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Nematodes are parasitic insect-eating round worms that feed on the grubs in the ground, providing an organic pest control method. Common nematode types include Steinernema Feltiae and Heterorhabditis bacteriophora. Place nematodes in the soil in late August or early September to begin treating the grubs for the next year. But you can apply nematodes as soon as there is evidence of Japanese beetle activity in your yard.  Make sure the nematodes are fresh because they are living organisms. Water the ground before you spread them out, and keep the soil moist after you apply. You can also apply nematodes in mulch. For 50 or 100 million nematodes mix, use 1/2 teaspoon per gallon of water. Beneficial nematodes only attack soil dwelling pests, such as Japanese beetles, fleas, German cockroaches, termites, and ants. They are not harmful to people, animals, plants, or earthworms.  You can purchase nematodes online, at garden centers, or at most major home improvement retailers. This method is effective for killing the pest in its larvae stage. You can spray the plants with Japanese beetle killer, found in most retailers and online. Sevin dust can also work, but it might kill beneficial insects along with the Japanese beetles.  Some ingredients to look for when purchasing an insecticide are carbaryl, acephate, and permethrin. A Pyrethrin-based insecticide is a safe way to control pests on your plants while getting rid of other beetles in the process.  Some insecticides are aimed at high Japanese beetle activity in your yard. Others are designed to be used in the early fall once the eggs have hatched and grubs are present. Make sure you read the label on the insecticide before you purchase it to find out what specific problem it targets. Neem oil is a natural pesticide that comes from the seeds of the neem tree, and is considered an anti-feedant because it reduces feeding of Japanese beetles when applied to the plants. Spray plants with neem oil to control the pests.  Azadirachtin, one of the components of neem oil, can also repel and reduce the feeding of nematodes. Use caution using neem oil if you have nematodes in your ground to combat grubs. You can purchase neem oil insecticides online and at most major retailers with a garden center. Soap-detergent sprays are contact pesticides, with no residual effects. They are similar to household detergents, but are specifically designed not to harm plants like household soaps can.  When applying insecticidal soaps, you must thoroughly wet the pest. Most pests needs to be sprayed every 4-7 days. The ingredient you are looking for in a spray is potassium bicarbonate.  Even soaps manufactured specifically for insects can harm certain plants. Check the label of the bottle, or do a test on your plant before spraying wide areas or repeatedly.

Summary:
Use nematodes to kill larvae. Chemically remove grubs with insecticides. Use organic neem-based insecticides. Spray Japanese beetles with insecticidal soap.