Besides fishing, there may be no hobby more steeped in folklore than gardening.
Place eggshells around your tomatoes to prevent blossom end rot. Plant corn on a new moon for bigger ears. Hang bars of soap on your fence posts to keep deer away.
If you've been gardening for any length of time, you've probably encountered advice like this. Some of it is based on generations of accumulated experience. Some of it is pure superstition. And sometimes it's difficult to tell the difference.
This seems especially true when it comes to companion plants and their supposed ability to deter insects. Garden books, blogs, and social media posts are full of recommendations, but supporting evidence is often hard to find. More than once, I've found myself wondering, "What is the original source of this information?" Follow the trail far enough, and it often leads right back to another gardening book repeating the same claim.
So I decided to investigate.
I dug into the scientific literature behind dozens of popular insect-deterring plants. While the evidence for many of the claims was surprisingly thin, a handful of plants stood out for having solid research supporting their effectiveness against specific pests.
In this article, we'll look at ten insect-deterring plants with some of the strongest scientific evidence behind them, focusing on those most relevant to North American gardeners.
1. Basil
For most gardeners and chefs, the sweet, spicy scent of basil is one of summer's great pleasures. For many insects, however, it's a warning sign.
Scientific studies have confirmed that basil possesses impressive insect-deterring properties. In one study, researchers found that gypsy moth larvae would rather starve than feed on leaves treated with basil essential oil. When the oil was separated into its individual compounds, feeding was reduced by as much as 94 percent.
And apparently some especially astute momma bugs recognize that a neighborhood with basil is no place to raise a family. Researchers have found that growing basil alongside tomatoes in close proximity to a basil plant reduced egg-laying in female leafminers by 50% or more (Kostic et al. 2008.)
Similar effects have been reported against mosquitos, tobacco cutworms, black cutworms, and even common pantry pests such as the Mediterranean flour moth and Indian meal moth. While basil won't eliminate insect problems entirely, the scientific evidence suggests it may be one of the more effective companion plants for discouraging a surprisingly wide range of pests.
2. Calendula
Also known as pot marigold, calendula has long been regarded as one of the most useful companion plants for pest control—and the research largely supports that reputation.
In one study, interplanting calendula with cabbage reduced diamondback moth larvae and pupae by an average of 88 percent across three growing seasons. Researchers observed similarly impressive results with aphids. In monocultured cabbage, aphid populations exploded to more than 400 aphids per plant by mid-season. When calendula was interplanted among the cabbages, populations peaked at just 50 aphids per plant, and 20 percent of the plants remained aphid-free through the end of the season.
Exactly how calendula achieves these results remains somewhat of a mystery. Researchers found similar numbers of beneficial parasitoid insects in both treatments, suggesting that increased parasitism was not responsible for the reduction in pests. One possibility is that calendula interferes with egg-laying or egg development. Supporting that idea, researchers have found that calendula extracts can dramatically reduce egg hatchability in several insect species. In one study, calendula extracts reduced tobacco cutworm egg hatch by as much as 98 percent.
Whatever the mechanism, calendula has some of the strongest scientific evidence of any companion plant on this list.
3. Nasturtium
Nasturtium has long been a favorite companion plant among gardeners. It's often planted as a trap crop for aphids, but research suggests its benefits may extend well beyond that role.
One of nasturtium's greatest strengths appears to be its ability to support beneficial insects. Researchers have found that nasturtium flowers provide food and habitat for important parasitoid wasps while offering little benefit to the pests those wasps attack. In one study, nasturtium selectively benefited the parasitoid Copidosoma koehleri without improving the survival or reproduction of its host, the potato tuber moth. Similar results have been observed with Trissolcus basalis, a parasitoid that attacks stink bugs, a common pest of beans, tomatoes, corn, and other garden crops.
Related Post: How to Grow Nasturtiums From Seed (And Why Less Is More)
Nasturtium may also help deter pests directly. In one study, companion plantings that included nasturtium reduced populations of striped and spotted cucumber beetles by as much as 50 percent over the course of the growing season.
While many companion planting claims rely heavily on tradition, nasturtium stands out as a plant with evidence supporting both the recruitment of beneficial insects and the suppression of important garden pests.
4. Thyme
Thyme oil is perhaps one of the most well-documented natural insecticidal extracts known to exist, so it is easy to understand why thymol, the major component of thyme oil, and its derivatives are often the basis for many commercial bioinsecticides. Thymol has been shown to cause greater than 90% mortality in larvae of the tobacco cutworm. Thymol and its derivatives have additionally shown insecticidal, larvicidal, or feeding inhibition in armyworms, Japanese beetles, cabbage loopers, aphids, thrips, and whiteflies. With such a wide range of pest-deterring abilities, thyme deserves a place in every garden. In fact, it probably deserves more than one place in every garden.
