Japanese beetles are a destructive pest that can cause significant damage to a wide range of plants in the garden. Native to Japan, these beetles were first discovered in the United States in 1916 and have since spread across the country, becoming a major problem for gardeners and farmers. Japanese beetles are most active in the summer months, feeding on the leaves, flowers, and fruits of plants, leaving behind skeletonized leaves, damaged fruit, and sticky excrement (frass). In addition to feeding damage, Japanese beetles also attract other beetles to the area, leading to further damage to plants.
The larvae of Japanese beetles, known as grubs, live in the soil and feed on plant roots. This feeding can weaken the plants and make them more susceptible to disease and other stressors. In addition, adult beetles often feed in groups, causing widespread damage to a single plant or multiple plants in the same area. If left unchecked, Japanese beetle infestations can cause significant defoliation, reducing the overall health and appearance of the garden.
Symptoms
It is important to monitor for symptoms of Japanese beetle infestations and take action as soon as possible to prevent widespread damage to your garden.
- Leaves and flowers of affected plants are partially or completely skeletonized, with only the main veins remaining
- Irregular holes in leaves, flowers, and fruits with a lace-like appearance
- Damaged or misshapen fruit, which may drop from the plant prematurely
- Heavy defoliation of affected plants, leading to bare branches and a reduced overall appearance
- A presence of adult beetles or larvae in the soil, including the emergence of adult beetles from the soil in late spring and early summer
- Sticky, sugary excrement (frass) on leaves and fruits of affected plants
- Attracted beetles may also feed on nearby healthy plants, causing new damage.
What is a Japanese Beetle
- Adult beetles are about 1/2 inch long and metallic green in color with a metallic sheen
- They have coppery-brown wing covers that overlap at the center of their backs, creating a V-shape when viewed from above
- Along the sides and underbelly of the wing covers, there is a row of white tufts of hair
- Their legs are short and brown with white joints
- The head of a Japanese beetle is small and dark in color, with large antennae that are feathered and easily noticeable
- Larvae are C-shaped, white grubs with brown heads and legs that live in the soil and feed on plant roots. They are about 1 inch long when fully grown.
Treating a Japanese Beetle Infestation
It may be necessary to use multiple methods to effectively control Japanese beetle infestations in a garden. For example, using physical controls, such as row covers and traps, in combination with cultural controls, such as plant selection and maintenance, and chemical controls, such as insecticides, can help reduce the overall damage caused by these pests and protect the health of the garden. It is important to monitor the infestation closely and adjust the control methods as needed to achieve the desired results.
Physical Control Methods
- Hand picking: This involves physically removing adult beetles from plants and dropping them into soapy water to kill them. This method is most effective when the infestation is small and localized. To hand pick beetles, use gloves or a tool to grasp the beetles and drop them into a container filled with soapy water. Repeat the process regularly throughout the day to reduce the number of beetles in the garden.
- Row covers: Using row covers made of lightweight, permeable material can help prevent adult beetles from reaching the plants. The covers should be placed over the plants before the beetles emerge in the spring and removed once the adult beetles have disappeared in the fall. To use row covers, simply drape the covers over the plants and secure the edges to the ground with stakes or weights. Make sure the covers are tight against the plants to prevent beetles from getting inside.
- Traps: Japanese beetle traps use pheromones and floral scents to attract adult beetles, trapping them inside. This method can be effective in reducing the number of beetles in the area, but it may also attract more beetles to the garden, causing additional damage. To use traps, place them in the garden according to the manufacturer’s instructions, making sure they are positioned away from the plants you want to protect. Empty the traps regularly and dispose of the trapped beetles.
Cultural Control Methods
- Plant selection: Choosing plants that are less attractive to Japanese beetles, such as plants in the onion family or plants with strong fragrances, can help reduce the damage caused by these pests. When selecting plants, consider their susceptibility to Japanese beetles and choose plants that are less likely to be damaged.
- Plant maintenance: Maintaining healthy, vigorous plants through proper watering, fertilization, and pruning can help make them less attractive to Japanese beetles and more resistant to damage. Regular watering and fertilization can help promote strong growth, while pruning can help remove damaged or infested parts of the plant.
- Crop rotation: Rotating crops in the garden can help disrupt the life cycle of Japanese beetles and reduce the overall damage caused by these pests. By rotating crops, you can reduce the number of beetles that are able to complete their life cycle and reduce the overall population of these pests in the garden.
Chemical Control Methods
- Insecticides: There are several types of insecticides available for controlling Japanese beetles, including contact insecticides, systemic insecticides, and insect growth regulators. Contact insecticides work by killing the beetles on contact, while systemic insecticides are absorbed by the plant and work from the inside out. Insect growth regulators disrupt the development of the beetles, preventing them from reaching maturity. It is important to follow label instructions carefully and apply the insecticide at the appropriate time to maximize its effectiveness. When applying insecticides, make sure to cover the entire plant, including the tops and undersides of leaves, to ensure maximum coverage.
- Neem oil: Neem oil is a natural, plant-based insecticide that can be used to control Japanese beetles. It is most effective when applied to the foliage of affected plants. To use neem oil, mix it with water according to the manufacturer’s instructions and spray it directly onto the foliage of the affected plants. Repeat the treatment every 7 to 10 days, as needed.
Biological Control Methods
- Natural predators: Birds, such as starlings, and other insects, such as parasitic wasps, feed on Japanese beetles and their larvae, helping to reduce the population of these pests in the garden.
- Microbes: Certain microbes, such as Bacillus popilliae, can be used to control Japanese beetles by infecting and killing the larvae in the soil.