Many specialist plant-eating insects use visual and volatile cues, essentially their senses of sight and smell, to find host plants. The practice of companion planting by mixing non-host plants among hosts can repel these pests. Non-host odours can also repel the insects.
Researchers looked at the effect of various essential oils on the Swede midge (Contarinia nasturtii), a serious pest on brassica crops, particularly broccoli and cauliflower, in Ontario and Quebec.
Stratton et al (2019) found that applications of certain essential oils, such as lemongrass, cinnamon, and oregano, had repellent properties. Adult midges were less likely to land on plants sprayed with these oils, and consequently the plants had fewer larvae than plants sprayed with water (the control) or the essential oils of caraway, coriander or niaouli. The smell of certain oils appeared to stress the midges (apparently stress is indicated when “adults rolled on their backs and shook their legs”).
A potential complication is that the essential oils of thyme, star anise and oregano caused severe damage to the plant (caraway, coriander, and cinnamon bark had only a mild effect).
The effect of essential oils on pests appears to vary between pests. The scientists conclude “Plant essential oils are an important tool to consider for sustainable pest management, for which we have barely scratched (and sniffed) the potential.”
Source: Chase A. Stratton, Elisabeth Hodgdon, Cesar Rodriguez-Saona, Anthony M. Shelton, Yolanda H. Chen. Odors from phylogenetically-distant plants to Brassicaceae repel an herbivorous Brassica specialist. Scientific Reports, 2019; 9 (1) DOI: 10.1038/s41598-019-47094-8
— Janet Wallace