Today I saw this year's first Japanese Beetles in City Park's rose garden next to the Museum. Japanese Beetles (Popillia japonica; length about 10 mm, 3/8 inch) are an invasive horticultural pest that should not be here and should not be able to survive in Colorado's dry climate. They arrived in 1911 from East Asia in New Jersey and spread rapidly throughout the East where they have been battled unsuccessfully for a century. Ninety years later, they arrived in Colorado and have been found every year since 1995. Meanwhile the beetle is established throughout the Front Range and feeds on hundreds of species of ornamental plants. Its larva lives underground where it feeds on grass roots in well-watered lawns. And this is the reason why this species that needs a much moister environment than Colorado can survive here: our lawns. Lawns are not native to Colorado, are not a part of our natural landscape. However, we do love them, and use 50% of water in Denver for outdoors, mainly for watering lawns. This way we create a wonderful artificial habitat for Japanese Beetles to thrive, and with all the introduced plants in our yards and parks, the adults can gorge themselves, getting well nourished to produce a new generation of Japanese Beetles every year. In the last years, the beetles were pretty common in City Park's rose garden (the photo is from last year). This year, already at the very beginning of their adult life, the Park seems to have sprayed: lots of dead ladybugs on the ground and dying beetles in the rose flowers. Good luck with that!
In Colorado, one of the few success stories in terms or eradicating a Japanese Beetle population took place: A localized outbreak in Palisade - the beetles love peach trees, too - was successfully defeated a few years ago, with strong community involvement and a combined application of soil insecticides and baited traps. Every property owner in the affected area needed to cooperate, or the agencies had the right to treat the land even against resistance. With the peaches at risk, resistance was sparse and the program a success. In the Front Range, the beetles are far too spread out for battling it in a concerted effort. We will have to share our yards and parks with Japanese Beetles for the time being, and might want to think more about xeroscaping and hand-collecting and then killing the adults (in the freezer or soapy water). A presentation on that issue I gave to the Denver Rose Society earlier this year has been written up and is available here.
I am compiling the distribution of the Japanese Beetle in Colorado for a paper to be submitted end of the year. If you have these beetles in your yard or neighborhood and want to contribute, please visit this Citizen Science webpage.