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Japanese beetles are about a half an inch long with iridescent copper and green bodies. They can heavily damage and even kill plants by eating the leaf material between the veins of plants. Many plants are susceptible from roses to pussy willows to cannas to crape myrtles.
Thankfully, they are short lived and only seem to be a real problem in my area in early to mid summer. However, in large numbers, they can even be a problem in their larvae stage where they can live in the soil for up to a year and will feed on the roots of grasses. I think it is much easier to control them at this stage, so that is what I will focus on first. You can view photos at this stage here.
Japanese Beetle larvae (also known as grubs) are susceptible to a bacteria called Paenibacillus popilliae or more commonly known as Milky Spore. You can now purchase milky spore in many local garden centers, hardware stores or discount stores and is readily available online from places like www.amazon.com and www.gardensalive.com.
Milky Spore will not harm other grubs or beneficial critters, like earthworms, only the Japanese Beetle larvae. Milky Spore comes in a powder form and is best applied in late summer (August for my area) when the larvae are at their most active. Follow the manufacturer's instructions on the container for application rates. Milky Spore is not a quick fix. It will work over time becoming fully effective in one to five years and remaining effective for up to 20 years in many climates. The Japanese beetle larvae will spread the bacteria amongst themselves, which is why it takes some time to work.
You will find Japanese Beetle traps in stores that contain pheromones to attract the beetles to the traps, but all you are really doing is attracting more of the beetles to your yard and you are only catching a small portion of them in the bag. While they are in your yard, they are laying eggs in your lawn and you will have an even bigger problem next year. So, in my opinion, it's not a smart idea to even put these traps in your yard, because, in the long run, you will be making the problem worse.
There are chemical controls that can be applied to the lawn or to the beetle themselves, but they will also kill off all the good bugs and beneficials that we need so badly for pollination, soil improvement, and plant health. So I would avoid them all together except for extreme infestations.
The only other form of control that really works is hand picking the beetles off your plants and drowning them in soapy water. They are not good fliers so are easy to pick off your plants and since they are only around for a few short weeks, hand picking is the easiest and safest control to use.
I personally applied milky spore to my lawn in 2005 and have very few beetles in my yard today. I will monitor the situation and as soon as it seems like populations are beginning to increase, I will apply milky spore to my lawn again, but that could be 10+ years from now.