Imagine a world free of ravenous Japanese beetles who eat every plant in sight. If you lived in America prior to 1912, you wouldn't have to imagine this scenario. As their name suggests, Japanese beetles (Popillia japonica) are native to Japan. The water surrounding the island and the insect's natural predators kept them contained there until the fateful day that some Japanese beetles were accidentally shipped to the United States along with a plant. The insects thrived in America, where their natural enemies were few, and became a bothersome agricultural pest. If you want to return your garden to a beetle-free paradise, you can take steps to reclaim your land.
It's a gross prospect if you're not a fan of insects, but the most effective way of controlling small Japanese beetle populations is to pluck them off your plants by hand and drop them in a container of water with a few drops of dish soap in it to kill them. If you prefer to avoid touching the beetles, cover your plants with a dropcloth in the morning when the pests are most active. After they've gathered on it, shake them off into a large bucket of soapy water.
Pick Your Plants
When planning your garden, choose plants wisely to prevent beetles. Japanese beetles aren't picky eaters, but they do have a few favorite plants, including roses (Rosa spp.), crapemyrtles (Lagerstroemia spp.) and Japanese maple trees (Acer palmatum). Putting these plants in your garden is akin to rolling out a red carpet. Instead, plant things the beetles dislike, such as holly trees (Ilex spp.) and magnolias (Magnolia grandiflora). If you love the same plants the beetles do, consider planting repellents, such as parsley (Petroselinum crispum) and garlic (Allium sativum) near your favorites to keep the beetles at bay.
You may also want to try sprinkling geranium plants (Pelargonium spp.) throughout your garden. Geraniums are toxic to Japanese beetles, but their delicious flavor often overpowers the beetle's self-preservation instinct. It is common to see Japanese beetles both feeding on and laying under geranium plants. In some cases, the plant will kill the beetles for you. In others, the beetles will become sick and fall to the ground, where you can easily sweep them up in a dustpan and kill them yourself.
Though it's benign to people, soap kills insects. To harness soap's killing power, spray your plant leaves with a mixture that is 1 teaspoon of liquid dish soap, 1 cup of vegetable oil, 1 quart of water and 1 cup of rubbing alcohol. Shake the solution well and apply it to your plants every 10 days. Apply the spray only when it is cooler than 90 degrees Fahrenheit and on cloudy days. The mixture may damage your plants if applied during hot weather or in direct sunlight. Avoid accidentally harming honeybees by spraying at dawn or dusk when the bees are inactive.
Homemade insecticide also works as grub control. Late in the summer, Japanese beetle larvae head underground where they live and grow until June, when they emerge as adults. Killing the beetles in the grub stage reduces the number of them in the garden come early summer. It also saves your lawn, which the grubs feed on as they grow. To control grubs, dilute 2 tablespoons of dish soap in 1 gallon of water and spray your lawn. This recipe makes enough spray to cover 1,000 square feet. When the spray reaches them, the grubs will crawl to the surface of the soil where birds will make a tasty meal of them. Apply this homemade solution to your lawn once in the fall and again in the spring.
Milky Spore and Nematodes
Dish soap isn't the only thing that will kill Japanese beetle grubs. Two other natural choices are milky spore and nematodes. When applied to the soil, milky spore and nematodes prevent Japanese beetles by killing the grubs before they reach adulthood and mate. Milky spore is a bacteria (Paenibacillus popillae) that kills grubs that ingest it. Most garden centers carry a powdered form of the bacteria that you sprinkle on the lawn when temperatures are between 60 and 70 degrees Fahrenheit. After it makes its way into the soil, milky spore spreads through the grubs and stays viable for 20 years or more. It may, however, take three to five years for milky spore to establish itself in the soil and provide adequate control.
You can also saturate your soil with nematodes (Heterorhabditis bacteriophora) to kill beetle grubs. Nematodes are microscopic roundworms. These parasites burrow into grubs and infect them with bacteria. As those bacteria grow, the nematodes feed off of it and kill the grub. Nematodes of the Heterorhabditis bacteriophora species are naturally found in American soil. They are not considered invasive and have no negative impact on beneficial insect species. It is , however, necessary to add more of them in order effectively control Japanese beetle grubs. It takes 10 million nematodes to treat a garden of up to 3,200 square feet. Fifty million will treat a 1/2-acre lawn and 100 million will treat one acre.
Always read and follow the instructions when using milky spore or nematodes and verify with local authorities that you are allowed to release the species you intend to use. For best results, apply nematodes to the lawn at night and give them water often. In warm climates, nematode populations are sometimes able to sustain themselves. In colder areas, a fresh infusion of nematodes may be necessary every spring.
Japanese beetle traps are sometimes effective at reducing beetle populations but can actually end up attracting more beetles, so use them only as a last resort. Traps use the scent of geraniums or female hormones to attract and detain beetles so you can dispose of them. You can also make your own trap with a can of fruit cocktail. Open the fruit can and set it in the sun to ferment for a week. Set the fruit cocktail can in a bucket, and fill the bucket with water until the liquid sits just below the rim of the can. Beetles will come for the fruit and get trapped when they fall in the water. If it rains on your cocktail, simply start again with a fresh trap.
No matter what type of trap you choose, set it at least 25 feet away from the plants you wish to protect. The goal is to draw the beetles away from the plants using more enticing bait. Unfortunately, beetle traps can also cause disputes with neighbors, because placing a trap 25 feet away from your plants may mean placing it close to your neighbor's garden.