What is the bling on those Japanese beetles?
Like most of you who garden, I have been plagued by Japanese beetles, Popillia japonica Newman, for the past 15 years or so. Up until then I had never seen one up here at 1,800' in northern Vermont. They show up in early July. I have tried milky spore in years past, but the expense is great if you are covering large areas. I used to put up traps with the pheromone attractants. It was soon obvious that all I was accomplishing was calling them in from the neighboring areas.
The beetles here seem to enjoy the roses more than any other flowers. Their go-to meal, however, is raspberries, both the plants and the berries themselves. They will also go after blueberries once their populations rise. I can spend hours early and late each day (they are docile and don't fly away during those times) walking up and down the rows of berries knocking the beetles off into a yogurt container with water in the bottom. This method can work to reduce the population, but it can eat up a lot of time. The hens and guineas love the practice though and are more than willing to eat up the contents. Recycling at its best!
Along comes mother nature. For the last several years I have noted small white spots on the back (thorax) of many if not most of the beetles. It turns out that the white spot is an egg deposited there by the tachinid fly. The larva that hatches will burrow into the beetle and consume it from the inside. The beetle will fall to the ground after a few days to be part of the natural life cycle of the fly, fertile ground if you will. What is the takeaway? Beetles with a white spot(s) should not be captured or destroyed. They are incubating the increasing numbers of tachinid flies which become predators for many different garden pests.
Be patient. Put those traps away and celebrate mother nature's remedy. The only ones to be disappointed will be the skunks as they try to dig up your sod to find beetle grubs. That is about as "organic" as you can be. For more on this subject read this piece from Savvy Gardening.