My sunchokes are attracting about twice as many japanese beetles as a sunflower would, should I do anything to control them, or just let them be?
In my experience, there aren't many options for controlling Japanese beetles. The choices you have:
- Hand pick them off the affected plants. This isn't difficult, especially if you go out in the morning while it is still cool. The easiest solution is to hold a bucket of soapy water under the plant, and then pick or push the beetles off the plant and into the water. The problem is that as soon as you remove the beetles, more beetles fly in from elsewhere.
- Leave them. I haven't actually seen Japanese beetles kill a plant. They skeletonize my Holly hocks every year, but the plants survive well enough to set seed. They almost completely defoliated 2 young Linden trees in my street terrace (and at least a dozen more in my neighborhood), but none of the trees died. The plants just look terrible and may be set back a bit.
- Remove the plants they're attracted to. If you don't mind losing the sunchokes, you could treat them as a trap crop and pull them now (drowning the beetles as you do. The hope would be that there aren't other plants in the yard the beetles are willing to move to after the sunchokes are gone.
- Convince your neighborhood to try milky spore. Milky spore is apparently quite effective in some areas for controlling the beetles. I've applied it to my yard in the past, but don't see a huge difference in the number of beetles I have compared to the number of beetles neighbors who did not treat have. From what I understand, it is far more effective if all of the neighbors surrounding you use it too.
This pheromone type trap will draw japanese beetles from just about anywhere. I once set one up about 100' from a japanese maple that was absolutely infested, and within 10 minutes there was a visible cloud of beetles flying off the tree and toward the trap. The thing must have finished the day with 500+ beetles in it. That said, they'll also draw beetles from the surrounding areas. So they work best if you can place them a fair distance from the area you are trying to protect - at your property line if you have the room. For several days the traps I set up had dozens and dozens of beetles flying around the openings and dropping into the bags (from which they did not escape). After a while the visitation numbers dropped, but so did my observations of beetles on my tree and garden crops. I didn't attempt to add more bait to the traps as my problem was solved to the extend I needed.
But I will never forget how impressive it was to see all those beetles leave the tree and head straight toward the trap.