Japanense Beetlemania Coming Soon To A Garden Near You

It was a rare occasion when on Memorial Day I was able to sit in front of the house and just gaze at my gardens. I had just finished cutting back my six rose of Sharon (shrub hibiscus) that are along the road. As I looked at the pruning job, I realized that in just a few short weeks my nemesis would return and begin to feed on them as well as my white birch and some of my roses. I knew that just several inches down in the soil these creatures were feeding on grass roots and slowly, ever so slowly, moving upward as they fattened up. Then, just before the Fourth of July they’ll emerge from the soil as the metallically colorful insect the Japanese beetle.

They return to these bushes every single summer, and we do battle. They have no known natural enemies that are particularly effective, and our arsenal of chemical and organic controls is very limited. But, if you understand their life cycle and the few things that do deter and even kill them, you can get the upper hand. And now, fellow gardeners, is the time to take up arms and have a plan.

It’s critical to understand the life cycle of this insect if you ever want to manage them. Then you need to know what works, how it works and when to use it. And even before all that, you need to figure out how averse you are to chemicals, as the chemicals that do the best job in JB control are not benign. They can affect soil organisms, bees, pollinators and a wide swath of nontarget, and possibly beneficial, insects. Good thing you have a few weeks to ponder this.

Let’s start with the JB life cycle. After feeding on your gorgeous roses, hibiscus, some of your vegetables and shrubs, the male beetles seek out the females, have some fun, and from mid to late July the females drop to ground and deposit their eggs in the soil. This is usually done very close to where they were feeding and explains why they seem to come back every year at the same places. These eggs mature for a period of weeks then grow into small white grubs that feed on roots of many plants—but they do love grass roots.

As the grubs grow and continue to feed, they move in a circle. This is how the telltale circles of dead lawn show up and expand. Crows and some other birds may be seen feeding at the edges of these circles as the grubs continue to feed and you may find areas of lawn dug up where opossums come out at night and dig for the grubs. The feeding continues until the soil cools down in the fall and the grubs dive for cover about a foot down where they remain in the winter. Then the following spring the grubs migrate up as the soil warms and begin feeding on the grass roots again until they emerge as beetles late in June or early in July.

There’s a common thread here and that’s the lawn. Plant your rose bushes in beds close to your lawn and you will start a cycle of grubs and beetles that will be hard to stop. Remember that important relationship between the grubs needing the lawn roots to feed on and where the females drop to the ground to lay their eggs. Perennial hibiscus plantings such as Rose of Sharon near the lawn can have the same result.

And this is where we talk about those yellow hanging Japanese beetle traps that are guaranteed to work. Well, not really. These traps have both a floral lure and a pheromone lure in them, offering just the right scent and just the right hint of sex to draw every JB in town right to your property. So, yes, these traps work at attracting huge numbers of beetles to your property and the traps, and that may give you some satisfaction as you empty them each day. But there are also thousands and thousands of beetles that are attracted from blocks away that head for the traps but get sidelined to other tasty treats in the area. Bottom line, there’s plenty of evidence showing that these traps are a very, very bad idea and should not be used.

Now, for those not averse to the use of chemicals there is one pesticide that’s very effective at controlling JBs, and it’s called imidacloprid (trade name Merit). This chemical is an nicontinoid and not without its share of controversy. It’s applied to the lawn late in the spring or in the early summer, and it kills the eggs and grubs of the beetles later in the season before they can cause damage the following year. The problem, other than the environmental ones, is that it has to be applied nearly a year in advance of potential beetle emergence. If it’s applied this year, your beetle problems can be dramatically reduced next year. You cannot apply it yourself though, and it must be applied by a professional certified pesticide applicator.

In spite of landscapers’ and lawn care company’s programs, I don’t think imidacloprid has to be applied every year. My feeling is that it can be applied once and after that only again when it can be shown that the white grub population in the lawn is high enough to require another application. That could be two, three maybe even four years later.

There’s another pesticide that can be applied to the lawn that’s called trichlorfon. (Grub Killer Plus is one brand name). There is a short window when this chemical is effective in early summer and again late summer when the grubs are near the surface of the soil. When the timing is right it kills the grubs on contact, but it’s effective for only about 10 days. Good for the environment but not so good if your timing is off.

And then as a balance there is soap and water. I put a few drops of Joy dish soap in about six ounces of water and just go from plant to plant tapping the leaf or bud the beetle is on. In most cases the beetle simply drops into the soapy water and dies. Simple, safe, but time consuming. This can also be difficult if you wait too late in the day because the beetles like to climb higher in the plants as the day warms up.

Pyrethrin is a botanical insecticide that can also be effective, but it works only on contact and has no residual effects. It will not work on sunny, hot days (over 80 degrees) and some plants may be sensitive to it. And while organic, it does not know if it’s been sprayed on a honey bee or a beetle, so be cautious.

Spinosad (Captain Jack’s Dead Bug and Monterey Garden Insect Spray) may—keyword is “may”—be effective on JBs, but I have had mixed results and have seen mixed reports from other gardeners. It must be ingested by the beetle, so it needs to be sprayed on the flowers, buds and foliage, and can take up to 48 hours to work.

Neem oil is another option, but again, there are mixed results. It probably won’t kill the beetles but it does seem to diminish their desire to feed on plants that it’s been sprayed on.

And lastly there’s carbaryl (trade name Sevin). This pesticide is very effective on these beetles and seems to have a slight residual effect, so it needs to be applied only a few times. The down side is that Sevin is very, very toxic to bees. If you are going to use it, take care to apply it early in the day or late in the day when bees may be less of an issue.

Sorry, no simple answers. But lots of choices and maybe one from column A and one from column B. Have a better solution? Please share it with us. Keep growing.