Here are some control strategies for all hop growers – organic and non-organic.
First concept: Beetles attract more beetles.
I do not consider Japanese beetles to be a threshold control type pest where the decision to spray is based on tolerating a certain level of pests before treatment or spraying is done. Female beetles excrete pheromones to attract males and mating is a gregarious affair – a “beetle orgy” for lack of a better term. Beetles fly into the wind and follow the pheromone trail to the plants and leaves where feeding and mating occur. Females apparently leave pheromones on the leaves; as I note that beetles arrive and continue to congregate on the same leaves – even if the original beetles are removed. However, a good hard rainfall seems to wash away this residual pheromone trail.
It is well known that hanging pheromone traps can attract large number of both male and female beetle from great distances. A population of mating beetles will grow exponentially as congregating beetles emit stronger pheromone trails for newcomers to follow to the orgy – just like a pheromone trap. Allowing the beetles to establish a population in a hop yard is exactly like hanging pheromone traps in the middle of your hop yard.