By Linda Peacock, Master Gardener, SCMG
I live in Clearview Township between Angus and New Lowell, Ontario (zone 5b) and up until last season I had hardly seen any Japanese Beetles (Popillia japonica) in my garden. Last season they started to invade my gardens, and this year they took over! I have learned quite a bit in the past two years, hopefully some may help your gardens too.
Last season I only noticed them on my roses. Which is a huge concern of mine since I have about 50 roses. Some are in a dedicated rose garden, but many are in among other perennials. My dedicated rose garden is in the back yard and seemed to be the hardest hit last year. I assumed it was because there were so many together that it made them more obvious to these little flying menaces – but I was wrong, they just hadn’t found the rest yet. This year they did. I also noticed them in the grape vine at the very back of the property and on the Birch and Maples in the front of the property. Later I had them on my edible legumes in the vegetable garden and on my raspberries. While walking my dog I noticed they were also on my neighbours’ Cherry, Pear, Apple, Crabapple and Linden trees. It turns out that they will eat up to 300 different species. How do you get rid of something that is attracted to so much?
I decided I needed to know my enemy to know how to go to war against it.
The adults emerge during the daytime hours to feed and mate from the last week in June to the last week in September. They live about 6 weeks. They chew the tissue between veins in foliage starting with the newest leaves at the top of a plant and eat their way down. Since many flowers and buds are at the top of a plant, they eat through those as well. In my garden they prefer the lighter roses. OMAFRA reports that there is no threshold of damage with these beetles. Most plants have time to recover each season after they beetles have gone, however, the affect on fruit trees (and beautiful roses) is still devastating to the owner. They are more active on warm, sunny days. They will curl a leaf around themselves and hide under petals in a flower. They are always in a group. They use pheromones to attract more adults to the food source and to mate. They fly best at approximately 21°C and 60% relative humidity. They are not able to fly if the temperature is above 35°C or 60% humidity. When they do fly, they easily travel for 1.6km (up to 8 km with good wind). The beetles return to the soil late evening. During July and August each female will lay 40 – 60 eggs in the grassy areas 5-10 cm deep in the soil around the food source. The eggs hatch within 2 weeks and grow into a white grub with a brown head that is 2 cm long. It is c-shaped and has 3 pairs of white legs. The eggs need moisture to hatch, and the larvae need moisture to thrive. They will not survive in very dry conditions. They will try going deeper to find moisture. In September they move to 15-25cm deep in the soil to over-winter. They will stay there until the ground above reaches 10°C around April and May where they will start feeding on roots.
Knowing the life cycle is our best defence with any pest. Getting rid of the adults is not easy. There are traps on the market which I did try this year. I put one about 30 yards away from my rose bed, I did not see much difference. I moved it to 100 yards away and I saw an improvement in the amount on my roses but there were still so many. There are pyrethrin-based sprays available, and I could kill some with it, however, the amount of spray you needed to affect them made it too time-consuming and too expensive. Since anything you spray could affect other insects and I did not like trying it. Picking or knocking them off into soapy water worked the best. I did get to really enjoy picking them off and squishing each one so I knew they were dead. My husband who was sitting on the patio having a few chips with an adult beverage did look at me in distain once and say, “I am not sharing my snacks with you now”. Getting rid of the grubs or larvae may be our best defence. The only way to do that is with nematodes. The suggested nematode for Japanese Beetles is Heterorhabditis bacteriophora or Hb. These are readily available and are successful on other insects that start as grubs. Where I applied them the ants that had been taking over my lawn also receded. Nematodes are easy to use but you need to follow these rules: The soil temperature must be able to remain at 10°C even at night for them to survive. They need moisture, make sure you water them into the soil well when you apply them and keep the area moist for two weeks after application. Since they are UV sensitive, I suggest doing it on a rainy day or late evening. (Yes, I have been seen in my rubber books and raincoat watering my lawn this fall, but honestly, I am applying nematodes). They do not like elevated nitrogen levels, avoid any use of fertilizer or fungicides in the area you wish to apply them nematodes for at least two weeks beforehand. The best time to apply them in this area is April-May and September to October when the soil temperature can remain at 10°C. If you give them the best conditions they will become established and they will multiply to help your soil for a long time. To get a good population built up you need to apply the nematodes spring and fall for 2 – 3 years. They are microscopic when you get them so you will not see them as you apply them. Be sure to keep them at the suggested temperature (usually in the refrigerator) before you apply them.
There are many other methods that I had discovered while I did my research. I had heard that Japanese Beetles do not like peonies, which is interesting because I do have one garden in the front yard with quite a few peonies in it and the roses there were the least hit of all. I will watch this next year to see if they just haven’t found that garden yet. One of my colleagues grew parsley plants with her roses and she had very few problems, I will try that next year in my dedicated rose bed. Last fall I planted garlic in one area of my perennial beds and that section seemed less effected, I am going to incorporate more garlic, chives, and onions in my gardens next year. I had heard white geranium would help deter them, but I did not have success with that. I had read that the Japanese Beetles had no natural predators here, however, I live in a wet area with a Red-Winged Blackbird population, and I watched them eat some of the beetles. Hopefully other things will decide they are edible and help us out with our fight!
So – for now we can enjoy our gardens with no beetles in sight. Try applying some nematodes, and enjoy an adult beverage on your patio. Maybe someone will share some snacks.
