I'm beginning to see adult Japanese beetles on my dahlias and other flowers. It doesn't look like a heavy infestation but I am wondering what are some non-chemical control methods. I've read that nematodes are one option. Are they effective? Are there others?
Milky Spore is another option. Apparently, it is very effective for some, not at all for others. I hear it is most effective if you can convince your neighbors to use it, as well, so a larger area is infected with the disease (which kills the grubs).
I put it down in my yard two years ago, as did a few of my neighbors, and I have noticed less beetles since, HOWEVER I put it down at the end of a boom season for the beetles (they almost completely defoliated all of the linden trees in the town I live in and the neighboring town, too), and I notice less damage all around. We had a record-breaking drought last year, too, which I'm sure killed a lot of the grubs. When I dig in my yard, I'm not finding many grubs at all, and have found none that appear to be infected, so I have no evidence that the milky spore is doing anything. I think we may be at in a "bust cycle" following the boom. I'll be interested to see how many beetles we have in a few years.
For smaller numbers of beetles, I hand pick into a bucket of soapy water. That seems to reduce the amount of damage I see in my yard, but during the boom year it was hopeless.