Neem oil, pressed from the seeds of the neem tree, is an excellent resource for killing an adult Japanese beetle. Japanese beetle larvae are hatched in the upper surface of the soil of a lawn and spend the first year of their life underground. To prevent larvae, you have to kill the adult beetles.
TIP: Our expert gardening advisor, Susan Patterson adds, "Neem oil also helps to deter spider mites and aphids from garden plants."
Step 1 - Make a Neem Oil Mixture
You will need a gallon of water, an ounce of neem oil, and a teaspoon of mild liquid soap. Mix the soap and water and then add the neem oil slowly while stirring constantly. A hand spray bottle is ideal for applying the mixture to the plants.
Step 2 - Spray Your Plants
The best times to spray are early morning and early evening. Shake the spray well before using. Make sure all of the plant's foliage is covered. Saturate the soil under the plants as well.
TIP: Susan cautions, "Do not spray neem oil directly on plant blossoms."
Step 3 - Repeat as Required
The spray needs to be used once a week. You should also re-spray after rain. The spray will not kill the insects (unless you manage to spray them directly), but it will make the plants smell and taste unappetizing. If the insects do ingest the mixture, it will start to attack the hard exoskeleton and the beetle will eventually die when it collapses. Insects that do not eat the leaves or stems of the plants will not be affected.
Step 4 - Prepare to Attack the Larvae
Now that you have made a significant reduction in beetle numbers you must turn to the larvae. Insecticides can be used against the larvae but they are not always safe or effective.
Step 5 - Use Milky Spore Powder
Milky spore disease is caused by bacteria and can now be bought in a dry powder form. Milky spore disease attacks and kills the larvae of the Japanese beetle.
Spread the powder carefully over your lawn and other suspect areas of your garden. The bacteria will be eaten by the larvae. The bacteria increase at a great rate inside the larvae and eventually kill it. As the dead larvae decompose, the bacteria are freed into the soil again and are ready to attack the next batch of larvae.
The milky spore bacteria will reproduce and maintain a high level of protection against the larvae for many years.
A combination of neem oil and milky spore will help to keep the numbers of Japanese beetles down, but it is not an instant killing solution. You will need to resort to removing beetles by hand if you see them starting up in particular areas. The major problem is that Japanese beetles can fly long distances, so there is always the risk of new beetles choosing your garden if you allow the neem oil treatment to fall behind.