Agricultural insect pests look for familiar aromas to find their host plants. However, they can also be repelled by odors from other plant species.
A new study from the University of Vermont published in Scientific reports It offers a novel framework to exploit plant odors to repel insect pests. The study is the first to show how the similarity of plant odors and the phylogenetic relationship can predict insect repellency.
The team applied this conceptual framework to swidge midge, a small fly that is becoming a major problem for growers in northeast broccoli, kale and other crops in the cabbage family. They discovered that particular essential oils (garlic, spearmint, thyme, lemon eucalyptus and cinnamon bark) were more effective in repelling midge. The findings are good news for organic farmers who do not have an effective solution to control the pest.
While essential oils have long been used in pest management, the determination of which oils are effective has followed a "trial by mistake" approach, said lead author Yolanda Chen, an associate professor in the Department of Plant Science and UVM soils.
"People often think that the most aromatic vegetable oils, such as mint, basil and lavender, repel insects, but there is generally no rhyme or reason to choose," said Chen, who is also a member of the Gund Institute for the Environment of the UVM. "It turns out that as we move forward in the family tree, plants that are more related to the host plant are generally more repellent."
Swede Midge is a recent invader on vegetable farms in the northeastern United States. Midge larvae must feed on the Brassica family of plants to survive, which includes many popular vegetables such as broccoli, kale, cauliflower, cabbage, Brussels sprouts, kohlrabi and cabbage. Making a mistake and laying eggs on the wrong floor would result in the death of the midge's offspring.
"The smell plays an important role in the host's location," said Chase Stratton, the lead author of the study, who recently completed his Ph.D. in the UVM. "Only one landing of a fly is enough to cause commercial damage," he said.
The larvae "sequester the plant's control system" and produce distorted growth, such as headless broccoli and cauliflower, wrinkled leaves and brown scars. Unfortunately for farmers, the damage is not observable until it is too late and the layer has already fallen from the plant. In areas where the midge has been well established, including parts of Canada, New York and northern Vermont, midge can cause crop losses of 100 percent.
To handle midge, conventional growers have turned to neonicotinoid insecticides, which have been implicated in the decline of bees. Without methods to kill the plague, some organic farmers have simply stopped growing vulnerable brassica crops. This led Chen and Stratton to explore new control options for organic farmers.
A sustainable solution
"It's hard to get away from the use of insecticides because they are good at killing insects," said Stratton, who is now a postdoctoral researcher at the Salina Land Institute, Kansas. "But plants have naturally defended themselves against insect herbivores for millions of years. Why are we arrogant enough to think we can do better than plants?"
Fascinated by the complexity of plant odors and species interactions, Stratton identified essential oils from 18 different plants that vary in their degree of relationship with brassica host crops. He and Chen hypothesized that plant oils that are more related to brassicas would have more diverse odors and would be more repellent. They thought that comparing the chemical structures of odors could have clues to predict repellency.
To test the theory, the researchers observed how female mosquitoes behaved when presented with broccoli plants that had been sprayed with each of the essential oils. They discovered that mosquitoes were less likely to deposit their eggs in broccoli plants that had been treated with essential oils, compared to untreated plants, and avoided flying towards certain oils more than others. In general, plant oils that were more distantly related to brassicas in the plant family tree were more likely to repel the layer. They also found that odors that were more chemically different were also more likely to be repellent. However, the oil that was more repellent, spearmint, actually had smells more similar to brassica cultivation.
"Biologically, it makes sense that mosquitoes are able to detect and avoid these plants because similar odors would make it easier for them to misinterpret these plants as hosts, which would be fatal to their offspring," Stratton said. "For swidge midge, garlic seems to be one of the most promising repellents, particularly because certified organic products that use garlic are already available to growers."
The study suggests a new sustainable solution for this new invasive pest and provides a new framework to test pest management strategies in other species.
The midge that eats more kale.
Scientific reports (2019). DOI: 10.1038 / s41598-019-47094-8
Garlic with broccoli: a smelly approach to repel an important pest (2019, July 23)
retrieved on July 23, 2019
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