Push–pull technology is an intercropping strategy for controlling agricultural pests by using repellent "push" plants and trap "pull" plants. For example, cereal crops like maize or sorghum are often infested by stem borers. Grasses planted around the perimeter of the crop attract and trap the pests, whereas other plants, like Desmodium, planted between the rows of maize, repel the pests and control the parasitic plant Striga. Push–pull technology was developed at the International Centre of Insect Physiology and Ecology (ICIPE) in Kenya in collaboration with Rothamsted Research, UK. and national partners. This technology has been taught to smallholder farmers through collaborations with universities, NGOs and national research organizations.
How push–pull works Push–pull technology involves use of behaviour-modifying stimuli to manipulate the distribution and abundance of stemborers and beneficial insects for management of stemborer pests. It is based on in-depth understanding of chemical ecology, agrobiodiversity, plant-plant and insect-plant interactions, and involves intercropping a cereal crop with a repellent intercrop such as Desmodium uncinatum (silverleaf) (push), with an attractive trap plant such as Napier grass (pull) planted as a border crop around this intercrop. Gravid stemborer females are repelled from the main crop and are simultaneously attracted to the trap crop.
The push
The "push" in the intercropping scheme is provided by the plants that emit volatile chemicals (kairomones) which repel stemborer moths and drive them away from the main crop (maize or sorghum). The most commonly used species of push plants are legumes of the genus Desmodium (e.g. silverleaf Desmodium, D. uncinatum, and greenleaf Desmodium, D. intortum). The Desmodium is planted in between the rows of maize or sorghum, where they emit volatile chemicals (such as (E)-β-ocimene and (E)-4,8-dimethyl-1,3,7-nonatriene) that repel the stemborer moths. These semiochemicals are also produced in grasses such as maize when they are damaged by insect herbivores, which may explain why they are repellent to stemborers. Being a low-growing plant, Desmodium does not interfere with the growth of crops, but can suppress weeds and help improve soil quality by increasing soil organic matter content, fixing nitrogen, and stabilizing soils from erosion. It also serves as a highly nutritious animal feed and effectively suppresses striga weeds through an allelopathic mechanism. Another plant showing good repellent properties is molasses grass (Melinis minutiflora), a nutritious animal feed with tick-repelling and stemborer larval parasitoid attractive properties.
The pull
The approach relies on a combination of companion crops to be planted around and among maize or sorghum. Both domestic and wild grasses can help to protect the crops by attracting and trapping the stemborers. The grasses are planted in the border around the maize and sorghum fields where invading adult moths become attracted to chemicals emitted by the grasses themselves. Instead of landing on the maize or sorghum plants, the insects head for what appears to be a tastier meal. These grasses provide the "pull" in the "push–pull" strategy. They also serve as a haven for the borers' natural enemies. Good trap crops include well-known grasses such as Napier grass (Pennisetum purpureum), Signal grass (Brachiaria brizantha), and Sudan grass (Sorghum vulgare sudanense). Napier grass produces significantly higher levels of attractive volatile compounds (green leaf volatiles), cues used by gravid stemborer females to locate host plants, than maize or sorghum. There is also an increase of approximately 100-fold in the total amounts of these compounds produced in the first hour of nightfall by Napier grass (scotophase), the period at which stemborer moths seek host plants for laying eggs, causing the differential oviposition preference. However, many of the stemborer larvae, about 80%, do not survive, as Napier grass tissues produce sticky sap in response to feeding by the larvae, which traps them, causing the death of the larvae. Recent large-scale field studies in East Africa show that maize grown in push–pull systems has higher levels of two benzoxazinoid glycosides, compounds known for their antiherbivore properties. These glycosides were present in greater abundance in maize leaves from push–pull fields compared to those from conventional fields.
Suppression of Striga
Desmodium also controls the parasitic weed, Striga, resulting in significant yield increases of about 2 tonnes/hectare (0.9 short tons per acre) per cropping season. In addition to benefits derived from increased nitrogen availability and competition for light, it was found that D. uncinatum strongly suppresses striga growth through allelopathy. These effects are thought to be related to isoflavanones produced in Desmodium roots, which can either promote the germination of striga seeds or inhibit seedling growth, depending on their structure. Together, these effects result in the phenomenon known as "suicidal germination", thus reducing the striga seed bank in the soil. Other Desmodium species have also been evaluated and have similar effects on stemborers and striga weed and are currently being used as intercrops in maize, sorghum and millets.
Improvement of soil quality Desmodium also enhances soil quality by increasing soil organic matter, nitrogen content, and soil biodiversity, as well as conserving moisture, moderating soil temperature and preventing erosion.
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