The Japanese beetle is a species of scarab beetle that’s native to Japan. Easily recognizable, the beetles have a metallic green head and coppery-brown wings. They are also known as garden pests that feed on and damage common leaves, flowers, and fruits. Japanese beetles are especially bothersome in the U.S. where they are considered an invasive species due to a lack of natural predators.
Life Cycle
Japanese beetles have several distinctive life stages. In the summer, female beetles repeatedly burrow into the ground to lay their eggs. They travel back and forth between feeding on plants and laying eggs in the soil until they have laid 40-60 eggs. When the eggs hatch, they enter into the larval stage, when they become white, C-shaped grubs. The grubs feed on organic material in the soil such as roots. By the fall, the grubs burrow deeper into the soil to hibernate for the winter. When the temperatures get warmer, the grubs enter their next stage, pupae. Finally, they undergo metamorphosis and adult beetles emerge from the ground during the late spring and summer, ready to continue the life cycle again.
Japanese Beetle Immigration
Japanese beetles were first discovered in the U.S. in 1916 at a plant nursery in New Jersey. They are thought to have been brought over accidentally in a shipment of iris bulbs sometime before 1912, when inspections on imported goods started. Since then, the population of Japanese beetles has skyrocketed in the U.S, where they have become menaces of gardens nationwide. Later in the 20th century, Japanese beetles also appeared in Canada and European countries such as Portugal, Italy, and Switzerland.
Natural Predators
In their native country of Japan, Japanese beetles have a diverse and well established range of natural predators such as spiders, birds, assassin bugs, and predatory stink bugs. While a majority of these species do live in the U.S., the ecosystem here hasn’t evolved in a way that keeps the population of Japanese beetles in check. This lack of natural checks and balances is why the beetles are considered an invasive species.
One such natural predator is the winsome fly (Istocheta aldrichi), a species of Tachinid fly. These flies are parasitic, laying their eggs on Japanese beetles. When the eggs hatch, the fly larvae burrow into the host beetle’s body, eventually killing the beetle. These flies are an important biological control because they constrain the population of many garden pests, including the Japanese beetle.
Japanese Beetles as Pests
As grubs, the beetles feed on roots in the ground, often grass roots. This damage to the roots of the grass can cause patches of dead grass, as the roots are needed to take in water and nutrients. If your lawn has brown spots of dead grass, you may have a Japanese beetle larvae infestation!
As adults, the beetles feed on leaves, fruits, and flowers on many different common plants. Some examples of their favorite plants include:
- Apples
- Cherries
- Peaches
- Plums
- Raspberries
- Roses
- Birches
- Grape vines
Beetles tend to skeletonize leaves by eating the tissue between the leaf veins while leaving the veins intact. While this feeding may cause damaged leaves to fall off, most healthy mature plants can survive skeletonization. However, an infestation of Japanese beetles will reduce plant yield and negatively affect the appearance of your garden!
Controlling Japanese Beetles
Starting in 1927, researchers in the U.S. started introducing insects that are natural predators of the Japanese beetle in an effort to minimize their population and destructive effects. The insects that were most successful were the winsome fly and a parasitoid wasp called the Tiphia vernalis. The experiments weren’t immediately successful in New Jersey, where the winsome fly’s skills were tested, because of the fly and beetles’ differing life cycle schedules. However, the flies became very effective predators in the cooler New England weather where they eventually migrated to. As populations of these flies and other natural predators of the beetles grow here in the U.S., the Japanese beetle invasion may eventually be eased.
What You Can Do
If your yard is prone to Japanese beetles, they will start appearing in late June and early July. You should start pest control management as soon as they start appearing because damaged plant leaves will chemically attract more beetles. It’s important to get the beetle population under control quickly. Here are some ways you can manage Japanese beetles:
- Physical removal: If there is only a small amount of beetles or if you have a small garden or yard, physically removing beetles from your plants will be easier. Once you pick the beetles off the plants, place them in a bucket of soapy water. If you choose this method, you should check your plants every day for beetles. They will be easier to remove in the evening or morning when it’s cooler out.
- Netting: A fine mesh netting can help protect your plants by acting as a physical barrier. Netting is most effective as a preventative measure, so if you are anticipating an infestation, place your netting before late June.
- Traps: Japanese beetle traps are usually not recommended, as they chemically attract beetles into the traps but also attract more beetles to your yard.
- Plants: Certain plants will attract or repel Japanese beetles. Aromatic herbs like catnip, chives, and garlic, and flowers like marigolds and chrysanthemums known to deter the beetles. Planting some of these may keep the beetles away from your prized plants. On the other hand, planting trap plants, which will attract Japanese beetles, can be used to draw beetles away from other plants so they can be collected and killed. Examples of trap plants are borage, grape vines, zinnias, and evening primroses.
- Natural Predators: For grub control, you can apply milky spores to your lawn. This bacteria will be ingested by the grubs when they are burrowed in the ground, giving them a fatal disease. Another organism that kills grubs is the parasitic insect-eating nematode which infects grubs with bacteria. The nematode heterorhabditis bacteriophora is commercially available. Introducing this organism to your lawn can target not only Japanese beetles, but other pests such as ants and ticks.
Reach Out to Professionals
If you are overwhelmed by a Japanese beetle infestation, reach out to a professional pest control service. They will be able to help you control the beetles and prevent them from returning to your yard.