When Japanese Beetles started appearing in Minnesota in 1968, I felt lucky and relieved that we hadn’t seen any at all this far north. And it went that way for several years - until it didn’t. And, I thought we might just escape the surge when they were first noticed in large numbers in the Twin Cities several years later. Obviously, I was dead wrong.
In 2021, many in the state know about Japanese Beetles and that the battle to keep them out of our gardens is mostly a lost cause. They are abundant in almost all counties but as of 2020, the western and north western counties seem to be Japanese Beetle free at least for now.
Science-based information helps us understand this pest and gives us insight how to protect your plants, minimize populations, or just live with having them be a part of gardening in Minnesota.
Be on the look out and know when they are coming. This typically happens in late June to early July and continues into late summer or early fall in some areas.
The average life span of an adult is short, only 30–45 days on average. They have a high rate of reproduction: females feed, mate and lay eggs every 24 - 48 hours. At each egg laying, female beetles deposit one to five eggs two to four inches deep in soil and one female can lay 40-60 eggs before she dies. The grubs will spend the next 10 months underground. So some grub control may help reduce adult numbers.
Control is difficult and can take time and money if you choose to use chemical control. And, applications of chemicals have to be repeated often. Hand-picking then dropping them into a pail of soapy water is 100% effective if you have the time for it.
Traps, which lure the beetles in using the scent (pheromone) of a female beetle or a sweet-smelling lure, have been shown over and over again to be ineffective because they lure more beetles to the area than are actually trapped.
Japanese beetles are very much attracted to certain plants which I will call “Diversion Plants” because they will draw the beetles to them in greater numbers reducing the numbers of beetles that would otherwise be on plants like roses, annuals, lindens, etc. It’s not a miracle cure but it helps.
These plants include wild grape vines, coral bells and hops vines. Plant them in an area away from your flower garden and give them room to spread. If your gardens are smaller, a cover material that lets sun and water in will help but it will have to be in place for months and that’s not a good option for enjoying a beautiful garden.
The following website will give you more complete information on Japanese Beetles. https://extension.umn.edu/yard-and-garden-insects/japanese-beetles.