Out of their habitat, Japanese beetles can be insidious garden pests; leave them unchecked and they will munch roses and precious flowers to lace by summer's end. Commercial traps kill beetles, but there are also common things around the home that, when combined, become effective at killing beetles. As a plus, these methods are inexpensive, natural and less harmful to the environment.
Handpicking is very effective, if time-consuming. Begin handpicking in June when Japanese beetle scouts appear. Fill a bucket, bowl or pan with soapy water — a good ratio is 1/4 cup dish soap to 1 part water. Pick off the beetles and drop them into the bucket, or shake them into it. Leave the bucket out in the area where attacks occur so beetles will catch the scent of their dead peers; the scent of remains will warn them your garden is a bad feeding ground. The soap also prevents mosquitoes from laying their eggs in the water. This method works best for a few affected plants. Go further by grinding some of the dead bugs and soapy water in a blender. Pour into a spray bottle and coat plants with the bug mixture.
Hot Pepper Garlic Spray
To make an all-purpose homemade Japanese beetle killer, combine 2 tbsp. hot sauce or 2 coarsely chopped hot peppers and 2 cloves coarsely chopped garlic into a blender. Pour into 1 pint water. Put on the lid and blend for one to two minutes, until pureed. Strain, then add 1 drop dish soap and 1 drop cooking or mineral oil. Pour the mixture into a clean spray bottle and let it stand overnight to strengthen the potency. Spray the mixture directly onto the affected plants early morning. Wear gloves to protect your skin. Keep in a cool, dry place.
Milk Jug Banana Trap
Into an empty 1-gallon milk container, pour in 1/4 cup hot water with 1/4 cup sugar. Shake well to combine and dissolve the sugar. Uncap the milk jug and drop in one half of a smashed ripe banana. Shake well to mix. Cover half of the jug's opening with tape. Place the trap away from the affected plants but still in the area of the garden. The beetles will be attracted to the scent of fermenting, rotting banana. They can easily fly into the trap but will be unable to escape. Remake and replace the trap as often necessary.
With a heavy-duty type vacuum, vacuum the affected plants. Discard beetles into a plastic trash bag, tightly sealed, and throw away.