The Colorado Department of Agriculture is to stop the spread of the Japanese beetle in western Colorado. The invasive pest was first spotted in Grand Junction in July 2022 likely brought to the Western Slope from the Front Range where 11 counties are "partially or completely infested."
Maeve Conran spoke with Melissa Schreiner, an entomologist in Grand Junction at Colorado State University Extension, Tri-River Area.
CSU Extension is leading efforts to raise awareness about the beetle and stop its spread.
How did the Japanese beetle come to the U.S.?
The Japanese beetle originated in the U.S. in the early 1900s, arriving on plant material from Japan to be shown at the World's Fair. The larvae came over in the potted soil and escaped the New Jersey area. With the moisture, precipitation, and humidity on the East Coast, they rapidly expanded and have been slowly chipping their way westward.
What should people be looking for when they are on the lookout for Japanese beetles? What do they look like and where should they be looking for them?
Japanese beetles are a type of scarab beetle. They're shiny with metallic elytra and have a green thorax and head capsule, which are also very metallic. They’re about the size of a coffee bean.
They are attracted to big flowering shrubs, including landscape plants like roses and Virginia creepers along fence lines, as well as fruit trees like crab apples, peaches, and cherries. They also love grapevines and linden trees. They're polyphagous, meaning they feed on many types of plants and are documented on hundreds of plants, though they have their favorites.
Large numbers of these beetles visit flowering plants, making them easy to spot in their adult stage. They likely feed during the day, defoliating and eating plant material.
In the younger stages of their life, they're grubs, feeding on grasses like turf grass. Japanese beetles could be living in a yard with Kentucky bluegrass or lawn. If that's the case, that turf grass may show signs of feeding damage because the roots are being pruned and eaten by the Japanese beetle from underneath. Signs of larval damage include turf grass that pulls up like a rug, lifting off the ground and appearing yellow and dry.
What should people do if they see a Japanese beetle or its larvae?
Reach out to your local . No matter where you are in Colorado, there's often an office in your county, if not a neighboring county.
You want to be sure that it’s a Japanese beetle and not a lookalike because many things are similar in appearance to these beetles. You can bring a sample to any extension office to confirm if you have the invasive Japanese beetle. There’s also a on the identification, biology, life cycle of this pest, and its host plants.
This is a hard beetle to manage because it feeds on so many things in the adult form. Some chemicals can be used to control them when they're younger, before they reach the adult stage.
If they are fully grown beetles, a great approach for adult management is to knock them into a bucket of soapy water since they freeze up when they’re disturbed and fall to the ground—in this case, into the bucket.
There are few products for because spraying a flowering plant with insecticide is often against bee-label language, as it will harm pollinators.
The extension office doesn’t recommend treating for the adult stage because it's just a band-aid to the problem. You want to treat larvae and eggs using more preventative, less reactive products—things that can prevent eggs from hatching and products that will attack the young stages of this beetle.
How concerned are you about the Japanese Beetle in Grand Junction and potentially spreading, given your proximity to Palisade, given the agricultural economy on the Western Slope?
We're starting to look in neighboring counties to secure more information. These other Western Slope counties are not being heavily trapped for the Japanese beetle.
If you're registered with and regulated by the you have more awareness about not spreading this beetle around through the plant trade. But many operating landscape contractors are not registered. Also if someone moves a plant from, say, Denver across the Continental Divide—that simple act of moving plant material can be devastating. Plus, auctions that occur for equipment, and plants sold at these auctions are also a potential threat.
What is CSU thinking about whether or not this can be kept under control in a way that hasn't been able to happen in the Front Range where we're seeing massive infestations of this beetle?
It all comes down to education and knowing that the movement of plant material is causing the spread of this beetle. But it's going to be challenging to slow down biology. An enormous effort is being made to get ahead of the game on this breeding population. It's a big year for securing more data to see if this has already spread around.
There are many approaches to figuring out how to stop this material from moving. There's a big public information campaign locally called , and it will be interesting to see if that's a reality for our area.
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