Questions about the Japanese beetle have dominated calls to the Purdue Extension office in Whitley County in the past 2-3 weeks. I suspect the same is true in many northern Indiana counties. Just where did these beetles come from, and why are they such a big a problem this year?
Dr. Cliff Sadof, Purdue entomologist, recently reported in the Purdue Landscape Report, purduelandscapereport.org, that Japanese beetles have once again become one of the more important problems in Indiana landscapes after almost a decade of low numbers.
“Although the reasons for this resurgence is unclear, part of the story is that for the past few years there has been enough moisture in the soil during the mid-summer egg-laying period to allow most of the beetle eggs to hatch into grubs,” he said. “These grubs flourished and grew into the adult beetles that emerged the following year.” In June, the warm wet weather created ideal conditions for last year’s grubs to become adult beetles and crawl out of the ground.
Sadof said that Japanese beetles rarely, if ever, uniformly infest a landscape. “There are always areas with heavy damage and areas with light or no damage at all,” he said. “You can determine if Japanese beetle will be a problem in your area by inspecting their favorite plants, like roses, grapes, elms, crabapples and flowering plums to see if they are in your area.”
Of course, by now most homeowners have noticed the damage caused by Japanese beetles in their own landscape. You may have noticed the selective feeding by these bronze and metallic green colored beetles on some plants, but not on others. You may also have seen the alarming leaf damage that results in what we call “skeletonized” leaves – or leaves that have only the veins remaining.
On smaller plants, you can tediously pick off beetles and drown them in soapy water. However, many homeowners will choose to use insecticides on adult Japanese beetles. If this option is chosen, what is the risk to pollinating insects and other beneficial insects?
“One of the best ways to protect pollinators and your flowers is reduce the number of times you spray your flowers,” said Sadof. “Rather than apply an insecticide when you see the first beetle, wait until you see some beetles starting to feed — then wait until more enough beetles arrive before you apply your second spray.” Sadof said this should reduce the number sprays during the spray period.
Adult Japanese beetles typically have a flight period from early June through August, with heaviest populations in July. In Indiana, Sadof said the peak flight lasts for about 6 weeks. Near the end of their adult life, they will mate, the females will lay eggs in the soil, and the adults will die.
When these eggs hatch into larvae, they become another familiar pest: white grubs. White grubs chew on grass roots, causing patchy death of turf. This may be a good year to consider applying a preventive or early curative grub control product on your home turf. Preventive products should be applied May through the end of July. Early curative products can be applied from early July through early September.
The additional challenge with these products is getting the insecticide into the root zone. This requires rain, or watering.
A basic strategic concept with pests is to attack them at their most vulnerable stage. For white grubs from Japanese beetles, this is shortly after egg hatch in mid-July through August.
If you have not experienced turf damage in the past, a reasonable strategy would be to monitor your turf for grubs, and treat only if needed. Dig a square foot patch about 3 inches down in 2 or 3 areas in your lawn and count the grubs (they will be very small).
The Purdue Extension publication entitled, “Japanese Beetles in the Urban Landscape,” states that insecticide applications are generally not required if soil sampling reveals 5 or fewer grubs per square foot.
Keep in mind that even if you have adult beetles in your landscape, it does not automatically follow that all those beetles will lay eggs in your turf. Adults can fly 1-2 miles for feeding or egg-laying.
White grubs come from Japanese beetles, but also from other insects. Other examples include the masked chafer (a brown beetle), and the green June beetle. Their grubs can also damage turf.
For more information, find the aforementioned publication on Japanese beetles at https://mdc.itap.purdue.edu/. Search for a related publication, “Managing White Grubs in Turfgrass,” at the same site.