The Japanese beetle (Popillia japonica) is an iridescent, plant-eating insect. A member of the Scarabaeidae family of scarab beetles, it is notorious for being a serious pest in both ornamental gardens and cash crop farms. In Japan, where it is considered a native insect, it is not known for being particularly destructive. In the US and Europe, however, it has proven to be an invasive bug.
Definitely small but terrible, the Japanese beetle can feed on hundreds of species of valuable plants, many of which become the hosts of its larval and adult stages. Its adults measure just 0.6 inches (1.5 cm) long and are distinguished by their copper-colored forewings and green heads. Its larval stage, in comparison, can grow to about an inch (2.5 cm) long. These mature in subterranean environments, where they feed on the roots of grasses.
Adult Japanese beetles tend to be most active during the warm months of the year. During this period, a large colony can quickly ravage miles of planted landscapes. Able to fly over considerable distances, they can move from one place to another as they skeletonize crops like beans, corn, strawberries, tomatoes, roses, and more.
Japanese beetles may be controlled by physical or chemical means, though these may be time-consuming, costly, and harmful to the environment. The most environmentally friendly means of managing their populations involves growing the right set of resistant or repellent plants. Note, however, that there’s no guarantee that these species will be effective against large, recurring colonies of pest insects.
1) Common rue (Ruta graveolens)
Also known as the herb-of-grace, common rue is widely touted as one of the most effective insect-repellent plants. It makes for a fine companion plant for crops that are frequently damaged by flying pests. Its scent, which is given off by the essential oils of its leaves and blooms, can mask that of nearby, vulnerable herbs. The aroma given off by the plant is strengthened when its cells are damaged. This means that regularly harvesting or bruising the leaves may help control beetle populations.
Also cultivated as a culinary herb, common rue is characterized by deeply lobed, blue-green foliage. In summer, it produces inflorescences of yellow blooms. These attract many beneficial insects, some of which are likely to parasitize both adult and larval stages of the Japanese beetle. Apart from the caterpillars of some butterfly species, insects are generally unlikely to favor the bitter taste of rue leaves.
Though common rue can function as a natural repellent, to an extent, it is unlikely to be effective against established beetle populations on its own. Make sure to grow it alongside other species that are unattractive to these pests.
2) Arborvitae (Thuja occidentalis)
Arborvitae is not necessarily an insect-repellent plant, but it does help limit the spread of Japanese beetles because it is not appealing to them as a source of nutrients. Beetles are less likely to perceive an arborvitae-rich area as an ideal feeding ground. Given a moderately wide stand of this species, ideally in medium to large-sized gardens, hungry insects may be repelled due to the coverage of a largely unpalatable canopy.
Often grown as an ornamental plant, this “tree-of-life” can measure as much as 50 feet (15 meters) tall. Its foliage is made up of complex leaflets with scale-like components. These evergreen leaves are tough to the touch and are rarely consumed by leaf-eating insects. Moreover, they contain essential oil with tropolones. Some of these phytochemicals have insect-repellent properties.
In its native range, arborvitae is most abundant in forested wetlands. It serves as a vital component in some coniferous swamps, where its roots exhibit an impressive tolerance for wet soils. Long-lived, they can contribute a wealth of environmental benefits to both residential and commercial properties with desirable conditions.
3) Garlic (Allium sativum)
Cultivated around the world as a culinary ingredient and as a source of its multi-purpose essential oils, garlic is one of the most important crops of the modern age. This flowering plant is distinguished by its underground bulb, which is packed with strongly flavored and aromatic phytochemicals. When the cells of garlic bulbs are damaged, their sulfuric compounds undergo a chemical process that produces a sharp, hot, and often overpowering taste.
The leaves of garlic likewise contain oils that emit a strong scent. Adult Japanese beetles are unlikely to graze on them unless they are the sole source of nutrients for miles around. Situating them around other susceptible crops may discourage the beetles from coming too close and laying their eggs in the surrounding substrates.
Alternatively, you may also make an organic beetle-repellent spray by mixing crushed garlic with water and other insecticidal oils. Spray these on the foliage and around the crowns of nutritious and vulnerable plants. The scent given off by the spray should deter both bugs and larger grazers from coming too close.
