A Pest Above the Rest – Japanese Beetles
Kristina Valle, Master Gardener
It’s July and our gardens are in their prime! But wait, what is that insect on my beautiful roses? And why do the leaves on my raspberries look like skeletons? Read this article about the dreaded Japanese Beetle and what you can do to minimize the damage they cause.
I moved back to Minnesota in the fall of 2018; I had not lived here since I was 6 years old. I was excited to be closer to family and to finally, put down some roots. My husband I had built a house and, over the winter, dreamed about how we would fill our blank landscape in the spring. In early May, I began shopping around at local nurseries and was drawn to and purchased a few flowering crabapple trees. I spent the rest of the month filling in the garden and by June, I was able to sit back and enjoy a complete yard. In early July, it was time to weed the garden. As I worked along the base of one of my crabapple trees, I bumped the trunk, which dislodged several bugs that I’d never seen before. Under closer examination, I realized that my crabapple had been taken over by these bugs. The Japanese Beetles had found my tree and were heavily at work, eating the once beautiful leaves.
Japanese Beetles are an invasive species that feed on the leaves, flowers and fruits of approximately 300 different types of plants. They are around one-half inch long and have a metallic green head and thorax with copper-brown wing covers.
Like all pests, Japanese Beetles find some plants more attractive than others. Some of the more susceptible plants include:
Adult Japanese Beetles typically arrive in our gardens at the end of June or early July and continue to feed on leaves through the month of August. While the beetles feed, they emit a specific odor that attracts more beetles to the plant or tree. The damage caused by the beetles varies but the pattern remains the same. Japanese Beetles “skeletonize” the leaves which means that they feed on the plant tissue between the veins, creating a lace appearance.
An established, healthy plant can withstand heavy feeding with only cosmetic damage done to the leaves, while younger, unhealthy plants may experience restricted growth or possibly death if the feeding is significant.
During this time, the beetles are also mating. Females will burrow a few inches into the soil, several times during the summer, to lay a total of approximately 60 eggs over the season. The eggs hatch about 2 weeks later and begin to feed on the roots of grass. Dryer soil conditions can help make the soil less favorable and may reduce the amount of new Japanese Beetles the following year. As temperatures begin to cool, the grubs burrow deeper into the soil to overwinter until the following spring. As temperatures rise, the grubs move closer to the surface of the soil and feed on the grass roots until they reach their adult phase.
There are many ways that we can minimize the damage caused to our plants by Japanese Beetles. Here are some control options:
You can cover smaller plants with cheese cloth or other fine netting to prevent access to the leaves. This will allow light and rain to filter through without exposing the leaves to the pest
Hand picking or shaking Japanese Beetles off the plants into a bucket of soapy water. While laborious, this is effective, especially if done in the early morning or in the evening when the beetles are sluggish.
If you visit your local nursery, you will find a variety of chemical options that are designed to minimize the presence of Japanese Beetles.
Remember to read the label – it’s the law!
Follow best practices:
Do not spray the plant on a windy day
Wear long sleeves, pants, protective eye wear and gloves
Wear a face mask to prevent inhalation
Ensure that the chemical will not harm beneficial insects in your garden
Commercial traps are readily available in many stores; however, this method is not recommended or effective.
Traps may attract more beetles than are actually caught.
Traps put other plants at risk that the beetles may have avoided in the past.
Consider adding plants to your garden that are not attractive to Japanese Beetles:
Conifers (e.g., arborvitae, spruce, fir, pine)
Japanese Tree Lilac
Red and Silver Maple
My first summer back in Minnesota left me with a lot of frustration, and I spent some of the winter months trying to understand how to prevent these pests from entering my garden in the upcoming spring. I learned a lot, but mostly, acceptance. As gardeners, pests, whether welcome or not, are part of the package, part of the experience that makes what we do always challenging, always an adventure.
Photo credits: University of Minnesota Extension (1, 2, 3, 4)