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.
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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. They don't bite or sting; but as a heads-up, their spiny feet may be offputting for some people. 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 spp.). 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.
Milky Spore and Nematodes
Two natural choices for controlling Japanese beetles are milky spore and nematodes. When applied to the soil, milky spore and nematodes prevent Japanese beetles from maturing to the adult stage by killing the larval stage (grubs) before they reach adulthood and mate. Milky spore is a bacterium (Paenibacillus popilliae), which kills grubs that ingest it. Most garden centers carry a powdered form of this bacterium 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 kills existing grubs and can remain dormant but viable in the soil for 15 to 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 beneficial 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 H. 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 beetle 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.
- The Old Farmer's Almanac: Japanese Beetles
- The Old Farmer's Almanac: Japanese Beetles: Best and Worst Plants
- University of Illinois Extension: Milky Spore Disease
- University of Minnesota Extension: Japanese Beetle Management in Minnesota
- University of Wisconsin: The Nematode Heterorhabditis Bacteriophora
- Buglogical Control Systems: Beneficial Nematodes - For Pest Insects
- U.S. Department of Agriculture: Plants Database
- Arbico Organics: Beneficial Nematodes - FAQ's