If you’re a gardener you’ll know just how many pests can attack all your hard work, but there’s a good chance the one that worries you most is the notorious Japanese beetle. These voracious insects are found across much of the eastern and Midwestern USA, and they’re spreading steadily west.
As the name suggests Japanese beetles aren’t a native species. Officially named Popillia japonica, it’s a colorful beetle that grows to just over half an inch long, and has a distinctive green head and thorax with copper-colored wing covers. In Japan it’s not a problem because it has enough natural predators to keep its numbers down, but in the USA things are very different. Since it arrived in the country in the late 19th or early 20th centuries it’s been able to spread almost unchecked because most of those predators don’t live here. Some native species have adapted to prey on it but, in general, the United States is a pretty safe environment for the beetle. That’s unfortunate for gardeners. The Japanese beetle sometimes eats fruit, but the real problem is its habit of turning leaves into skeletons. They feed on the leaf material, leaving only the veins – and a leaf that’s been mutilated like this is no use to the plant.
Most of the common garden pests give you a bit of time to react and deal with the problem, but it’s not always this way with Japanese beetles. They tend to turn up in groups, and if one beetle starts munching on your yard you can bet others won’t be far behind. The reason this happens is that when a Japanese beetle finds a good food source it starts releasing an attractant pheromone, signaling the good news to every other beetle downwind of it. Then they lay eggs in the soil, which hatch into grubs. That makes it vital to identify the problem quickly, deal with it then take steps to deal with the next wave.
The first thing to do is to make your garden as unattractive as possible to Japanese beetles. That’s not easy, because they’re notoriously unchoosy about what they eat – so far they’ve been recorded as eating more than 200 species of North American plants. Unless you’re willing to concrete over your whole yard you’re not likely to deprive them of a food source. There are a few things you can do, though. The adult beetles emerge and start feeding in June, so you can cover particularly vulnerable plants with fine netting or row covers. That will make it hard for the pests to get to them – and if they don’t find food they won’t send out signals to bring in more beetles.
Many birds eat Japanese beetles, so if you can attract them to your garden they might take care of the problem for you. Consider bird feeders or nesting boxes, and make sure your trees provide plenty of perches. At the same time plant shrubs and small trees that the beetles don’t like – holly, lilacs, white and red oak, Southern magnolia and boxwood are all resistant.
Another popular method is to set up traps. These actually exploit the beetles’ signaling system, as well as another characteristic of the species – they can fly pretty well, but they’re not so good at landing. This means it’s possible to catch them in a trap with a vertical surface baited with signaling pheromones, and a bag below it. The beetles fly into the surface then drop into the bag and can’t get out again. It’s a simple and ingenious method, but unfortunately not a very effective one. Researchers have investigated these traps and found out they work very well at attracting the beetles, but aren’t as good at catching them. Instead the insects tend to land on plants near the trap and start munching, so deploying these devices will probably lead to more damage. You’ll end up being bothered by beetles that wouldn’t have come near your property if you hadn’t used the traps.
If Japanese beetles do make it into your yard you can spray your plants with insecticidal soap. This kills them, but unfortunately it kills beneficial insects too. The best solution is to remove them by hand. This is tedious but effective, and not that difficult. The best time to do it is in the morning, not long after dawn, when the beetles are cold and sluggish. Half fill a bucket of water and add a splash of dish soap, put on your gardening gloves and start patrolling your plants. When you find a beetle, catch it and throw it in the bucket. Once you’ve caught all the visible ones you can find, leave the bucket where the infestation was worst – don’t tip the contents away. Repeat the process next day then as often as it takes to keep the visible infestation down. The more beetles you can get in the bucket the better, because they’ll drown in the water then start decomposing. When that happens they release a different pheromone, one that warns other beetles to keep away, so your beetle death trap will become an effective deterrent. Oh, the dish soap? That’s to stop mosquitoes breeding in the standing water.
Catching the beetles works, but do you want to have to do it on an even bigger scale every year? Probably not. Unfortunately you might have to. Japanese beetles lay their eggs in the soil, where they hatch into grubs that live underground through the winter. Sometime in early summer they pupate, then emerge (usually in mid-June) as adults. The grubs are usually impossible to see – but in fact this is the time the beetle is most vulnerable.
The simplest way to attack the grubs is to dilute two tablespoons of dish soap in a gallon of water then spray your garden with it, one gallon per thousand square feet. Do this late in spring, and again late in fall. The mixture will drive the grubs to the surface, where they’ll provide a tasty meal for birds. Alternatively you can use biological warfare on them. Most garden stores sell milky spore, a bacteria solution you can spray on your yard. Treat it annually and after three or four years there will be enough in the soil to infect and kill any grubs that hatch. You can also buy parasitic nematodes, again as a solution; pour it on the most infested areas and the tiny worms will kill the grubs.
Japanese beetles are a real nuisance and can cause a lot of damage, and the way they can quickly gather in large numbers means this isn’t a problem you can be relaxed about dealing with. It is possible to get rid of them though, and targeting the grubs will lighten the load in the future.