5. Peppermint
Peppermint has a long reputation as an insect-repelling plant, and the scientific evidence suggests there may be some truth to the claim.
In a study comparing 41 different essential oils, peppermint oil was one of the most effective at reducing the attraction of Japanese beetles to field traps. Traps treated with peppermint oil captured roughly 70 percent fewer beetles than untreated controls. Presumably, at least some of this effect is exhibited by the plant itself. Just be careful where you plant it—peppermint is famously invasive. A pot sunk into the ground near pole beans or other Japanese beetle favorites may be the safer option.
One of peppermint's most interesting defenses involves a compound known as (E)-β-farnesene. This chemical happens to be identical to an alarm pheromone produced by aphids when they are attacked by predators. When nearby aphids detect the signal, they stop feeding and move away. In other words, peppermint appears to mimic an aphid distress call, convincing would-be pests that danger is already present.
Finally, peppermint and spearmint have long been recognized for their ability to repel mosquitoes. While they won't eliminate every mosquito in the neighborhood, they may help make patios, porches, and outdoor seating areas a little more comfortable on summer evenings.
In a test of 41 different essential oils, peppermint oil was found to be one of the most effective treatments for reducing attraction of Japanese Beetles to field traps. Traps that were treated with peppermint oil had roughly 70% fewer beetles in the trap compared with controls. Presumably this effect would, at least to some extent, be exhibited by the peppermint plant itself. Just be cautious as peppermint is known for being an enthusiastic spreader. It might be worth a try to sink a pot of peppermint into the ground near the base of your pole beans, as these plants can be especially attractive to Japanese beetles.
| I'm Getting Mixed Signals Here |
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One of the mechanisms by which companion planting works is to confuse a pest's navigation system by interfering with the intricate biochemical interactions that occur between the pest and its host plant. Certain companion plants, such as marigolds (Tagetes spp.), release allelochemicals like limonene and alpha-terthienyl, which are known to disrupt insect olfactory receptors. These compounds interfere with the insects' ability to detect host plants, effectively confusing them and reducing pest damage. Additionally, studies have shown that companion plants like basil (Ocimum basilicum) emit volatile organic compounds (VOCs) such as estragole and linalool, which interfere with insect navigation and feeding behavior. By harnessing these chemical cues, gardeners gain not only a beautifully scented garden, but also a sophisticated defense mechanism to help protect their garden against destructive pests. |
6. Summer Savory
Although not among the most popular herbs for the kitchen garden, summer savory (Satureja hortensis) deserves a closer look for both its culinary quality and amazing insect deterring properties. In fact, while scouring the scientific literature for insect-deterring plants to include this article, time and time again I found summer savory tested against common pests, often alongside other plants, and time and time again it was found to be the most effective. For example, researchers in one study found that the essential oil of summer savory elicited a 100% mortality rate in adult broadbean weevils in just 36 hours after treatment. Similar effects have been observed for the cowpea beetle, tomato leafminer, cotton whitefly, maize weevil, and at least a dozen more common pests.
7. Rosemary
Are you catching a trend here? There seem to be a lot of aromatic herbs on this list, and the next insect-deterring plant is no exception. Rosemary is a fragrant, woody-stemmed plant that is native to the Mediterranean. Research has shown that interplanting rosemary with sweet pepper significantly reduces the prevalence of three common pepper pests: thrips, green peach aphids, and whiteflies. Furthermore, while the presence of rosemary was a deterrent to these costly pests, no adverse effects were observed on the populations of their natural enemies, indicating a selective effect that may further improve integrated pest management strategies.
Possibly most interesting to gardeners is the effectiveness of rosemary at deterring the diamondback cabbage moth. This garden nuisance is a constant challenge to gardeners, laying its eggs on cruciferous vegetables like broccoli, cabbage, and kale. Once emerged, the larva chew holes in the leaves and sometimes florets of these plants and are quite an unappetizing sight to the person responsible for cleaning and preparing the produce. The diamondback moth was one of the first pests to show resistance to pyrethrin and has since developed variable resistance to Bacillus thuringiensis (Bt), so its control can be difficult.
