In a recently published study, researchers have identified the natural insect repelling chemical produced by marigold, reinforcing what farmers have culturally used for years as a tool to prevent or reduce whitefly infestations.
Whiteflies are a common pest in many areas of the world, notably across Africa and Asia. These small flies have major impacts on global food production, especially with tomato crops. The pest is now considered to be one of the most serious crop protection threats, causing millions of dollars in losses. As adults, these pests are small, moth-like insects that feed on plant sap. This can cause severe yield losses under high infestations. Whiteflies can also transmit a number of plant viruses which can result in further yield losses, plant death and promote the development of mould on infected plants.
Lead researchers for this study, Dr Colin Tosh said in ScienceDaily, “Direct feeding from both adults and larvae result in honeydew secretion at a very high rate. Honeydew covers the leaves and reduces the photosynthetic capacity of the plant and renders fruit unmarketable.”
Researchers from Newcastle University published their findings last month in the journal PLOS ONE, stating that the leading agent released by marigolds is limonene which results in reductions in pest presence. The findings of this study could potentially lead to the development of cheaper alternatives to pesticides which have a much smaller impact on their immediate environment.
To support the future implementation of limonene within agricultural management, the chemical does not kill the target insect, as such, there would be little risk of the development of chemical resistance as well as negatively impacting beneficial insects. The study also suggests that the use of limonene would not affect the quality of crop produce.
In terms of cultural practices, it is quite simple to apply the natural insect repellent properties of marigold within a field. The common use of this plant is either as a border crop or interspersed within the target crops. A more direct alternative would be the positioning of limonene (e.g. in pots) among the crop so the chemical can disperse in the air throughout the surrounding area and be exposed to whitefly pests.
On a commercial basis, the research team noted the possibility of producing a product based on common air fresheners, containing pure limonene which could be placed within greenhouses, plantations and open fields to dispel whiteflies.
PhD student, Nial Conboy who led the research team alongside Dr Tosh said, “We spoke to many gardeners who knew marigolds were effective in protecting tomatoes against whiteflies, but it has never been tested scientifically. We found that the chemical which was released in the highest abundance from marigolds was limonene. This is exciting because limonene is inexpensive, it’s not harmful and it’s a lot less risky to use than pesticides, particularly when you don’t apply it to the crop and is only a weak scent in the air.”
The study involved carrying out two large greenhouse trials using French marigold. In the first experiment, the repellent properties of the plant were confirmed. The second experiment involved using newly developed technology to analyse the gaseous chemicals being released by marigold. It was through this that the team were able to determine that limonene was the key player in repelling whitefly pests.
“There is a great potential to use limonene indoors and outdoors, either by planting marigolds near tomatoes or by using pods of pure limonene. Another important benefit of using limonene is that it’s not only safe to bees, but the marigolds provide nectar for the bees which are vital for plant pollination. Any alternative methods of whitefly control that can reduce pesticide use and introduce greater plant and animal diversity into agricultural and horticultural systems should be welcomed,” said Conboy.
Further studies are in discussion to develop a three-component plant mixture that will repel three major insect pests of tomato (thrips, spider mites and whiteflies). On a longer-term basis, the researchers aim to publish a guide focusing on companion plants as an alternative to pesticides, which would be suitable for use across a wide range of horticultural issues.
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