When Japanese beetles discover your garden, it can be frustrating. I stopped counting after dunking more than 300 into soapy water. The beetles enjoy eating the leaves of more than 200 different plants and trees, and in our garden they feast on one particular rose bush and a corkscrew hazel.
There seems so little one can do other than stand guard to spy the beetles and then knock them into a bucket of soapy water. If allowed in your area, some suggest spraying with neem oil. Neem oil is used as a bio-pesticide in organic farming to repel a wide variety of pests, including aphids, cabbage worm, thrips, whiteflies, mites, fungus gnats, beetles, moth larvae, mushroom flies, leafminers, caterpillers, locust and Japanese beetles.
The damage is visible on the leaves of the trees and shrubs attacked by the beetles. And there are the patches in the lawn where last year’s grubs devoured all the roots of the grass.
As summer goes on, however, the beetles have been mating and laying eggs in the soil. Those eggs will hatch in late August, and the young grubs (white, C-shaped) will be active in the soil this fall until hibernating over the winter.
How to control and minimize Japanese beetle populations
We spray the soil in September with a product called Canadian Lawn Guardian from NIC (Natural Insect Control) in Stevensville, Ont. NIC says Lawn Guardian has beneficial nematodes — microscopic worms — native to Canada, meaning they’re fresh and more targeted to Canadian pests and 100% organic. The nematodes seek out an insect host (the grubs) and release bacteria to initiate an infection. The infection causes the insect to die, and nematodes to feed and reproduce. The NIC entomologists say that the product is safe to handle and there is no harm to pets, plants or friendly insects such as ladybugs.
To target the “bad guy” grubs, Lawn Guardian is a mix of two nematodes, one that stays close to the surface (approximately three inches/8 cm) and another that goes deeper (approximately seven inches/18 cm).
I’ve asked different people why beetles seem to be on the increase. Some attribute it to Ontario’s year-old ban on pesticides, which ended the use of products that kill grubs in lawns. Sandra Mitchell, owner of NIC, says we’re on the up-curve of the natural insect cycle of beetles, and that they will be worse next year before beginning to decline. Her company has been supplying natural products for 20 years and shipping Canadian-produced nematodes since 2008. Here are some of her suggestions:
- Wait for the right timing to apply nematodes. In the fall, the grubs are small and close to the surface, so a fall application of nematodes will likely kill more grubs. But the soil must be moist to below the level of the grubs. We’ve had a relatively hot, dry summer, so wait for some soaking rains (or water deeply).
- Apply in the evening, not in direct sun, because the sun’s rays can reduce the effectiveness of the nematodes.
- After applying, water well to help flush the nematodes into the soil.
- If it’s hot, water for 2 or 3 days afterward.
- Nematodes can be applied as long as the temperature is above 10° C (50° F).
- The Lawn Guardian product has an expiry date (60 days after being made) so check the expiry date before buying. NIC also has online ordering.