Improving food safety by reducing pesticide residues: developing a pheromone alternative to insecticides for control of thrips on legumes in Kenya

Cowpea is rarely eaten in the UK, but it is a major crop across Africa south of the Sahara, where it is estimated that 38 million households (194 million people) grow it and harvest a total of 6 million tonnes per year. Cowpea is mainly grown by smallholder farmers and is an important source of protein to the urban and rural poor who cannot afford meat, fish or milk products. Unfortunately, many smallholder farmers apply chemical insecticides too frequently while struggling to control pests, so that crops for domestic consumption often contain unacceptably high levels of pesticide residues. In contrast, produce for export to Europe is required to have minimal residues, so access to pesticide-free produce is not equitable.

The main pest of cowpea in Kenya is the bean flower thrips, which is a very small insect that breeds fast and is resistant to many insecticides. Smallholders end up spraying the crop repeatedly with insecticides, and this misuse and overuse by smallholder farmers has negative consequences for local consumers, particularly as cowpea is a staple food.

The aim of the project is to provide an alternative to insecticides for managing the bean flower thrips in cowpea and other similar crops in Kenya. This will reduce the amount of pesticide used and so improve food safety. It will also lead to higher yields, better safety for farm workers, reduced environmental impact and more sustainable agriculture.

Male thrips form aggregations for mating. They do this by releasing special chemicals (pheromones) that attract males and females. We have previously identified two male-produced chemicals in the bean flower thrips and preliminary experiments show that they attract males and females. We will synthesise these chemicals and then test them in the laboratory and in crops to understand how they affect thrips behaviour. We will then exploit the pheromone in various ways to develop a control method for the bean flower thrips. The use of the pheromone will be developed to attract large numbers of thrips to sticky traps for mass trapping and to disrupt and prevent mating. Another approach we will test is the use of the pheromone to attract thrips away from the crop to areas that can be spot-sprayed with insecticide. Although this still uses insecticide, it uses far less and it is not sprayed on the crop, which avoids residues. We will also develop the use of the pheromone for a method known as "lure and infect" in which we will attract thrips to a device where the thrips pick up spores of a naturally occurring fungus that kills thrips. The thrips then disperse with the spores and spread them to other thrips on the crop.

The research will involve researchers from Keele University and Harper Adams University in the UK, working with researchers from the International Centre of Insect Physiology and Ecology, known as icipe, in Kenya. This will build the capacity for thrips research in Kenya and provide an opportunity for student placements and exchanges between Kenya and the UK.

Cowpea (Vigna unguiculata) is widely grown in sub-Saharan Africa, where the main pest is the bean flower thrips (Megalurothrips sjostedti). The insect is resistant to most insecticides, so smallholder farmers end up spraying too often, leading to unacceptably high pesticide residues. Our aim is to develop a pheromone-based control method that will reduce pesticide use and thus improve food safety by reducing pesticide residues in food. It will also lead to higher yields, better safety for farm workers, reduced environmental impact and more sustainable agriculture.

Adult male thrips produce aggregation pheromones that attract both males and females. We discovered the first aggregation pheromone in thrips, which was in the western flower thrips (Frankliniella occidentalis), and it is used commercially for monitoring, but little attempt has been made to understand the effects of the pheromone on mating behaviour in order to develop the potential for control. We have previously identified two compounds that are present in the headspace volatiles of adult male (but not female) M. sjostedti, which indicates that they are pheromones. Preliminary experiments show that both compounds attract male and female thrips to sticky traps. We will synthesise these compounds and investigate their effects on mating behaviour, by means of laboratory bioassays and field trials, using a range of controlled release rates and ratios. We will then test and develop their potential for (a) mass trapping, (b) mating disruption, (c) spot treatment of thrips attracted to pheromone hot spots away from the crop, and (d) auto-dissemination of spores of the entomopathogenic fungus Metarhizium anisopliae (strain ICIPE 69), which kills M. sjostedti.

The project uses our world-leading UK research strength in thrips biology. It will be carried out in Kenya in conjunction with the International Centre of Insect Physiology and Ecology (icipe), which is based in Nairobi.

The research will produce cheap and effective pheromone-based methods for control of bean flower thrips (BFT) on cowpea and other grain legumes (e.g. French bean, pigeon pea) in Kenya. These alternatives to insecticides will be used by smallholder farmers and commercial growers. The main impact of the research will be on the people of Kenya who will benefit from having lower levels of insecticide residues in the locally produced cowpeas and other grain legumes that they eat. Insecticide residues often reach unacceptable levels because of misuse and overuse. Since cowpea is a staple food across much of sub-Saharan Africa and the BFT is a major pest there, pheromone-based methods can be extended from Kenya across the region, to countries such as Nigeria and Tanzania. It is estimated that across sub-Saharan Africa 38 million households (194 million people) grow cowpea over an area of 10 million hectares and harvest a total of 6 million tonnes per year. Many more people there eat cowpeas regularly and will benefit from safer food. Nigeria is the largest producer with 3.4 million tonnes per year, while Kenya produces 0.13 million tonnes per year. African production is worth about £2.7 billion per year and a conservative 5% loss to BFT costs £127 million per year. Insecticide reduction of 2 sprays per year would save smallholders about £23 per hectare.

Cowpea is an important source of protein to the urban and rural poor who cannot afford meat, fish or milk products. They are more dependent on cowpea and have it as a larger part of their diet, so they will particularly benefit from lower levels of residues. Cowpea is not just used for the grain (seeds), but is commonly used as a leaf vegetable by the poorest people and their families in subsistence farming systems, especially in East Africa. Insecticides are sprayed directly on leaves, so residues will be very high in leaves, and the residues will also be high because minimum times to harvest for consumer protection are often ignored. This will affect the poorest farmers most and so they have more to gain from lower levels of residues.

Smallholder farmers and commercial farmers will benefit from having cheap and effective alternatives to insecticides because BFT is difficult to control with insecticides, leading to misuse and overuse. Current control methods for BFT do not work well and will become less effective in future as insecticide resistance increases. More effective control methods will benefit farmers by giving higher yields.

A reduction in insecticide use will give better safety for farm workers who are at most risk from acute and chronic exposure to insecticides. Studies have shown that symptomatic occupational pesticide poisonings are very common among agricultural workers in developing countries, and symptoms include headache, body ache, fatigue, skin rashes, nausea, dizziness and tremors. This affects not just farmers, but sprayers, weeders, planters and harvesters. Many of these farmers and other workers are female and of childbearing age and so are particularly vulnerable to chronic effects of insecticide exposure.

Reduction in pesticide use will also reduce the environmental impact on non-target organisms, including beneficial insects such as natural biological control agents and pollinators, which will benefit farmers and the environment. It will also reduce the residues in air, soil, and water, which lead to indirect exposure in the general population.