It’s that time of year when Japanese beetles start coming out of the ground and our phone starts ringing off the hook with concerned gardeners asking for advice in how to deal with them. I wish I had a fool-proof, easy answer because, if I did, I would be a very rich man. The truth is that the Japanese beetle has long been a source of garden stress and there are a lot of different opinions on how to deal with them. Like most advice that we give, we try the most nontoxic approach and recommend using a combination of methods to tackle them.
Before we address their demise, for this pest, we need to focus for a minute on their life cycle. Japanese beetles start out as eggs and then hatch as grubs (the larval stage) that live just below the surface of your soil. That’s right, grubs — those disgusting, milky-white, wormy things (that’s the scientific description) that you find when planting in the spring, those are adolescent beetles on the brink of adulthood. Adult beetles emerge from the ground anywhere from mid-June through the end of summer — but grubs are really in the soil year-round and this is the best place to start addressing your problem. Not only do the grubs turn into beetles, but they will also eat the roots of your plants (especially grass) making the pest twice as destructive.
The best organic way to get rid of grubs is to use biological means. There are particular types of bacterial spores such as Paenibacillus popilliae that you can use which the grubs ingest and then become infected with a disease known as “Milky Spore” that will eventually kill the beetle larva. Over time the bacteria will build up in your soil and spread, reducing your grub count from year to year. Another biological weapon are beneficial nematodes. These are parasitic worms that will enter the grub through small body openings and then, basically eat the thing from the inside out. Unlike bacterial spores, however, beneficial nematodes need to be reapplied year after year.
The best time to do either of these tactics is in the spring and fall, when the larvae (grubs) are closest to the surface of the soil and feeding. But what can you do right now? Those grubs are all grown up and starting to pop out of the soil in droves, salivating for your garden.
Start by picking them off and throwing them into a bucket of soapy water. Like me, Japanese beetles are sluggish in the morning, so if you get out early, you can usually just shake them off into the bucket and avoid touching them if you aren’t into the idea of picking them off by hand. The sooner you start getting rid of the beetles the better. They give off pheromones that attract other beetles, and then those beetles release more pheromones and attract even more beetles, etc. It is for this reason that you should avoid those unsightly, yellow beetle traps. Don’t let them fool you — yes, they are filled with dead beetles — but they use a very strong pheromone that attracts them to the trap, usually from miles around. The beetles you are catching are ones you are actually attracting to your yard, not deterring, making the problem that much worse.
If you want to use an organic pesticide, try a Neem-based solution or insecticidal soap. If your problem persists, you might want to try avoiding plants that the Japanese beetle prefers. This includes roses, Japanese maples, crab apples, and linden, birch, cherry, peach, and plum trees.
The chances of you completely eradicating your Japanese beetle problem are slim, however, with the right kind of care, you can significantly reduce their numbers and the amount of damage they do to your garden.