Ecological chemistry of pest control in push-pull intercropping systems: What we know, and where to go?

Push-pull technology (PPT) employs mixed cropping for sustainable intensification: an intercrop repels or suppresses pests of the focal crop (push), while a trap crop attracts pests out of the field (pull), where they may be targeted for control. Underlying chemical-ecological mechanisms have been demonstrated in controlled settings, primarily for some of the best-established cereal PPT systems developed in east Africa. Yet, many questions remain regarding mechanisms, and strategies to adapt PPT for different crops and locations. We conducted a systematic review of scientific literature on PPT and related practices for biological control of pests of food and fodder. Of 3335 results, we identified 45 reporting on chemistry of trap- or intercropping systems for pest control, of which 30 focused on cereals or African pests. Seven of these reported primary chemical data: measurements from glasshouse and laboratory studies (5), or of field-collected samples (2). From these 30, we provide a database of compounds, discussing degrees of evidence for their mediation of push-pull. We depict hypothesized spatial distributions of selected compounds in PPT fields from physical properties and emission/exudation rates, and design of the east African cereal PPT system, and discuss influences on activity in field settings likely to affect success

Determining smallholder farmers’ preferences for Push-Pull technology dissemination pathways in western Kenya

The push-pull technology (PPT) has widely been disseminated to control stemborer (Chilo partellus and Busseola fusca) and Striga weeds (Striga hermonthica and Striga asiatica) in maize fields in Kenya. This study examined farmers’ preferences for various dissemination pathways in order to proffer better targeting of resources in an optimal dissemination strategy. The pathways considered were public meetings (barazas), radio, farmer field schools (FFS), field days (FD), farmer teachers (FT), the fellow farmers (FF) and print materials. Using a weighted score index and ordered probit regression, the different pathways were sequentially ranked as FD, FT, FFS, FF, print materials, Radio, and barazas. Marginal effects from ordered probit showed that farmers had the least preferences for baraza and radio pathways. The farmer categories with the highest preference for particular pathways were: less educated farmers for FD, farmers with small land sizes for FT, farmers belonging to groups for FFS, and young educated farmers for the print materials. This information is extremely important for targeting the different segments of farmers.Push-pull technology, Stemborer, Striga, Dissemination pathways, preference, Research and Development/Tech Change/Emerging Technologies,

Ecological Chemistry of Pest Control in Push-Pull Intercropping Systems: What We Know, and Where to Go?

Push-pull technology (PPT) employs mixed cropping for sustainable intensification: an intercrop repels or suppresses pests of the focal crop (push), while a trap crop attracts pests out of the field (pull), where they may be targeted for control. Underlying chemical-ecological mechanisms have been demonstrated in controlled settings, primarily for some of the best-established cereal PPT systems developed in east Africa. Yet, many questions remain regarding mechanisms, and strategies to adapt PPT for different crops and locations. We conducted a systematic review of scientific literature on PPT and related practices for biological control of pests of food and fodder. Of 3335 results, we identified 45 reporting on chemistry of trap- or intercropping systems for pest control, of which 30 focused on cereals or African pests. Seven of these reported primary chemical data: measurements from glasshouse and laboratory studies (5), or of field-collected samples (2). From these 30, we provide a database of compounds, discussing degrees of evidence for their mediation of push-pull. We depict hypothesized spatial distributions of selected compounds in PPT fields from physical properties and emission/exudation rates, and design of the east African cereal PPT system, and discuss influences on activity in field settings likely to affect success

Classification and influence of agricultural information on striga and stemborer control in Suba and Vihiga Districts, Kenya

This paper reports on findings of a study to examine the sources used by farmers in search of agricultural information on striga and/or stemborers control technologies and factors that influence acquisition of such information in Western Kenya region. A random sample of 476 households in Suba and Vihiga districts were interviewed and 15 information pathways were identified. Using principle component analysis (PCA) to derive few latent variables that encapsulate maximum variance in the pathways, two components (latent variables) proxying for ‘agricultural knowledge’ were extracted. Type I-knowledge (first component) loaded heavily with sources that had ‘group’ information searching. Type II-knowledge (second component) loaded heavily with sources requiring individual farmer search. Both types of knowledge positively and significantly influenced the likelihood of households using improved technology to control stemborer, while only Type-II knowledge and social economic factors were important in influencing the farmers’ likelihood of using an improved technology to control striga. This study shows that information is an important factor in the households’ likelihood of using improved technologies in the control of striga and stem borer in Vihiga and Suba, Kenya. Methods of individual interaction are important to striga control.Agricultural information, improved technology, striga, stemborers, control technologies, Kenya, Agricultural and Food Policy, Community/Rural/Urban Development, Demand and Price Analysis, Farm Management, Food Consumption/Nutrition/Food Safety, Food Security and Poverty, Institutional and Behavioral Economics, International Relations/Trade, Marketing, Productivity Analysis, Research and Development/Tech Change/Emerging Technologies, Resource /Energy Economics and Policy,

