Diesel fumes from tractors or pollution from nearby roads or factories could be reducing the effectiveness of some naturally occurring biocontrol agents that help suppress aphid populations on field brassica crops, according to research at the University of Reading.
The research used special equipment to deliver controlled amounts of diesel exhaust and ozone – both pollutants found in emissions from vehicles and industrial processes – to study their impact on the numbers of aphid parasitic wasps on oilseed rape plants, though it has implications for vegetable brassicas too.
Even when the pollutants were applied at levels lower than current ‘safe maximums’ set by environmental regulations, the numbers of parasites on the crop fell. They were less able to find their aphid hosts and reproduced more slowly.
Research leader James Ryalls said that while most of the parasitic wasp species in the study were adversely affected by diesel fumes and ozone, one actually did better. He says this may be because the pollutants stimulated the plants to produce more of the defensive compounds that give brassicas their distinctive bitter or peppery flavours.
“That parasite, Diaeretiella rapae, particularly likes to prey on cabbage aphids, and we know that some of the compounds in brassicas are converted into substances that attract D.rapae,” he said. “So, we could speculate that the stronger smell attracts the wasps and they are more successful in finding and preying on aphids.”
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