Aromatherapy, essential oils, and such, have no medical benefit in humans, but they do work against some pests, if you don't want to use something better.

The process is simple enough for simpler creatures; insects are attracted to some smells and repelled by others. So use the repellent on plants you want to keep. 

Like garlic on broccoli.

A recent paper on how the similarity of plant odors and phylogenetic relatedness can predict insect repellency applied this conceptual framework to swede midge, a tiny fly that is becoming a major problem for Northeastern growers of broccoli, kale and other cabbage-family crops. They found that particular essential oils - garlic, spearmint, thyme, eucalyptus lemon and cinnamon bark - were most effective at repelling the midge. The findings may come as good news for "organic" farmers who are limited to old pesticides and so are without an effective solution for managing the pest.

A headless broccoli plant that has been impacted by swede midge, which can cause 100 percent crop losses. Since organic farmers limit themselves to old pesticides that won't work, many of them have given up on these kinds of food. Aromatherapy against insects won't be great but perhaps the higher profit margins will offset the losses. Image: Insect Agroegology and Evolution Lab / University of Vermont

Headless crops
Swede midge is a recent invader on vegetable farms in the Northeastern United States. Midge larvae must feed on the brassica plant family in order to survive, which includes many popular vegetables like broccoli, kale, cauliflower, cabbage, brussels sprouts, kohlrabi and collards. Making a mistake and laying eggs on the wrong plant would result in the death of the midge's offspring.

The larvae "hijack the plant's control system" resulting in distorted growth, such as headless broccoli and cauliflower, puckered leaves, and brown scarring. Unfortunately for farmers, the damage is not observable until it's too late and the midge have already dropped off the plant. In areas where the midge has become well established, including parts of Canada, New York, and Northern Vermont, the midge can cause 100 percent crop losses. 

To manage the midge, conventional growers use neonicotinoid insecticides, but organic farmers have simply stopped growing vulnerable brassica crops because neonics are not allowed under the organic manufacturing process.

A boutique solution for the organic process

The organic community are not strangers to essential oils and belief in aromatherapy and at least when it comes to plants and insects this can help, even if it will never allow them to be competitive with their science-based competition. 

To help them out, the group identified 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. Comparing the chemical structures of the odors might hold clues for predicting repellency, they thought.

To test the theory, the researchers observed how female midges behaved when presented with broccoli plants that had been sprayed with each of the essential oils. They found the midges were less likely to lay their eggs on broccoli plants that had been treated with essential oils, compared to the untreated plants, and avoided flying towards certain oils more than others. In general, oils from plants that were more distantly related from brassicas on the plant family tree were more likely to repel the midge. They also found that odors that were more chemically different were also more likely to be repellent. However, the oil that was most repellent - spearmint - actually had odors more similar to the brassica crop.