The Japanese Beetle: A Gardener’s Reference

By Brad Sylvester

Popillia japonica, commonly known as the Japanese beetle, can do a tremendous amount of damage to your flower and vegetable gardens and, surprisingly, to your lawn as well. Those nasty white grubs that you find whenever you dig around in your lawn are Japanese beetle larvae. The beetle larvae grow to about an inch in length. These grub worms eat the roots of grass and can do tremendous damage. Not only do the Japanese beetle grubs kill the grass directly by chewing through the roots, but this damage weakens the resistance of the grass that does survive to such disease as milky spore. According to the US Department of Agriculture, the Japanese Beetle and its larva do in excess of $450 million dollars worth of damage every year in the United States.

Japanese Beetles: Non-native invasive species

The Japanese beetle would be almost pretty if it didn’t do so much damage. The shell is a metallic copper color accented with deep green. The adult beetle measures just under a half inch. Natural Predators of the Japanese Beetle The Japanese beetle is native to Japan and is a prime example of a very successful and damaging invasive species in the United States. The first report of Japanese beetles in America dates back to a single bug munching happily on tender American plants in a New Jersey greenhouse. That was somewhere between 1916 and 1919, depending on who you ask. In less than 100 years, they have spread to almost the entire US except a few states in the southeast. In Japan, the Japanese beetle is prey to a parasitic fly that keeps their numbers under control. In the United States, skunks are one of the largest predators of Japanese beetle grubs. Unfortunately, to get at the beetle larvae, skunks will often dig a number of holes in your lawn. If they find the grubs plentiful, they will come back night after night, digging more holes. A more yard-friendly predator of the Japanese beetle is the cardinal, which eats the adult beetle rather than the grub. You can also buy entomopathogenic nematodes (insect-eating roundworms) such as Grub-Awayr Nematodes to keep Japanese beetle grubs under control. I’ve never tried these so I can’t personally vouch for their effectiveness. They are quite popular though.

Japanese Beetle Damage

The damage done by Japanese beetles is one way to identify them as the problem. They typically eat the soft part of the leaf while leaving the harder veins. This is called skeletonizing because of the bare bones look of the leaf when they are done. They start in the middle of the leaf, not at the edge the way a caterpillar might. They will also go for the tender flower buds before they open. Last year, we had Japanese beetles all over the grape vines. They ate the leaves down to almost nothing. The newly planted grape vines barely survived. No matter many beetles I picked off by hand, there would always be another batch within the hour. They didn’t stop at eating the grapes, though. They also went after the string beans, tomato plants, and various other flowers and vegetables throughout the yard. They also liked the leaves on our pin cherry trees, but the trees are big enough and tough enough to be little affected by a few chewed up leaves. We don’t have any planted yet, but roses are a particular favorite of the Japanese beetle and they can often be found nestled snugly between the petals.

Japanese Beetle Traps

This year, I wised up and put up two Spectracide Bag-A-Bug traps up in the yard. Both of them are working great. I use the Spectracide Bag-A-Bug Japanese Beetle Trap, which is working great. This product comes with a two piece plastic frame, a specially shaped plastic bag (and a spare one), a wire twist-tie hanger, and a two way scent lure. The Spectracide scent lure has a floral scent that attracts hungry beetles and a pheromone scent that attracts Japanese beetles with other pressing desires. I hung one from the branch of our biggest pin cherry tree, and the other from a maple downwind of the garden as directed by the package. Both traps are filling up with beetles. I’d estimate a hundred beetles per trap have been removed as threats to my garden. The traps don’t catch them all, but they have greatly reduced the numbers of Japanese beetles that I’m finding on my plants. The grapes are doing well this year, growing and expanding. The pin cherry is still getting chewed on, but that’s partly because one of the traps is so close to it (against the manufacturer’s kind suggestions). I couldn’t find a link to the Spectracide brand, but it’s not much different than this Japanese Beetle Trap which should work just as well. It also has the floral and pheromone lures, but only comes with one bag. The Spectracide Bag-A-Bug package warns that after a while the trapped beetles will have a repellent effect and ruin the Bag-A-Bug trap’s effectiveness. Then, it’s time to use the spare bag that came in the package and throw the used one away. Some people just hang any long, narrow plastic bag on the bracket, but if I fill up two bags before the season is over, believe me, I’ll have no problem going to the store and buying another Spectracide Bag-A-Bug Kit.

Tough Little Bugs

 The Japanese beetle can travel huge distances considering its diminutive size, and they may come from a mile away to find your garden. The female can dig as much as three inches deep through the soil in your lawn to lay her eggs down below the roots upon which the larva will feed when they hatch. Grubs will be actively eating the grass roots from about May to September with a respite in the June-July period when most of the population has matured into beetle form. Once the eggs hatch in the July heat, they’re right back destroying your lawn. Many of the chemical pesticides effective against Japanese beetles are also toxic to other beneficial creatures such as birds, fish, and honeybees, so use them only as a last resort if at all. Besides, unless all your neighbors use the same pesticides, you’re going to have Japanese beetles anyway. Managing the population and baiting them away from your garden with traps is probably your best bet.