The Sauk County Gardener — scouting for Japanese beetles

Jeannie Manis, Sauk County Master Gardeners Association (SCMGA) President

“Bugs are not going to inherit the earth. They own it now. So we might as well make peace with the landlord.” — Thomas Eisner

May’s weather has definitely been unpredictable. We had high temps and humidity a few weeks ago, cold and rain the next and then Memorial Day weekend – frost! I know I’m going to be “eating crow” the next time I see a couple of my fellow gardener friends as they both asked if I was sure that May 17th was the last expected frost date. Well, it was the last EXPECTD frost date, but it obviously was not the last date for 2021. If you just planted your tender annuals and newly planted tomatoes, peppers, and other warm weather crops, I hope you were able to protect them from the frost. I had planted my peppers and tomatoes and number of tender annuals the weekend before, so mine needed to be protected. Luckily, my husband and in-laws were able to get them covered and bring in most of my container plants.

With this unusual weather, it is important to pay attention to degree days and indicator. When the lilacs are in full bloom, it is time plant beans, cucumber, and squash. It is time to plant tomatoes when lily-of-the-valley are in full bloom. My irises are also in full bloom and it is the time to transplant eggplant, melons, and peppers. Following these common plant phenological indicators, I knew the conditions were good for planting. However, it is still important to keep an eye on the weather – as our late frost was proof of that. Now I am starting to watch for insect pests. The closest growing degree days monitoring location for me is Madison. On May 27th, the growing degree days were 551. Those are currently ideal conditions for the Codling moth and European corn borer. Late May/early June is also the time when June beetles emerge and they can damage turfgrass, some ornamentals, various vegetable and field crops, and more. The grubs tend to be the most damaging life stage as the grubs feed on the roots of plants and disrupt the plant’s ability to take up and use nutrients and water. These tend to be localized so don’t be surprised if you don’t see them. If you find them, use proper irrigation and fertilization to help minimize damage. It’s important to note that June beetle grubs resemble the grubs of Japanese beetle so proper identification is important before doing any treatment.

Japanese beetles emerge need about 1,030 growing degree days to complete development and will continue to emerge until around 2,150-degree days. This means they should start showing up around mid to late June so be prepared to watch and destroy their scouts. Scout beetle will do the reconnaissance on your garden and report back to the rest of the Japanese beetles that your garden is now an open buffet. Look for the scout beetles in the afternoon (I do this when I take my after-work walk in the garden) and hand pick and dispose of them. I use a bucket of soapy water and drop them in the water where they go to a watery death. I used to like to leave the bucket of soapy water and drowned beetles out in a sunny location as a warning to the others that I am a ruthless Japanese beetle hunter and to go looking for food elsewhere. However, that is not a good idea as mere presence of them (even the trapped beetles) attract other Japanese beetles to your yard. This is why it is not recommended to use Pheromone lure traps as the simply attract more Japanese beetles to that location. The only reason I’d use them is if I didn’t like my neighbors and then I’d put the trap close to their yard. Luckily, for my neighbors, I like them and won’t be using any traps.

Both grubs and adults die off when the soil is dry so hold off watering your lawn if possible and that can help reduce the grub population. If you fi nd the need to use insecticides, use one that contains Bacillus thuringiensis galleriae (Btg) to help protect foliage from the adult beetles. It is safe to use a variety of fruits and vegetables. To control the grubs, a reduced risk insecticide that contains Chlorantraniliprole can be used on trees, shrubs, and perennials. It can help control Japanese beetles for 28 days and has minimal impact on beneficial insects. Finally, consider choosing plants that Japanese beetle do not typically like. If you decide to use an insecticide, be sure to follow the directions for proper use. As they like more than 350 different plants and flowers, the list of plants they don’t like is quite short. They tend to dislike clematis, dogwoods, lilac, arborvitae, and several types of trees. To learn more about controlling Japanese beetles, visit Here is also a source to learn more about degrees days: and

This week’s article is written by Jeannie Manis, a Wisconsin Certified Sauk County Master Gardener Volunteer. If you have any gardening questions, please contact the Extension Sauk County by emailing or calling the University of Wisconsin Madison Division of Extension Sauk County office at 608-355-3250.