Drought-tolerant Desmodium species effectively suppress parasitic striga weed and improve cereal grain yields in western Kenya

Abstracts The parasitic weed Striga hermonthica Benth. (Orobanchaceae), commonly known as striga, is an increasingly important constraint to cereal production in sub-Saharan Africa (SSA), often resulting in total yield losses in maize (Zea mays L.) and substantial losses in sorghum (Sorghum bicolor (L.) Moench). This is further aggravated by soil degradation and drought conditions that are gradually becoming widespread in SSA. Forage legumes in the genus Desmodium (Fabaceae), mainly D. uncinatum and D. intortum, effectively control striga and improve crop productivity in SSA. However, negative effects of climate change such as drought stress is affecting the functioning of these systems. There is thus a need to identify and characterize new plants possessing the required ecological chemistry to protect crops against the biotic stress of striga under such environmental conditions. 17 accessions comprising 10 species of Desmodium were screened for their drought stress tolerance and ability to suppress striga. Desmodium incanum and D. ramosissimum were selected as the most promising species as they retained their leaves and maintained leaf function for longer periods during their exposure to drought stress conditions. They also had desirable phenotypes with more above ground biomass. The two species suppressed striga infestation, both under controlled and field conditions, and resulted in significant grain yield increases, demonstrating the incremental capability of Desmodium species in striga suppression. These results demonstrate beneficial effects of Desmodium species in enhancing cereal productivity in dry areas

A climate-adapted push-pull system effectively controls fall armyworm, Spodoptera frugiperda (J E Smith), in maize in East Africa

Fall armyworm, Spodoptera frugiperda (J E Smith), an economically important pest native to tropical and subtropical America has recently invaded Africa, causing substantial damage to maize and other crops. We evaluated functionality of a companion cropping system, ‘climate-adapted push-pull’, developed for control of cereal stemborers in drier agro-ecologies, as an added tool for the management of fall armyworm. The technology comprises intercropping maize with drought-tolerant greenleaf desmodium, Desmodium intortum (Mill.) Urb., and planting Brachiaria cv Mulato II as a border crop around this intercrop. Protection to maize is provided by semiochemicals that are emitted by the intercrop that repel (push) stemborer moths while those released by the border crop attract (pull) them. 250 farmers who had adopted the technology in drier areas of Kenya, Uganda and Tanzania were randomly selected for the study during the long rainy season (March-August) of 2017. Each farmer had a set of two plots, a climate-adapted push–pull and a maize monocrop. Data were collected in each plot on the number of fall armyworm larvae on maize, percentage of maize plants damaged by the larvae and maize grain yields. Similarly, farmers' perceptions of the impact of the technology on the pest were assessed using a semi-structured questionnaire. Reductions of 82.7% in average number of larvae per plant and 86.7% in plant damage per plot were observed in climate-adapted push-pull compared to maize monocrop plots. Similarly, maize grain yields were significantly higher, 2.7 times, in the climate-adapted push-pull plots. Farmers rated the technology significantly superior in reducing fall armyworm infestation and plant damage rates. These results demonstrate that the technology is effective in controlling fall armyworm with concomitant maize grain yield increases, and represent the first documentation of a technology that can be immediately deployed for management of the pest in East Africa and beyond

Removing constraints to sustainable food production: new ways to exploit secondary metabolism from companion planting and GM

The entire process of agricultural and horticultural food production is unsustainable as practiced by current highly intensive industrial systems. Energy consumption is particularly intensive for cultivation, and for fertilizer production and its incorporation into soil. Provision of nitrogen contributes a major source of the greenhouse gas, N2O. All losses due to pests, diseases and weeds are of food for which the carbon footprint has already been committed and so crop protection becomes an even greater concern. The rapidly increasing global need for food, and the aggravation of associated problems by the effects of climate change, create a need for new and sustainable crop protection. The overall requirement for sustainability is to remove seasonal inputs, and consequently all crop protection will need to be delivered via the seed or other planting material. Although genetic modification (GM) has transformed the prospects of sustainable crop protection, considerably more development is essential for the realisation of the full potential of GM and thereby consumer acceptability. Secondary plant metabolism offers wider and perhaps more robust new crop protection via GM and can be accomplished without associated yield loss because of the low level of photosynthate diverted for plant defence by secondary metabolism. Toxic mechanisms can continue to be targeted but exploiting non‐toxic regulatory and signalling mechanisms should be the ultimate objective. There are many problems facing these proposals, both technical and social, and these are discussed but it is certainly not possible to stay where we are in terms of sustainability. The evidence for success is mounting and the technical opportunities from secondary plant metabolism are discussed here

An indirect defence trait mediated through egg-induced maize volatiles from neighbouring plants

