Japanese beetles, when infesting rows of plants of the same variety, usually occur unevenly on the individual plants. Some of the factors have to do with the vigor or color of the tree. In my observation on peach, I have repeatedly seen a sickly, yellow and half-wilted tree with thousands of beetles in it, while other similar but healthy trees in the same row averaged only a few hundred beetles. You can make one branch of a tree more attractive to the insects than the rest of the branches by partly girdling it or permitting borers or cankers to damage the base of the branch. This observation suggests that the increased sugar content raises the attractiveness of the leaf. It coincides with what is already known that extracts of plants preferred by the Japanese beetle have, in general, a higher sugar content, or more of a fruit-like odor than those not attacked. (Metzger et al, Jour. Agric. Research, ~49~ (11): 1001-1008. 1934. Washington, D. C.)
There are other observations you can easily make yourselves. The Japanese beetle avoids shade, except on the hottest days, and its feeding in dense trees shows up most in the tops; its feeding on uniform plantings tends to show up most in the edge rows. Nursery-size trees are more extensively defoliated than larger ones. At this point we must consider that the insect usually has to fly into a planting from the outside, for it breeds chiefly in lawns and meadows. If the foliage mass of the nut planting is small and the grass areas nearby are large, the beetles are likely to do heavier damage than where the planting is very large and grass areas negligible. A small planting in a suburban area, beside a large golf course, cemetery or dairy farm, is going to be more heavily attacked than a large one set in a clearing in the woods.
~Control of the adult:~ The safest, most direct measure is to pick or knock the beetles off the plants, preferably in the early morning, when they are cool. They may be dropped in a pail with a little kerosene in it. Some plants can be shielded with thin nets which can be placed on them by day. We do not recommend Japanese beetle traps. These yellow traps, which are baited with geranoil and other essential oils, can draw beetles in from a considerable distance but we have found that, possibly because many beetles miss the trap, the population of beetles remains high near the trap, in spite of heavy daily catches. Although the use of one trap to the acre on a block 10 miles square would probably get results, the use of a few traps on a small nut planting is likely to be disappointing.