Native to Japan, the first Japanese beetles were introduced into the United States in 1916 (in New Jersey). Although the introduction was accidental, the beetles found an excellent climate to thrive. From the North to the South and then westward, the beetles continued their spread. Today, they may be found throughout the U.S. These pests can do great damage to vegetable gardens.
These beetles begin life as white grubs in the soil. Adult females burrow into the soil and deposit a total of 40 to 60 eggs over their 30- to 45-day lifespan. The eggs hatch into grubs, which live in the soil for the next 10 months. They feed on grass roots and vegetable seedlings. The adults will emerge from the soil and start feeding on plants around June each year. For four to six weeks in June and July, the adult beetles wreak havoc on 300 different varieties of plants, as well as rotting or damaged fruits.
The grub stage of this beetle also feeds on your lawn and garden. Soil insecticide is useful against them. Using an organic insecticide will cause the least environmental damage. Eliminating grubs from your soil will not necessarily get rid of the adults, since they may fly in from other areas.
There are numerous broad-spectrum insecticides used to kill Japanese beetles. However, these insecticides may be harmful to the environment; and are especially dangerous in a vegetable garden, since they also kill beneficial insects and pollinators and may contaminate the soil and vegetables.
Botanical solutions include Neem products, such as Azatrol or Neem-Away (Gardens Alive). Another botanical solution is Pyola (which contains pyrethrins in canola oil). These keep the beetles away for three to four days at a time per application. According to University of Kentucky Extension, the following home remedies are not effective: insecticidal soap, garlic sprays, hot pepper solutions, orange peels and companion planting. Bt (Bacillus thuringiensis) is widely used as an organic pesticide / insecticide.
While pheromone beetle traps have been shown to work in attracting and trapping adult beetles, they may have the unintended effect of luring more beetles to your garden than otherwise would have found you. If you do use a lure, place it far away from your vegetable garden, orchard or rose bushes.
In areas where there is a high infestation of Japanese beetles, you may consider not planting the plants this pest prefers. These include flowering shrubs like Althea, roses and a number of trees. In vegetable gardens, the beetles particularly like green bean leaves, peach tree leaves, blackberry bushes and grape vines. Covering these plants with bird netting or fabric during peak beetle season may be helpful.