Attack of plants by herbivorous arthropods may result in considerable changes to the plant’s chemical phenotype with respect to emission of herbivore-induced plant volatiles (HIPVs). These HIPVs have been shown to act as repellents to the attacking insects as well as attractants for the insects antagonistic to these herbivores. Plants can also respond to HIPV signals from other plants that warn them of impending attack. Recent investigations have shown that certain maize varieties are able to emit volatiles following stemborer egg deposition. These volatiles attract the herbivore’s parasitoids and directly deter further oviposition. However, it was not known whether these oviposition-induced maize (Zea mays, L.) volatiles can mediate chemical phenotypic changes in neighbouring unattacked maize plants. Therefore, this study sought to investigate the effect of oviposition-induced maize volatiles on intact neighbouring maize plants in ‘Nyamula’, a landrace known to respond to oviposition, and a standard commercial hybrid, HB515, that did not. Headspace volatile samples were collected from maize plants exposed to Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) egg deposition and unoviposited neighbouring plants as well as from control plants kept away from the volatile emitting ones. Behavioural bioassays were carried out in a fourarm olfactometer using egg (Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae)) and larval (Cotesia sesamiae Cameron (Hymenoptera: Braconidae)) parasitoids. Coupled Gas Chromatography-Mass Spectrometry (GC-MS) was used for volatile analysis. For the ‘Nyamula’ landrace, GC-MS analysis revealed HIPV production not only in the oviposited plants but also in neighbouring plants not exposed to insect eggs. Higher amounts of EAG-active biogenic volatiles such as (E)-4,8-dimethyl-1,3,7-nonatriene were emitted from these plants compared to control plants. Subsequent behavioural assays with female T. bournieri and C. sesamiae parasitic wasps indicated that these parasitoids preferred volatiles from oviposited and neighbouring landrace plants compared to those from the control plants. This effect was absent in the standard commercial hybrid we tested. There was no HIPV induction and no difference in parasitoid attraction in neighbouring and control hybrid maize plants. These results show plant-plant signalling: ‘Nyamula’ maize plants emitting oviposition-induced volatiles attractive to the herbivore’s natural enemies can induce this indirect defence trait in conspecific neighbouring undamaged maize plants. Maize plants growing in a field may thus benefit from this indirect defence through airborne signalling which may enhance the fitness of the volatile-emitting plant by increasing predation pressure on herbivores

Push-pull farming system controls fall armyworm: lessons from Africa

Fall armyworm (FAW) Spodoptera frugiperda invaded Africa, with the first detections being reported in Central and Western Africa in early 2016, and now affects at least 40 countries in Africa, causing up to total crop loss amounting to over $6.2 billion p.a. FAW is an invasive polyphagous pest that causes damage to economically important crops and has recently been reported in the Indian sub-continent. Effective control of FAW through use of synthetic chemical pesticides and genetically modified crops such as Bt maize faces challenges including improper use, unaffordability by smallholder farmers and development of resistance by the pest. Additionally, dispersal of FAW larvae into the lower maize plant canopy keeps them out of reach of topical insecticide applications. Integrated pest management (IPM) packages like the push-pull technology which eliminate pesticide use, and deploy natural processes are more suitable and cost-effective. Push-pull is a farming system intensification approach that involves attracting insect pests with trap plants (pull) such as Napier grass (Pennisetum purpureum) or Brachiaria grass, while driving them away from the main crop using a repellent intercrop (push), Desmodium spp., commonly known as desmodium, and attracting natural parasitoids and predators to the field. In the rhizosphere, chemicals secreted by desmodium roots inhibit attachment of germinated striga to maize or sorghum roots and abort germination of striga seeds which are rapidly depleted in the soil. Moreover, it improves soil fertility by fixing nitrogen, improving carbon sequestration, organic matter, moisture retention, and soil biota, and prevents further degradation of soil. The climate-adapted push-pull technology significantly reduces plant damage by FAW and is the first IPM management tool for the pest in Africa, and is well suited to agro-ecosystem intensification needs of smallholder mixed farming systems in Africa and beyond. Both the African and the Asian continents provide favourable climatic conditions for sustained reproduction of the FAW, which is expected to result in severe damage to crops; and being a new pest in both continents, it might have few natural enemies. Conventional control methods have limited effectiveness, as explained above. Therefore, an IPM approach that is compatible with mixed cropping farming systems of small and medium scale farmers is necessary. The climate adapted push-pull is the first demonstrated IPM management tool for the FAW in Africa. The technology combines this with other concomitant benefits including control of stemborers and the parasitic striga weeds, improvement of soil health through factors such as nitrogen fixation by desmodium, natural mulching, moisture retention and improvement of soil organic matter and soil biota. The combined benefits, including control of FAW, result in ecologically sustainable higher crop yields, and well suited to agro-ecosystem intensification needs of smallholder mixed farming systems in Africa and beyond