Conversations about Japanese beetles seem to have calmed down in 2020, but that doesn’t mean they aren’t still present.
Bryan Jensen, entomologist with the University of Wisconsin-Madison, said many farmers are simply growing accustomed to seeing the invasive insects, which has garnered them fewer headlines.
“The first few years it was a topic of conversation, like one over a cup of coffee,” Jensen said. “Now, people are getting used to seeing them. They realize what the level of concern may be.”
Jensen said he saw higher numbers of beetles in southwest and west central parts of Wisconsin, but now people have a better understanding of what damage can occur from the insects.
In his town of Jacksonville, Illinois, Ken Johnson said Japanese beetles weren’t as much of an issue in 2020. The University of Illinois Extension entomologist said population trends are often cyclical, and people may have simply started getting used to the pests. He said fluctuations will still occur, but feels the peak populations seen in recent memory aren’t likely.
“When they start moving into an area, you get a real big spike,” Johnson said. “It builds up and up and then those numbers start to decline until you reach an equilibrium which is a lot lower than it was when they first showed up. In Illinois, we are kind of getting to that.”
Johnson said wet summers during the egg-laying period are beneficial to the beetles, as it gives them more areas to lay their eggs. Larvae tend to struggle more in drought conditions. Over the winter, he said hard freezes are helpful for killing off larvae as well, but having snow cover will likely keep the ground insulated and protect the insect.
Defoliation is the major issue facing farmers. Jensen referred to the economic threshold for soybeans and corn. For soybeans, if the plant is showing 30% defoliation prior to bloom, or 20% defoliation after blooming, an insecticide may be needed. The mark for corn is three or more beetles per ear with silks clipped to one-half inch while pollen shed is under 50% complete.
Jensen said anticipation is the most important aspect of managing Japanese beetles, but waiting isn’t going to be too detrimental to a crop.
“The big thing is getting it on the calendar,” he said. “The good part is they are showy, so you can easily scout for them. Along field edges is always a good first place to look. People need to realize, though, it’s not catastrophic. If you don’t make a decision today, the economic thresholds are on the conservative side, so you don’t have to get too excited. Just be scouting on a regular basis.”
Outside of crops, Johnson also discussed protecting yards and flower beds from the insects. He said hand picking tends to be the most effective way to manage Japanese beetles. For those with rose bushes, another option is to cover them to keep the bugs off.
“If you do a good job of keeping those beetles off your plants and keep the populations low within the first several weeks, you tend not to have as much damage,” he said. “As those plants get damaged, they release pheromones that draw in more beetles, so it just builds on itself.”