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Scientists build on folklore to help protect our tomatoes.

Byline: TONY HENDERSON ENVIRONMENT

THEY are a favourite of gardeners, cooks and a mainstay in the kitchen, and are also big business.

But tomatoes are also a tasty treat for a pest which is all too familiar to greenhouse and vegetable growers - whitefly.

It has long been part of gardening folklore that growing marigolds alongside tomatoes deters the tiny sap-sucking insect.

Now researchers at Newcastle University's school of natural and environmental sciences have shown that the flowers are a valuable ally in protecting tomato crops.

Domestic gardeners have a choice of more than 30 tomato varieties in their seed catalogues.

Tomatoes are also the second most important edible horticultural crop by production in developed nations but, under glass, pest infestations are dominated by the common glasshouse whitefly, which is known to cause severe yield losses through transmission of viruses and enabling the growth of mould on the plants.

Newcastle University's Dr Colin Tosh said: "Direct feeding from both adults and larvae results in honeydew secretion at a very high rate. Honeydew secretion that covers the leaves reduces the photosynthetic capacity of the plant and renders fruit unmarketable."

But now the Newcastle team has identified limonene - released by marigolds - as the main component responsible for keeping whitefly at bay. The insects find the smell of limonene repellent and are slowed down by the chemical.

The findings of the study have the potential to pave the way to developing safer and cheaper alternatives to pesticides.

Since limonene repels the whitefly without killing them, using the chemical shouldn't lead to resistance, and the study has shown that it doesn't affect the quality of the produce.

All it takes to significantly deter the whiteflies is interspersing marigolds with tomatoes, or hanging little pots of limone amid the plants.

The team, led by Dr Tosh and PhD student Niall Conboy, has shown that it may be possible to develop a product, similar to an air freshener, containing pure limonene, which can be hung in glasshouses to confuse the whiteflies.

Niall 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 it is only a weak scent in the air.

"Most pesticides are sprayed on to the crops. This doesn't only kill the pest that is targeted, it kills absolutely everything, including the natural enemies of the pest."

Limonene makes up around 90% of the oil in citrus peel and is commonly found in household air fresheners and mosquito repellent.

Dr Tosh said: "There is 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 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."

The researchers carried out two big glasshouse trials. In the first experiment, they established that the repellent effect works and that marigolds are an effective companion plant to keep whitefly away from tomato plants.

Marigolds are grown next to tomatoes throughout the growing period and whitefly population growth was followed from the seedling stage over a 48-day infestation period.

It was found that association with marigolds significantly slows whitefly population development.

The second study looked at the impact of marigolds when used as an "emergency" measure after whitefly populations were allowed to build to a high density on unprotected tomatoes.

The emergency marigold companion planting yielded minimal reductions in whitefly, but the use of limonene dispensers was more effective.

Tomatoes are typically produced in monoculture, thus rendering crop areas of limited value to wildlife, so any alternative methods of whitefly control that can reduce pesticide use and introduce greater plant diversity into agricultural and horticultural systems is a positive.

Further studies will focus on developing a three companion plant mixture that will repel three major insect pests of tomato - whiteflies, spider mites and thrips.

Longer term, the researchers aim to publish a guide focusing on companion plants as an alternative to pesticides.

CAPTION(S):

Whitefly and their larvae can decimate crops

Studies show limonene can protect glass house tomatoes

Marigolds have long been known to deter whitefly
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Publication:The Journal (Newcastle, England)
Date:Mar 9, 2019
Words:769
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