Japanese beetles (Popillia japonica) are voracious feeders that can quickly destroy a plant. Adult beetles feed on the foliage and flowers of many plants, especially roses, tomatoes, beans, corn and fruit trees. The immature larvae (white grubs) feed on plant roots and organic matter in the soil, particularly under turf.
Adult Japanese beetles mate and deposit eggs in the soil several times a year, laying 40 to 60 eggs by mid-August. The eggs hatch in eight to 14 days and the white grubs overwinter in the soil. They emerge in the spring when the temperatures warm up and develop into beetles to repeat the cycle every year.
Japanese beetles are hard to control because they move from plant to plant, and from garden to garden. Chemical pesticides have to be applied every three or four days for effective control and alternative pesticides have to be applied more often.
Milky spore is a type of bacteria (Bacillus popillae) developed to control Japanese beetles in the larvae stage. Spores spread on the ground live in the soil and organic matter around the roots of plants. When the grubs eat the spores, the bacteria germinate in the grub's gut and infect the bloodstream and cells. As the spores reproduce, the grub turns milky white. When infected grubs die, millions of spores return to the soil. Milky spore needs to build up in the soil for two to three years for the best control and is more effective in the eastern part of the U.S.
Neem oil derived from the Neem tree contains two insecticidal compounds that affect Japanese Beetles--Azadirachtin and salannin--and some fungicidal compounds. Neem is an insect growth regulator that interferes with the normal life cycle of an insect; it does not quickly kill insects. It also acts as a feeding repellant. Neem is considered a botanical or organic insecticide and is not highly toxic to mammals.
Pyretheum is one of the most widely used and safest botanical insecticides in the U.S. It contains pyrethrins extracted from chrysanthemum plants, which penetrate the nervous systems of insects, including Japanese beetles, but are non-toxic to most mammals. Pyretheum knocks down most insects at low concentrations, but may not effectively kill all of them. It breaks down quickly especially in the sunlight so the effect typically lasts only one or two days.