Controlling Japanese Beetles

Lisha Utt
Frederick County Master Gardener Program

Japanese beetles are a major pest in Maryland with adult beetles feeding on over 275 different species of plants and grubs (larvae) that feed on the roots of turfgrass such as Kentucky blue grass, fine fescues, rye grasses or bentgrasses.

Japanese beetles usually start to emerge in mid June and peak in early July. They remain active for 4-6 weeks and lay most of their eggs in July. In about 10 days the eggs hatch and the grubs begin feeding on the turf roots. The grubs grow to be about one inch long and are cream colored with brown heads. When grub feeding is severe turf can be rolled back like a rug. The grubs move down into the soil 6-18" by mid-October and are inactive until spring.

Below are some control options you may want to consider:

  • Hand pick beetles and drop them into soapy water. While effective, this option becomes impractical with extremely high populations or when beetles are feeding in difficult to reach spots such as the top of your cherry tree.
     
  • Plant non-favored host plants. A good idea for future plantings but not helpful for the "favored" plants you already have in your garden.
     
  • Use Japanese beetle traps. While these traps can capture large numbers of adult beetles, they usually do not provide effective control. Typically a single trap can attract beetles from the entire neighborhood, concentrating the damage in the area close to the trap. Research has demonstrated a 31-40% increase in adjacent plant damage when traps were used close to susceptible plant species (Home and Garden Mimeo #78).
     
  • Use a currently registered insecticides such as insecticidal soap/pyrethrin blends. Be aware that even the best insecticide will only be toxic to the beetles for a few days and will not protect any new growth. Two to three spray applications may be required. Be sure to read and follow all label directions.
     
  • A final option is to control the grubs in the ground. This won't help you now but can be effective in controlling the number of Japanese beetles you will face next season. In early August inspect your lawn for grubs. By then most of the grubs will have hatched and be " to " long. Chemical control should only be considered if you have more than 6 to 8 grubs per square foot. Use lawn insecticides that contain either imidacloprid or halofenozide as the active ingredient. Again follow all label directions for application rates, timing and safety.

So there are several options for you to try. You may find that a combination works for you. Picking off the beetles and putting them in soapy water may be all you need to do for your geraniums and basil, while spraying your cherry tree may be required. But don't despair, many plants will tolerate moderate defoliation and fill back in once the Japanese beetles have gone for the summer. Good-luck!!

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