Chemicals

Push-pull strategy for pest control

By Dr Shahzad Maqsood Ahmad Basra, M. Tariq Javed & Dr Irfan Afzal

April 09 2007: AGRICULTURAL production is limited by many factors like soil, water, genetic potential of crops and organisms that feed on or compete with food plants. Food crops, the world over, are damaged by more than 10,000 species of insect-pests and overall estimated yield losses from different pests are reported to be $500 billion globally and Rs40,000 billion in Pakistan alone. About 42.1 per cent of attainable production is lost due to pest attack. However, if no control measures are adopted the figure would be 69.8 per cent.

Insects? control has relied heavily on insecticides. Beside other drawbacks, these insecticides induce acute and chronic posing in human and other living organisms. Chronic exposures to organophosphorous insecticides inhibit enzymes activity in human. Chronic exposures to organochlorine pesticides accumulate residues in human body. The assessment of human exposure to selected organochlorine compounds, in Belgium, China, India, Israel, Japan, Mexico, Sweden, USA and Yugoslavia showed the pesticide residues in human milk sample. Higher level of these chemicals in mother?s milk is the reflection of their increased burden and their translocation passage. These alarming facts have prompted efforts on the safer, reduced-risk and environmental compatible method of pest control with the objective to maximise crop productivity.

Push-pull strategy, which is a behavioural manipulation technique, is best suitable in this scenario. First of all, Australians coined the term push-pull, as a strategy, for insect-pest management. They investi gated the use of repellent and attractive stim uli, deployed in tandem, to manipulate the dis tribution of Helicoverpa species in cotton, thereby reducing reliance on insecticides, to which the insects were becoming resistant. Push-pull strategies use a combination of behaviour-modifying stimuli to manipulate the distribution and abundance of pest and or beneficial insects for pest management.

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Strategies targeted against pests try to reduce their abundance on the protected resource, for example, a crop or farm animal. The pests are repelled or deterred away from this resource (push) by using stimuli that mask host appearance or are repellent or deterrent. The pests are simultaneously attracted (pull), using highly apparent and attractive stimuli, to other areas such as traps or trap crops where they are concentrated, facilitat ing their elimination.

In plant-based systems, naturally generated plant stimuli can be ex ploited using vegetation diversification, in cluding intercropping and trap cropping. Push stimuli can be delivered by intercrop ping with non-host plants that have repellent or deterrent attribute appropriate to the target pest. Intercropping reduces pest density in crops, principally by disrupting host loca tion through reducing the visual appearance of the host plant, by repellent or de terrent.

Trap crops, therefore, represent a key element of plant-based push-pull strategies. However, the relative attrac tiveness of the trap crop compared with the main crop, the ratio of the main crop given to the trap crop, its spatial arrangement and the colonisation habits of the pest are crucial to success and require a thorough understanding of the behaviour of the pest.

The most successful push-pull strategy, indeed the only example currently used in prac tice, was developed in Africa for subsistence farmers. Although directed at resource-poor farmers, lessons can be learned and applied to organic or low-input agricultural systems. Maize and sorghum are principal crops for millions of the poor in Africa and Asia, and stem bor ers cause yield losses of 10 per cent to 50 per cent. Agricultural advisory services in the region rec ommend the use of chemical pesticides, but this is uneconomical and impractical for poor, small-scale farmers.

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Thousands of farmers in Africa are now using push-pull strategies to protect their maize and sorghum. The strategies in volve the combined use of intercrops and trap crops, using plants that are appropriate for the farmers and that also exploit natural enemies. Stem borers are repelled from the crops by repellent non-host intercrops, partic ularly molasses grass, silver-leaf desmodium, and green-leaf desmodium (push), and are concentrated on attractive trap plants, pri marily Napier grass and Sudan grass (pull).

Molasses grass, when intercropped with maize, not only reduced stem borer infestation, but also increased parasitism. Molasses grass showed attractive compounds similar to those found from maize. These had already been identified from herbivore-damaged plants and were repellent to stem borers. Desmodium intercrops also produce these compounds and furthermore, when intercropped with maize or sorghum, suppress the parasitic African witch-weed, a significant yield constraint of arable land in Africa and Asia.

A trap crop of Sudan grass also increased the efficiency of stem borer natural enemies. Although stem borers oviposit heav ily on Napier grass, it produces a gummy substance that restricts larval development, causing few to survive.

The push-pull strategy has contributed to increased crop yields and livestock pro duction, resulting in a significant impact on food security in Africa. However, wherever these approaches are developed for the specific needs of local farmers, it is essen tial that the scientific basis of the modified systems be elucidated.

Several new technologies may help de velop and improve future push-pull strategies. Because we better understand the be haviour of pest and beneficial insects, enabled by advances in analytical techniques, synthe sis procedures, and formulation science, we may have a larger and more effective armoury of semio-chemicals and other stimuli for future use. In plant-based strategies, the use of induced defences and plants that produce the desired semio-chemicals themselves, rather than applying them to the plant, would help make the strategies more sustainable and available, especially for resource-poor farm ers.

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The use of push-pull strategies in almost all agricultural production systems (sustainable agriculture, horticulture, and forestry) is easy, feasible and economical, returning high output to growers. These strategies are also environmental friendly and have public acceptance. So there is a dire need to educate the farming community about these techniques and their practical application in agricultural production systems.

Courtesy The DAWN

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