Japanese Beetles

By Shannon Moreau

 It’s time to start thinking about Japanese Beetles and what options you have to prevent them or deal with them once they appear in your garden. While this invasive species may have come onto our radar in Northern Colorado fairly recently, Japanese beetles have actually been in North America for roughly 100 years. They first appeared on the East Coast and have slowly made their way throughout North America. These hungry beetles enjoy a wide variety of plants and can cause some serious damage at both their larval and adult stages if they are left to flourish unchecked. There are a few look-alikes for Japanese beetles so identification is crucial! If you aren’t sure about an ID, bring a sample (in a sealed bag or jar) or photo of the insect in question into your local extension agency or garden center for help.

Japanese beetles undergo complete metamorphosis, starting as grubs in your lawn and developing into round, metallic- sheened, hard bodied beetles. One sign that you may have grubs developing in your garden are large dead patches of turf grass. These grubs thrive in short, well irrigated lawns. One prevention method is keeping your lawn taller and adjusting your watering system allowing for your tuft to dry between watering. A benefit we will find as more homeowners convert lawns into water wise perennial gardens we are minimizing their ideal habitat. There are a few beneficial insects that are great to have in the garden and take care of Japanese beetle grubs. One example are the Tiphia wasps. These parasitoid wasps lay their eggs in the grubs, defending your garden and ecosystem before the beetles have time to pupate into their adult forms. 

Once they are finished maturing these beetles are easy to find, they are large and like to eat and find mates. Controlling them can be time consuming but don’t get discouraged! July – September I like to bring a small bucket of soapy water (just a bit of dish soap and tap water) into my garden with me in the mornings and afternoons. When I see a troublesome beetle (gray blister beetles and Japanese beetles) I gently shake them into the bucket. As grim as it may be, it is a helpful way to catch them right away. Be sure to knock in any beetles you may see in high areas on foliage. For treatment options Grubx and Beetle Gone are excellent for control. It is a biological control method and is species specific so it is safe for beneficial insects and pollinators. These products can be found at your local garden center along with helpful information on application and timing.

Originally published on July 5th, 2023. Updated on July 15th, 2023.