With IPM, it is critical to understand the life cycle of a pest to know how to treat the problem. Adult Japanese Beetles emerge from the ground in July, feed on leaves and flowers and mate, until late September when they lay eggs in the ground. The larvae of JBs live and feed on turfgrass roots in late summer-early fall and then move downward in the soil where they overwinter. In spring, the larvae move back up in the soil and feed on turfgrass roots again until July and then pupate and emerge as adults.
Foliar sprays are very effective at killing JBs. You can find a variety of ready to use sprays at your local garden center. Sevin, Spectracide Bug Stop, and Bayer Advanced Lawn & Garden Control sprays will do the job. Since they are broad spectrum they kill honeybees and other beneficial insects. They can also be harmful to birds, aquatic species and humans, so please limit your use.
This type of pesticide is applied as a root drench or injected into the trunks of trees. The pro of systemics is that the plant takes the chemical up internally, which means it won’t wash off when it rains, as with topical sprays. Larger plants take more time to move the chemical up to the top of the plant, so you may notice a delay in effectiveness in those plants. With systemics, the insect must ingest the plant before it is killed, so you will still see beetles browsing. Systemics do also harm honeybees and some other beneficial insects, so do limit your use.
There are some natural remedies and even though they may be less effective than harsh chemicals, they are definitely worth trying. As stated earlier, JBs don’t have major predators in the US but Milky Spore fungus and nematodes are biological agents that can be effective controls.They are used to kill grubs. Other products worth investigating are Neem and Cedar oil, which work to repel JBs.
This is the most environmentally safe method, and can be done pretty easily on smaller shrubs and flowers. Pick off beetles and chuck them into a bucket of soapy water as you go. They will die in the soapy water, and you won’t have to go to the mess of smashing them between your fingers.
Plant species less commonly eaten by JBs
JBs browse hundreds of species but there are a few plants they love and some that we know they don’t go for as much. To have fewer JBs in your yard, avoid planting species they love the most.
- Plants that attract JBs: Rose, Apple/Crabapple, Birch, Cherry, Plum, Norway Maple, Linden, Walnut, Horse Chestnut, Aronia.
- Plants less commonly eaten: Common Lilac, Red Maple, White and Red Oak, Spruce, Fir, Pine, Tuliptree, Holly, Hemlock, Arborvitae
Three Common Myths:
Myth 1 If you treat your turfgrass with grub insecticide, you will have fewer adult JBs in your yard. Reality JBs can fly up to 5 miles to find food, so the majority of beetles on your plants may not have come from your own turf. Don’t bother treating your turfgrass unless you know you have a grub problem. To scout for grubs pull up a few 12” x 12” squares of grass and count how many grubs you have. It’s recommended to not spray unless you have 7-15 grubs per square foot. Do this in April- May or September.
Myth 2 You should install a pheromone trap in your yard to lure and kill large amounts of JBs.
Reality Pheromone traps use the scent of roses and a JB sex hormone to attract and trap adult beetles. What usually happens is that you will actually attract more JBs to your yard than what your plants attract on their own, without the traps. Many of these beetles never end up in the traps. Some still argue that traps may be effective on large areas of land like acreages, where you are able to place the traps a far distance from plants you are trying to protect.
Myth 3 JBs are introduced to your landscape from plants purchased at garden centers/nurseries.
Reality JB adults lay eggs in turfgrass, so there are no eggs or larvae in other type of plants. It may be possible for a stray adult beetle to hitch a ride in a plant on the way home, but one beetle is negligible in comparison to the thousands that fly in from nearby turf.
Summary: It is important to know that you will probably never be able to get rid of every Japanese Beetle. One of IPM’s main principles is to accept a certain amount of loss. Yes, it’s annoying and unsightly, but it’s good to know that JB damage is almost always aesthetic and doesn’t kill affected plants.
Horticulturist, Bentley Ridge Tree Farm