Catbird. where did you get your traps? and do they have the female lure inside? I have been hand picking mine off and throwing them in a bucket of soapy water. It's weird, but I have only one rose bush that attracts them, a tall growing one that I have trained up a support. It's in the middle of my mixed border of bedding roses, daylilies, blackeye susans etc. Beetles never even look at them or my crepe myrtles. My beetles are very choosy about their menu.

I just get the standard ones from Lowe's and, yes, they do have the scented hormone lure (whatever it is). There are two pieces of yellow plastic that interlock at the top, the lure that fits into those, and a long bag that hangs underneath. It really pulls 'em in.

when did you put out the traps?
if you did this just before your post, the fact that the numbers are declining might be because it is about past time for them to be here. they are only around for about 2 months in the adult stage.

I put the traps out over a month ago, so I'm hoping I got most of them before they laid eggs for next year's crop.

I had the same experience up here, but will have to wait until next year to see if the numbers decrease as a result. Problem is that I am surrounded by National Forest, so no one to help with them.

So far here in North Alabama (Huntsville), this year's jap beetles attack was much smaller in numbers than last year's. So IMHO the overall trend would make it difficult to discern if the traps in a single location last year had much effect there this year.

On the other hand, if one could tear up an entire lawn, count the Jap Beetle grubs, for several years running, with and without traps, that might yield useful information.