When finding their plant hosts, agricultural insect pest always seeks familiar scents. But they can also be repelled by odors from other plant species, according to new research, which offers a new framework for exploiting plant odors to repel insect pests.

Broccoli, one of the affected crops. Credit: Living in Monrovia (Flickr)

A team at the University of Vermont worked on the swede midge, a small fly which has become a problem for farmers from the Northeast that work with cabbage-family crops like broccoli and kale. They discovered that a set of essential oils were effective at repelling the midge, such as garlic and spearmint.

“People often think more aromatic plant oils, like mint, basil and lavender will repel insects, but usually there is no rhyme or reason for choosing,” says senior author Yolanda Chen. “It turns out that as we go along the family tree, plants that are more distantly related from the host plant are generally more repellent.”

In order to survive, the small fly feeds on the brassica plant family, which includes a set of popular vegetables such as cabbage and brussels sprouts. If the midge laid its eggs on the wrong plant, it would mean the death of its offspring, according to observations by the researchers.

The midge’s larvae affect the plant’s control system, causing distorted growth – such as brown scarring. But, unfortunately for farmers, they can’t observe the problem until it’s too late and the midge has dropped off the plant. The tiny fly is known to cause crop losses of up to 100% in some areas of the US and Canada.

Trying to deal with the midge, farmers have turned to insecticides, which has been associated with a decline in bees. Organic farmers found no methods and just stopped growing the vulnerable crops. This led to the team at Vermont University to find new methods to control the small fly.

“It’s hard to get away from using insecticides because they’re good at killing insects,” said lead author Chase Stratton, who is now a postdoctoral researcher at The Land Institute in Kansas. “But plants have been naturally defending against insect herbivores for millions of years. Why are we so arrogant to think we can do it better than plants?”

Stratton and her colleagues were able to identify essential oils from 18 different plants that vary in their degree of relatedness to brassica host crops. They hypothesized that oils from plants that are more distantly related to brassicas would have more diverse odors and be more repellent.

They spent time observing how midges acted when facing broccoli plants that had been sprayed with each of the essential oils. The small fly, they discovered, was less likely to lay eggs on broccoli plants that had been treated with essential oils, compared to the untreated plants.

“Biologically, it makes sense that midges would be able to detect and avoid these plants because the similar odors would make it easier for them to misinterpret these plants as hosts, which would be deadly for their offspring,” said Stratton. “For swede midge, garlic appears to be one of the most promising repellents, particularly because certified organic products using garlic are already available for growers.”