With all of these challenges mounting, any approach that can keep these pests at bay is welcomed. That's why I was so excited to discover from a 2022 study, that rosemary essential oils showed a high degree of effectiveness at killing larva and deterring oviposition (the laying of eggs) in adult diamondback moths. A 72% reduction of egg-laying was observed in this study. Similarly, another study found that including dried, crushed rosemary leaves in samples of dried beans reduced the egg-laying behavior of bean beetles by more than 87%.
8. Sage
Rosemary and sage don't just go together in Simon and Garfunkle songs. It turns out their essential oils are both effective at killing diamondback moth larva and inhibiting egg-laying behaviors in adults. What's more, neither of them had any detrimental effects on the parasitoid, Cotesia vestalis, a parasitic wasp that lays its eggs in diamondback moth larva. With the possibility of further synergistic effects by planting them together, sage definitely deserves consideration if you're already planning on growing rosemary near your Cole crops.
If you end up with a few extra seedlings and happen to have some space near your patio, sage and rosemary might also be worth a try there, especially for their ability to repel the bothersome blowfly. Blowflies are attracted to organic matter (like that found in compost piles), meat, and manure from pet droppings. In one study, the essential oils of sage and rosemary were both found to completely deter female blowflies from depositing their eggs for up to 24 hours after application. I'm wondering if this might be another reason why sage has for centuries been one of the most popular herbs for seasoning meat.
9. Onions and Garlic
Intercropping onions with cabbage is a commonly practiced technique for keeping pests at bay, and this is one companion planting strategy with plenty of scientific support. Researchers in one study found that intercropping cabbage with either onions or garlic significantly reduced pest populations, decreased leaf damage, and increased overall cabbage yields.
Interestingly, researchers found that onions were most effective when planted between cabbage rows rather than among the cabbages themselves. Cabbages are known to communicate with neighboring plants using airborne chemical signals when under attack. Could onions be disrupting those conversations? I don't know, but it's a fascinating possibility. (Read more about natural plant defenses here.)
These benefits have been confirmed in numerous other studies (Baidoo et al. 2016). Intercropping onions and garlic has also been effective at reducing pests in potato, strawberry, and mustard, to name a few.
Furthermore, scientists have identified a specific compound within garlic that seems to have especially potent insect-deterring properties. Allium sativum leaf agglutinin (ASAL) has been shown to have antifeedant and insecticidal effects against a variety of sap-sucking insects. Some researchers have even explored genetically engineering crop plants to produce this compound in order to improve resistance to common pests.
If you live in the South, you may appreciate that garlic provides protection against several common garden pests, including spiny bollworms, cotton bollworms, red cotton bugs, and tobacco cutworms. In one study, eggs of these species displayed slowed or arrested development when exposed to volatile compounds produced by garlic.
10. Marigold
No article on insect-deterring plants would be complete without mentioning marigolds. Long regarded as the workhorse of companion planting, marigolds (Tagetes spp.) are among the most thoroughly studied pest-control plants in the garden.
In fact, researchers have spent more than 80 years investigating their ability to suppress plant-parasitic nematodes, microscopic pests that attack the roots of many common vegetables. Because nematodes can be difficult to control once established, prevention remains one of the most effective management strategies available to home gardeners.
Scientists have proposed several mechanisms to explain marigold's effectiveness. Depending on the species and situation, marigolds may act as a trap crop, produce compounds that inhibit nematode development, or encourage beneficial organisms that attack nematodes. These effects may occur individually or in combination.
Research suggests that marigolds are most effective when grown as a cover crop and incorporated into the soil before planting vegetables, although interplanting can also provide benefits. More importantly, those benefits often translate into higher yields. In one study, planting marigolds before tomatoes increased yields by 50 percent, while melon yields increased by as much as 95 percent. Significant yield increases have also been reported in potato, carrots, and cowpeas.
Conclusion: The Best Pest-Deterring Plants for the Home Garden
Well, there you have it—ten companion plants with scientific evidence supporting their effectiveness against a variety of common garden pests.
Does that mean every companion planting claim you've ever heard is true? Probably not. But it does suggest that some of the gardening wisdom passed down through the generations may have more substance behind it than many people realize.
If you're looking for natural ways to protect your garden, I hope this article has given you a few new ideas to try. And if you've been battling a particularly troublesome pest, perhaps you've found the next tool to add to your gardening arsenal.
As with everything in gardening, experimentation is key. Every garden is different, and the only way to know what works in your particular conditions is to give it a try. Fortunately, you don't have to start from scratch. Thanks to generations of gardeners—and a fair amount of scientific research—we have a few good clues about where to begin.