4) Catnip (Nepeta cataria)
Known for its curious ability to attract and excite felines, catnip is a fantastic herb for just about any type of garden. This flowering perennial is a member of the Lamiaceae family, so it has all of the wonderful properties of fragrant mint plants. One of its most beneficial uses in both ornamental and commercial gardens is its capacity to repel a wealth of pests. The mechanism behind its pest-repellent activity lies in the formulation of its essential oils.
Catnip oil contains a phytochemical called nepetalactone. When isolated from its oil and tested against mosquitoes and cockroaches, this compound has been shown to rival the effects of DEET (an industrial-grade repellent). A substantial dosage can severely affect the nervous system of insects such as aphids, squash bugs, and flies that frequently attack vegetables. It’s highly unlikely that Japanese beetles have defenses against this compound.
To maximize the deterrent effects of catnip, grow it as a companion plant around susceptible crops. You may also cultivate rows of this herb around flowering ornamentals. Apart from producing unappetizing foliage, its blooms should draw in pollinators and parasitoid wasps. These beneficial insects should help keep beetle populations at bay by either consuming them or producing parasitic larvae.
5) Zonal geranium (Pelargonium zonale)
Zonal geraniums don’t exactly repel Japanese beetles in the typical sense, as their presence may actually draw them into ornamental gardens. Their capacity to control these pests lies in the toxic compounds found in their blooms. Instead of managing beetle populations by creating a smell that repels them, their petals have properties that attract them. Researchers have found that in as little as 30 minutes after consuming geranium petals, Japanese beetles can become paralyzed.
The toxins in geranium petals are not necessarily able to kill Japanese beetles. However, the beetles are as good as dead when they are paralyzed in outdoor locations. Unable to move, they become easy prey for larger insects, birds, insectivorous mammals, and reptiles. In a sense, this highlights the capacity of geraniums to serve as a trap crop instead of a repellent.
If you experience the occasional summer visit from hungry Japanese beetles, yet wish to continue cultivating some of their favorite host plants, growing geraniums should be a great way to minimize their spread. Place a few well-established rows of these flowering plants around plots of susceptible species. Note that their blooms would need to be present for them to be effective as trap crops.
6) Common boxwood (Buxus sempervirens)
One of the most popular and versatile plants for landscaped gardens, common boxwood can be transformed into a natural fence, a well-rounded shrub, or a whimsically-shaped accent. It doesn’t appeal to plant-eating insects like Japanese beetles because of its tough shoots and its poorly-tasting foliage. In the presence of other tender host plants, beetles will likely avoid even the most well-established boxwood stands.
To discourage Japanese beetles from laying waste to your garden, you can grow boxwood hedges around susceptible, low-growing ornamentals. It isn’t a foolproof solution to their infestations, but it may definitely dissuade them from making your property their home. Able to grow to an impressive height of 30 feet (9 meters) tall, boxwood can serve as a screen against all sorts of animals. It is also resistant to grazing from the most persistent pests as its shoots are able to recover fairly quickly.
Is the Four O’Clock Flower (Mirabilis jalapa) Toxic to Japanese Beetles?
The four o’clock flower is often brought up by members of the horticultural community as a form of defense against Japanese beetles. This ornamental plant, which is native to the tropical regions of the Americas, contains phytochemicals with a wealth of medicinal and antimicrobial properties. However, there has yet to be a study that proves its effectiveness against plant-eating insects like Japanese beetles.
Rumors that claim this species is able to kill Japanese beetles do not appear to be sourced from actual evidence. While beetles may avoid consuming this plant in outdoor gardens, they can feed on it without fatal consequences in a laboratory setting. While the plant does produce oils that may cause irritations upon contact and unpleasant symptoms after ingestion (by humans), it does not appear to have any significant effects on beetles.
Plant-Derived Oils for Repelling Japanese Beetles
If Japanese beetles continue to plague your garden despite the presence of insect-resistant and repellent plants, consider using plant-derived essential oils. These tend to have a more targeted, direct, and stronger effect on beetles. You may dilute them with water or incorporate them into an organic, homemade spray made for controlling a broader range of pests and pathogens.
You may plant the species from which these oils are sourced, but they may not be as effective as repellents in their intact forms. Below are some plant oils to consider adding to your arsenal of eco-friendly pesticides:
These can be sprayed onto the foliage and flowers of susceptible plants. For large-scale crop farms, using these essential oils may be an extremely costly means of pest control. Consult your local agricultural unit for more advice on how to manage Japanese beetle infestations in densely-planted vegetable gardens, orchards, and farms.
