A sign falsely suggesting that plants will keep certain insects out of the garden.
Jackie Jordan, ©2024, Clemson Extension
Do Certain Plants Repel Mosquitoes?
Basil, lavender, rosemary, peppermint, marigolds and catnip are all aromatic plants, but is their scent strong enough to keep mosquitoes at bay? Citronella plants, scented geraniums and lemongrass plants are also commonly touted as mosquito-repelling plants. Unfortunately, there is more to the story. A meta-analysis in the Malaria Journal looked at 62 scientific studies examining the effectiveness of plant essential oils and their ability to repel Anopheles species. Only 6 studies were performed in the field, and the other 56 were completed in laboratories. None of the studies examined landscape plantings and their ability to repel mosquitoes. Unfortunately, the fragrance emitted by the plants dissipates into the air and can be minimized with a breeze. To be effective, mosquito repellent, the plant’s leaves must be crushed, or the essential oils must be rubbed directly onto skin. CAUTIONS: Some essential oils can cause irritation, especially in high concentrations. Studies have also looked at the effectiveness of plant essential oils and the length of time they can repel mosquitoes. Rosemary oil protected test subjects for about 15 minutes, and lemongrass oil lasted about 40 minutes. The best way to control mosquitoes is through source reduction and chemical control. For more information, see HGIC 2435, Mosquito Control In and Around the Home.
Can Certain Plants Repel Other Insects?
How Can Mixed Plantings (Polyculture) Disrupt Pests?
Many plants produce volatile organic compounds or scents that can attract insects. However, insects find their preferred host plants through a combination of sensory clues. Visual clues such as the color and shape of plants and flowers are used in combination with scent and taste to confirm an insect’s preference for a particular plant. Claims that certain plants will mask the scent of crops to keep pests away haven’t held up to scientific scrutiny. A research study looked at Brussels sprouts interplanted with sage and thyme to determine if the scent of the herbs could disrupt diamondback moth egg laying and found that disruption only happened if the moth landed on the leaves of the herbs.
The practice of mixed plantings can disrupt certain insect pests. Insects land on plants to determine if they are host plants. In addition to planting non-host plants, research has shown that placing green sheets of paper around crop plants also disrupts insect pests. The more non-host plants around a preferred host plant, the more time an insect’s energy is diverted from finding a host plant. The practice of polyculture, mixing different plant types, helps to limit pests by spacing out vulnerable plants and making it more challenging for insects to find their preferred host plants.
How Can I Attract Beneficial Predatory Insects?
Luckily, many beneficial predatory insects also feed on pollen and nectar. Incorporating a selection of herbs, annuals, and perennials that offer blooms throughout the growing season will help to keep the problematic insect population in check by supporting populations of predatory and parasitic insects.
For a list of predatory insects and the pests they eat, see HGIC 1721, Incorporating Beneficials Into the Gardener’s Toolkit. For recommendations on plants that attract beneficial insects to the landscape, see HGIC Blog: Attracting Beneficial Insects.
