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There are two classes of insect repellents: synthetic, and natural. For example, DEET, the main ingredient in insect repellent, is synthetic, while plant oils, like eucalyptus oil, are natural insect pest repellents. DEET is useful for repelling several insect pests, such as mosquitoes, gnats, flies and ticks, but each plant oil targets only one or a small number of insect pests. Many people, especially gardeners and landscapers, have probably heard that certain ornamental flowers and herbs have certain properties that can repel annoying airborne insect pests, including mosquitoes, no-see-ums, black flies and gnats. There exists some controversy regarding the effectiveness of natural plant oils as insect repellents, but a large body of research has shown that certain flowers and herbs produce volatile compounds that repel insect pests. However, this does not necessarily mean that planting insect repelling flowers and herbs in a garden will work to repel insect pests.

The mere presence of flowers and herbs that produce compounds that are toxic to insects will not work to keep a property free of insects; instead, the volatile compounds must be extracted, or somehow dispersed from the plants in order to see results. Otherwise, the compounds will remain within the plant. According to Walter Leal, a biochemist and professor at the University of California, plant-derived chemicals work to repel insects, but only for a short time. The most well known plant-based repellents include oil of lemon eucalyptus and oil of citronella. Unlike plant-based repellents, DEET is a chemical of low volatility, making it last longer, and therefore, does not need to be applied repeatedly in order for it to be effective. Gary Bachman, a horticulturist working at the Mississippi University Extension Office claims that lemongrass, lemon balm, bee balm, and lemon thyme are all scientifically proven to deter mosquitoes, and rosemary, lavender, basil, mint, citronella, and catnip work to repel a variety of insect pests. Evidence also suggests that ornamental flowers like marigolds, alliums, chrysanthemums, petunias, and geraniums will repel insect pets, but not if they are just sitting in a pot on a porch or within a home. In order to exploit the repellent compounds in these plants, the oil must be spread on the skin by rubbing the plants on the body, but these oils will cause skin irritation, so this is not recommended. Ironically, synthetic insect repellents are less harmful to the skin than natural repellents. Bachman states that plant-based repellents are not nearly as effective or convenient as DEET-based insect repellents.

Have you ever purchased flowering plants with the hope that they would repel insects?