This spring’s cool, wet weather may have delayed the emergence of the dreaded Japanese beetle – but emerge, they finally have – and they’re voraciously chewing through roses, ornamental vines, and almost any other kind of plant you have in your landscape. The iridescent, green- and copper-colored beetle is a highly destructive pest that can be notoriously difficult to control. We spoke to Alison O’Connor, a horticulture specialist with CSU Extension in Larimer County, for more on where they came from and how to deal with them without harming pollinators.
What is the Japanese beetle, and why should we be worried about it?
The Japanese beetle is actually a beautiful insect. If we're talking about how pretty insects are, it’s one of the most attractive that you'll find in the landscape. It is obviously an introduced insect that came over from Asia. We’ve seen it in Colorado probably for about 20 years or so, but it was introduced into the United States in the early 1900s on the east coast. So, it's been in the U.S. for a while, but it's a relatively new insect to Colorado.
The problem with Japanese beetles, and this is unusual for a lot of insects, is that there are two damaging life stages. There's the adult that feeds on a lot of our landscape plants, plus vegetables and ornamentals. And then there's the grub stage, the little creamy white grubs that are in your turf, that are eating your turf roots.
And is there a particular plant that it likes more than others?
That is the worst part about this insect. It loves more than 200 different ornamental plants. It loves a lot of our flowering plants like roses and grapevines. It loves vegetables; it’s a huge fan of lindens. It has an enormous appetite. So, it's not a specific insect like emerald ash borer where it only affects ash trees. This is an insect where there's 200 different ornamental plants it can feed on and be very, very happy in the landscape. Essentially our landscapes are kind of buffets for this beetle, and you probably have plant material that it will feed upon.
Well - how do we cut them off from the buffet? Is there a way to fight them that’s preferably not damaging to pollinators?
The problem with this insect is that it tends to affect plants that are in flower. So, turning to insecticides and things like that are generally not a good recommendation because it could harm beneficials and our pollinators. And again, you have these two life stages. You have the adult beetle, you have the larvae, and so you have to really approach them in two different ways. For the adult beetle in the landscape, it sounds crazy, but hand-picking can be extremely effective. In the early morning, the beetles are sluggish; they’re insects that really like it to be warmer and they need sun in order to be active. And so, in the early morning, you can just go and knock those beetles off plants into a bucket of soapy water. This was research done at the University of Kentucky by a wonderful graduate student, and it was found to be really effective in helping control the populations. And for every beetle that you're able to manage or control, you're helping prevent that insect from laying eggs. So you're kind of paying it forward.
We don't recommend using any sort of traps. You might see them at garden centers or other areas, but traps can actually attract beetles to your landscape. And so we don't generally recommend that you use them, because you might actually get more coming to your property. Then there are also things that are predators, like if you have chickens or you have turkeys or guinea fowl, they will do a really good job helping control some of the adults as well.
We’ve had so much rain this spring and summer. Are there other insects we need to be looking out for?
I mentioned the emerald ash borer; it is back in the news. That was first confirmed in Boulder back in 2013. And compared to other parts of the United States, especially to the east coast, it's had a much slower spread. The thought is that we get those really late spring freezes, which are horrible to our tomato plants, but really good at helping kill the emerging beetles from our ash trees. And what we're recommending with that insect is that if you have ash trees, make a plan as to what you're going to do. It's going to continue to spread. Fortunately, it's not spreading at the rate that we expected. But this will be an insect that we'll be dealing with for our lifetimes.
Other insects that are pretty easy to manage, especially this year with all of our rain and all that awesome succulent growth we've had, aphids are huge and their populations are simply exploding. And the cool thing, if you're interested in insect biology, is that aphids are born pregnant so they can reproduce so fast. And so if you think, oh my gosh, in two days I had this explosion of aphids, it's really because of how quickly they're able to reproduce. Aphids are pretty easy to manage. They don't tend to do a lot of damage to plants, so squirting them off with the hose is great. You can use insecticidal soaps and those kind of things. The worst part about aphids is probably the honeydew they create, which is kind of like sticky insect poop. So you might see that on plants; and then if you're walking under trees, you might get a little bit of a facial as well.
Yikes. I know you're a horticulture professor and not an entomologist, but can you explain why the Japanese beetle is such a beautiful adult insect -- but the grubs are so gross?
The grubs are disgusting. I'm not going to lie. We call them a C-shaped grub. So if you peel back your turf, if you're getting weird brown spots and things like that, you can look to grubs. Generally, we try to say that brown spots are probably from some sort of irrigation coverage; but if you're able to peel up the turf, you might see these grubs and they kind of curl into a C shape, highly unattractive. The beetles themselves, though, people make jewelry out of them. They’re kind of bronze colored. They have these little white tufts along the abdomen and they're just very attractive. When you're talking about an insect that's as devastating as the Japanese beetle, you have to give it some sort of credit. So, I guess it's pretty – but also it's super devastating.
So, what’s the main takeaway here when it comes to protecting our landscape from Japanese beetles?
I think that when you're looking at helping control the insect, get to know the plants that it prefers. It doesn't mean that all roses are off the list. While roses are a preferred host for this insect, it doesn't mean that it eats all of our roses. There are studies out there that tell you the cultivars that it doesn't prefer. And then going forward, you know, getting an idea of the hosts not to plant. So instead of planting beetle buffets, we really want to encourage people to look beyond that. And because this insect has been in the United States for over a century, we have great research that we can look to from our partners on the east coast and other places.
In your own landscapes, if you have questions, you can always reach out to your local extension office through Colorado State University. We can help you with recommendations; we can help you with other control options. There are biological nematodes. There are some insecticides that you can use on your turf, and those are just conversations that are probably better at a personal level.
More reading and resources:
How the town of Palisade fought the Japanese beetle – and won [Colorado Sun]
A trove of helpful information from longtime Front Range master gardener Betty Cahill
Betty Cahill’s recent piece on her battle with the beetle [Denver Post ]
