The York Street gardens will close at 3 p.m. on Wednesday, July 17 for a private event and close at 3 p.m. on Thursday, July 18 for a concert.
Japanese beetles (Popillia japonica) have become numerous around Denver Botanic Gardens – I can’t walk near grapes, roses or other flowering plants and not see them. Even though these insects are beautiful like glimmering green and copper jewels, the damage they cause is not.
Unfortunately, it’s challenging to use insecticides to manage this insect because these insects feed on flowers near pollinators. Spraying insecticides may have more impact on the pollinators and other beneficial insects than the beetles. The most effective method is to collect the insects off the plants and either crush them or drop them in soapy water.
In northern Japan where the Japanese beetle is native, they have a number of natural enemies that keep the populations in check. When the beetles were first detected in the United States in 1916, they had no natural enemies and were able to grow unchecked.
Starting in the 1920s, researchers began studying the Japanese beetle’s natural enemies, looking for species that could be introduced to help control the U.S. population as biological control. The natural enemies they selected are specific to the Japanese beetle, and do not move off target – they only seek out Japanese beetles.
On Friday, July 13, 2018, in conjunction with Dr. Whitney Cranshaw of Colorado State University Extension and the Colorado Department of Agriculture, I helped release over 100 infested beetles that had been parasitized by the winsome fly (Istocheta aldrichi). This fly lays eggs on the beetles, and once they hatch, the larvae crawls inside the beetle. The beetle buries itself shortly thereafter, and the larvae completes its life cycle inside the beetle carcass. In the late spring or early summer, the fly emerges, ready to begin the cycle anew.
The hope with releasing this insect is that the fly will develop a population over subsequent years in order to assist in controlling the Japanese beetle. Eradication where the beetles are already well established (like Denver) is very challenging and highly unlikely. But, this is one tool in our toolbox that we are using to help control this nuisance pest.
This is not a random, sporadic release of insects to "see what happens.” Instead, it is a highly regulated, specific release with decades of research to back it up. There are a few sites that have been identified in the area that are part of this study. Hopefully over the next few years, we start seeing a decline in the sheer volume of Japanese beetles feeding on our beloved plants.
If you have other questions about the Japanese beetle, the biological control of it, or what you can do as a homeowner with this same problem, check out Dr. Cranshaw’s Questions and Answers about the Japanese Beetle.