Help! The Japanese Beetles ate everything this summer!

By Kathy Conner Cornell
Southside Master Gardener

If you think that Japanese Beetles were as bad this year as you can ever remember, you were right on target. Generally I only see them munching on my roses and sometimes the blackberries. This year they decimated our apple trees, wild muscadine grape vines and tomatoes. Let’s take a look at what caused this “perfect storm” this summer.
A little background is in order. Japanese Beetles were first found in the United States in 1916. The pest is from Japan as its name indicates but there it has natural enemies so it is not considered destructive. By the 1970’s the pest had traveled to 22 states east of the Mississippi including Virginia. Since it is a pest of lawns, ornamental plants and fruit trees, a lot of money is spent trying to control the beetles.
To completely understand what kind of control will be effective, it is important to be familiar with the life cycle of the insect. Japanese beetles have what is called a complete metamorphosis, meaning it has 4 main life stages – egg, larval, pupal and adult. We are more familiar with this type of metamorphosis with butterflies and moths. I have a cartoon called “A Bug’s Life” where an insect is reading a to do list listing Emerge, Eat, Mate and Die. This is pretty much the beetle’s life. When the beetles are buzzing around and chopping on our plants, they are also mating. During this time. which here is usually late June and July, the female will dig small holes about 3” deep in your lawn and lay a few eggs. She will generally lay 40 – 60 eggs in the season. At this point the work of the beetles is complete so they will die.
Meantime, the eggs hatch into grubs in a short time and start to feed on your grass roots. There will 5 instars or molts until the grub reaches its final size of 1”. When that first hard frost hits, the grubs will bury into the ground about 4 to 8” and remain inactive all winter.
Unfortunately, our soils never get cold enough to have a winter grub kill. Grubs can survive Minnesota and Chicago winters, pretty resilient pest.
In the spring, the grubs come back higher in the soil and again feed on your grass roots. Sometime in late May the grubs form a pre-pupa which basically spills its guts and becomes translucent. The pupa is formed in the split skin of the pre-pupa in an earthen cell. The pupa resembles the beetle except the wings and legs are held tightly. In about 2 weeks the adult beetles emerge. I don’t need to tell you what happens next as the cycle starts all over.
So what happened to make this summer’s beetle population so large. Virginia Tech Entomologist Eric Day explained, "Last year we had good rainfall in July, so the Japanese Beetles had very good establishment. We had good rainfall early in June, so then they had good emergence as well, too. So it kind of was a perfect storm of these weather conditions."
We had a pretty dry July this summer so that might help keep populations down next year.
Some control options are available. The Japanese Beetle traps are questionable. There is no doubt that they do trick the beetles to come to the lure and crawl in the bag. However, they bring in more beetles than they kill oftentimes. When I went to horticulture school, we were told by numerous instructors, if you want to use beetle bags place the trap in your neighbor’s yard. For sure, do not place the bags near their favorite treats, 50 feet away is good. I get a plastic container and fill half way with water and some dish soap. Early in the day when there is still dew, the beetles are a bit lethargic so it is pretty easy to take the lid and push them in the water. This is usually enough control most summers. Have to admit it thrills my soul to get them while mating! As with cancer, control is best when caught early. Adult beetles are difficult to kill by spraying. It also helps control the population if you forego irrigating your lawn when beetles are active.
Japanese Beetles fly long distances to feed so just because you have an abundance of beetles doesn’t necessarily mean you will have enough grubs to damage your yard. Damage is pretty obvious with patches of dead grass. To get an accurate count you need to survey your lawn. Do this in August/September or April/May when the grubs are feeding high. Dig up random samples of patches 8” by 8” and 3” deep near damaged sections of lawn. Turn this over on a piece of paper and search the roots and the hole for grubs. Ten grubs per patch might be cause for concern. Be sure to only count Japanese Beetle grubs. If you look closely at the raster or hind end, it is rounded and would appear as if tiny hairs are growing from it.
If you have decided you want to take active control action, contact your Extension Agent for effective methods and more importantly, timing of application. The window is narrow and applying a pesticide outside of that time frame is wasting product and money and harming beneficial insects. Doing nothing is always an option. For more information on Japanese Beetles download Virginia Cooperative Extension Japanese Beetles publication at