Japanese beetle season is here! Defoliated plants – in some cases entire trees – look horrible and no matter what you do they seem to keep coming. So, what’s the best strategy for managing them?
Below are some tips to help minimize damage in your landscape. But first – realize that when Japanese beetles first come into an area, beetle numbers build for about 5 to 8 years, reaching a peak level. After that time, beetle populations drop and, although they never disappear, damage becomes less severe. So there is light at the end of the tunnel, in that the amount of plant damage seen during the peak years will lessen naturally in time.
Some areas of eastern Nebraska have been dealing with Japanese beetles for several years already, while others still haven’t seen their first beetle. Even within Lincoln, some sections of town have had problems for several years, while other areas haven’t seen them yet. So gardeners across Lincoln are at different points on the Japanese beetle infestation timeline.
What Do They Look Like
Both adult and immature Japanese beetles are damaging to landscapes. The adult beetles are shaped similar to our common June beetles, but a little smaller – about ½ inch long. Their head and thorax is metallic green, their wing covers are copper brown. But you have to look closely sometimes to see these colors. At first glance they often just appear dark brown. They also have a series of five white tufts of hairs on both sides of their abdomen, distinguishing them from a native insect called the false Japanese beetle or spring rose beetle.
Immature Japanese beetles are similar to white grubs. They are creamy white colored with a brown head and three pairs of legs on the front part of their body. As immature insects, the white grubs feed on plant roots and are a frequent lawn pest. Root feeding causes browning and death of the grass.
Adults feed on over 300 common plants in our landscapes, including Japanese and Norway maple, hollyhock, Rose-of-Sharon, birch, cherry, plum, peach, rose, American elm, American linden, as well as marigolds, grape, Virginia creeper and Boston ivy. Adults feed during the day, preferring hot weather and plants in full sun. They have chewing mouthparts and consume tissue between the leaf veins, leaving behind a lacy or skeletonized leaf that quickly turns brown.
Don’t Put Out the Welcome Mat
Do not use Japanese beetle traps advertised in gardening magazines or found at some hardware stores. Research has shown these traps attract many more insects than they catch, resulting in more harm than good for your landscape.
For more information:
Discouraging the Use of Traps for Japanese Beetle Control, https://lancaster.unl.edu/pest/resources/japanesebeetletraps.shtml
Tolerate Some Damage
Linden, crabapple, cherries and birch are favored tree hosts and adult feeding damage is often severe. Fortunately, healthy trees tolerate this loss of foliage well and will not be killed. Even if the entire canopy of leaves is skeletonized, plants will re-leaf next year since the underlying twigs and branches are not damaged. Don’t overreact by pruning out branches or removing entire trees.
Less favored trees can be planted to minimize problems. That doesn’t mean they will have no beetle damage, but it will not be severe. These include boxelder maple; shagbark hickory; Japanese tree lilac; black, northern red, scarlet or white oak; redbud; sweetgum or tuliptree.
Protect Landscape and Food Plants
If Japanese beetle damage becomes an annual event in your landscape, consider swapping out the plants they feast on for less preferred ornamentals. If only one or two plants in your landscape are affected, handpick beetles and dropping them into a can of soapy water as pictures above.
Before applying insecticides for control on blooming plants, understand how to use the product correctly and not damage pollinators. Follow all label directions; do not spray on windy days or during the day when pollinators are present. Instead, spray late in the day, near dusk. As always when using pesticides, read and follow all label directions including the use of personal protective equipment.
The organic pesticides Neem and Pyola will give some control of the adult beetles, but only for about 3-7 days. However, Neem and Azadiractin products can be effective repellants, reducing foliage damage when applied regularly (but no more than weekly).
When using conventional insecticides, typically two applications during peak adult flight periods, late June through early August, are needed for control. The first application should be made before plant damage becomes intolerable, but when adult beetles are abundant. Carbaryl, bifenthrin, cyfluthrin, permethrin and lambda-cyfluthrin provide about 2 weeks of control following a thorough application. Follow the label restrictions for use on food crops and harvest timing after application.
At this point in the season, it’s too late to use systemic products like imidacloprid as a soil drench. These products require time to be taken up by the plant and should have been applied around Mother’s Day. They can be used next year if you have trees or shrubs suffering significant damage. These products cannot be used on Linden trees.
Prevent Lawn Damage
Adults lay eggs in the soil for about a 4 to 6-week period beginning in late June. Eggs hatch out 10-14 days later and tiny grubs begin feeding on turf roots. Most turf damage appears in late summer, from mid-August through September.
Turf products for the control of our common white grub also provides control of Japanese beetle grubs and should be applied from mid to late June. If extensive damage occurs and grub control products were not applied earlier in the summer, then Dylox works well in late summer as a rescue treatment.