four MS), and identity was confirmed using electrophysiologically active green leaf authentic samples (Fig. 1). volatiles released from the trap crop A general hypothesis developed plants within the two hours of the during this work on insect pests is that scotophase, the time at which non-host plants are recognized by stemborers fly and most oviposition colonizing insects through the release of occurs. Although stemborers oviposit repellent or masking semiochemicals, heavily on Napier grass, it produces a although it is almost inevitable that gummy substance which restricts larval compounds also produced by hosts will development and only few survive to be present (Fig. 2). In this case, the host adulthood. cereal plants and the non-host M. A trap crop of Sudan grass also minutiflora would be expected to have a increased efficiency of stemborer number of volatiles in common as they natural enemies. In a maize field are both members of the Poaceae surrounded by a border of Sudan grass, family. For M. minutiflora, five new the parasitization of stemborers peaks with EAG activity were identified, increased significantly relative to fields in addition to the attractant compounds without grass borders. and others normally produced by members of the Poaceae. These are: Benefits of push-pull strategy (E)-ß-ocimene, α-terpinolene, ß- The principles of the push-pull caryophyllene, humulene, and (E)-4,8- strategy are to maximize control dimethyl-1,3,7-nonatriene. Ocimene and efficacy, efficiency, sustainability, and nonatriene had already been outputs, while minimizing negative encountered as semiochemicals environmental effects. Although each individual component of the strategy Reduced soil erosion and increased soil may not be as effective as a broad- fertility. Soil erosion and low fertility are spectrum insecticide at reducing pest very common problems in eastern numbers, the efficacy is increased Africa. The push-pull strategy has through tandem deployment of push and exploited some of the existing practices