I think it's a hard question to answer. I think you have to interject gut feel.

If your weather is dry/hot & silks are growing slow....then only a couple of beetles can do a lot of damage. If you've gotten recent rains or have good moisture a couple beetles can't keep up w/silking-silks will outgrow beetle damage.

I personally think if there's enough beetles that they're keeping silks trimmed to 1/2" or less...then I'd spray. That's personal feeling. If there's Jap beetles also piled on w/CRW beetles, then I'd definitely spray.

When scouting-keep in mind that CRW beetles are skittish. They see you coming usually (I guess) and will fly. You'll often see/count less beetles than there are. If you walk slow-or look a few plants away, you may have a better chance. You might 1st call local aerial applicator & ask him "how long" it would be before he got there IF you wanted to have it sprayed. That may influence decision also.

If you have beetles and have NOT used insecticide, it's hard to believe that you don't also have pruned or damaged roots from CRW. Beetles don't travel very far they tell us. They came up out of the ground below the plant.