Smelly plants could be natural pest control

30 April 2014 by Harriet Jarlett

Bombarding whiteflies with smells from different plants may stop them damaging crops, say scientists trying to understand traditional organic farming techniques.

TomatoesFor centuries gardeners and farmers have been using a technique called companion planting, where they surround flowers or crops with strong-smelling plants to deter pests.

Scientists have suspected this works because too many smells confuse nuisance insects, but the science behind this has never been proven until now.

A new study, published in Agronomy of Sustainable Development, aimed to assess this confusion effect by exposing whiteflies that were trying to feed on a tomato plant with smells from plants like cucumbers, cabbages and courgettes. The smells are caused by gases released by the plant, known as volatiles.

The scientists found that when the whiteflies were bombarded with a range of volatiles from different plants it took them much longer to feed than when the other plants weren't around.

"It's like a teacher trying to mark their class's homework. If someone switches on the telly, another person turns on the radio and then someone else starts speaking to them, they won't be able to concentrate," explains Dr Colin Tosh of Newcastle University, one of the lead authors on the study.

"The plants we remove smells from are actually pleasant, attractive smells to the whiteflies. You might expect bombarding the insects with a pleasant smell would stimulate their feeding, but it does the opposite because they are confused by the abundance of information," he explains.

Whiteflies feed on plants by pushing their long mouth parts into their leaves to reach the phloem - where the nutrient-rich sap flows through the plant.

"The insects have quite a complicated task ahead of them to push their mouths through the cells in the leaf to reach the sap. They have to detect gradients and different types of tissues so having different smells around seems to be just too distracting," Tosh says.

Tosh and his co-author, Dr Barry Brogan of Newcastle University, studied the confusion effect using a miniature greenhouse. This clear Perspex box contained four tomato plants with whiteflies and a tube which sucked the volatiles from the other vegetable plants and into the greenhouse.

"The confusion effect caused a delay in the time the flies took to reach the phloem of the tomato plant. In the control test, with no other smells the whiteflies had all reached it within 15 hours and started feeding, but barely any of confused flies managed in this timeframe," says Tosh.

Whiteflies are a major pest in greenhouses and on farms worldwide. The honeydew they produce sticks to plants and triggers the growth of a sooty mould fungus that can cause damage worth hundreds of millions of pounds a year. Currently most people treat them using a variety of chemical pesticides and parasitoids, but Tosh suspects that creating a confusing environment could be the first step in finding a more sustainable method of pest control.

"While the smells delayed their feeding, it made little difference to the amount of honeydew they produced overall," says Tosh. "So the confusion effect wouldn't be effective as a standalone pesticide, but if you combined it with other effects - such as triggering the natural defences of the crop being attacked - it could be useful to deter their feeding further."

"These volatiles are all things you'd be eating and smelling anyway, so it should be completely harmless. Using plant smells just seems to be a bit smarter than GM. We're not firing some kind of insect death gene into the plant, instead it's about working with the plant's natural biology to enhance a natural processes," he concludes.

'Control of tomato whiteflies using the confusion effect of plant odours' - CR Tosh, B Brogan. Agronomy for Sustainable Development.