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Hungry Japanese beetles are a formidable foe for Wisconsin vineyard owners
Since they first appeared in the United States over 100 years ago, Japanese Beetles have made their presence known in gardens, orchards and vineyards across much of the eastern and midwestern states.
Just the sight of the shimmering, metallic green and copper bodies devouring the leaves, flowers and fruits of over 300 different plants on their menu is enough to raise the blood pressure of gardeners and farmers alike.
Among the destructive insects' favorite foods are fruit trees and grapevines. Vineyard and orchard owners across Wisconsin have been waging a constant battle against the voracious appetites of these mobile pests that can defoliate plants, impacting the yield and quality of the fruit.
Fruit growers battle beetles and disease
For years, Adam Rabe has been helping Wisconsin vineyard owners protect their precious crops against the imported pests. Rabe, who owns Dirt Road Vineyards, is a vineyard management and consulting business in southeastern Wisconsin. His business provides consulting services as well as installing vineyards, brokering fruit, handling pest and disease control and harvesting the grapes.
He spends a lot of time on the tractor, prowling the rows of his client's grapevines, scouting for pests and disease. While the growing season started dry and slow, constant rain showers have amped up the disease pressure.
'A lot of growers earlier had been dealing with the black rot and now we're seeing a lot of downy mildew that thrives with heat and moisture,' Rabe said.
For the past three years, Rabe says pressure from Japanese beetles has been minimal in the southern part of the state.
'I've been working at Cambridge Winery just southeast of Madison this week, and I haven't had to spray for beetles at all this year, or last year,' he said. 'However, my clients in the driftless region have definitely been spraying.'
Christelle Guédot, a fruit crop entomologist and extension specialist in the Department of Entomology at the University of Wisconsin–Madison, says that Japanese beetles were once the bane of gardeners and farmers in the southern half of the state, but their range has since spread northwards.
'Over the past 10 years, they have been detected by the Wisconsin Department of Agriculture, Trade and Consumer Protection up in Bayfield. We also trap many hundreds in Spooner and we see them now in Door County,' Guédot said. The beetles have also been wreaking havoc in the northeastern counties, including Oneida and Vilas.
While many growers swear the beetles seem to be increasing in number, Guédot says that's not the case.
'Their populations fluctuate from year to year depending on temperature and moisture levels, particularly in the spring and some in summer. If it is too dry in the spring, as it was in 2023, they may not emerge from the soil in high numbers,' she said, noting the low population of beetles across the state due to drought.
Battling the small but mighty beetle
Controlling and eradicating Japanese Beetles can be a tall task. These invasive species breed rapidly and feed for approximately six weeks during July and August during which they destroy plants — including roses, raspberry bushes, perennial hibiscus, fruit trees, and grapevines and more.
If they run out of a food source in one area, they take wing and head elsewhere. Rabe says Japanese beetles are active flyers capable of covering up to five miles with help from the wind.
Once they settle in a vineyard, beetles set their sights on the upper part of the canopy where established vines can generally tolerate some major damage. However, young vines are more susceptible as beetles devour the green plant tissue, until the skeletonized leaf resembles a fine doily. According to the UW Fruit program, severe defoliation of grapevines by Japanese beetles can weaken the plants, potentially impacting yield and fruit quality.
Guédot says some growers see Japanese beetles as helpers for the thinning of the canopy after fruit set to expose the clusters of grapes to the sun. However, commercial growers will spray insecticides when defoliation levels reach a certain threshold.
"The threshold for treatment for some growers may be 5-10 beetles on a vine, but I'd say 30% defoliation on a leaf is about the average threshold,' Rade said. "For me, spraying insecticides is on an 'as-needed' basis. And it's tricky to find spray windows. I try to spray in the morning as much as possible or in the late evening when the beneficial (insects) and butterflies aren't around as well."
Handpicking is the strategy recommended for home gardeners. For commercial growers with acres and acres of vines, there are other weapons in the arsenal.
Many wineries across the country take pride in sustainable farming practices. Battling Japanese beetles may include a multi-prong approach.
Some growers may install row covers to physically block beetles from reaching the vines during peak infestation periods. While it is effective, it's costly.
Rabe says pheromone traps, advertised to attract and capture adult beetles, may bring more beetles into the area instead of decreasing the numbers in your vineyard. However, some growers may use the bags to attract beetles to some sacrificial rows on the edges of the vineyard, then spray them in that concentrated area.
Other growers may opt to use biological controls. Beneficial nematodes may be introduced into the soil to target the larvae stage of the beetles, preventing them from maturing into adults. Natural predators such as birds like robins, starlings, and crows are known to eat both adult beetles and their larvae (grubs), according to Orkin. Certain insects, such as predatory stink bugs, assassin bugs, and ground beetles, also prey on Japanese beetles.
UW-Madison entomologist PJ Liesch told the Green Bay Press-Gazette there's also some evidence that a small parasitic fly might be helping with Japanese beetle populations. The female winsome fly lays her eggs just behind the head of the beetle. Once those eggs hatch, the larva burrows into the beetle and kills it.
Guédot says many vineyards have grass growing between the rows and around the perimeter. However, it creates ideal conditions for Japanese beetles to lay eggs since they prefer to lay eggs in mown grass and in moist soil. She says research shows that tilling or managing ground covers in the row middles may impact beetle populations.
Rabe says a large percentage of vineyards use some kind of insecticides to control pest populations and plant disease. Many take care to select and apply chemicals to minimize the impact on insects and the environment.
"There are a lot of chemicals on biological and organic sprays out there that are coming on the market, as that's the kind of demand out there," he said. "Unfortunately, in our area, pesticides are a necessary evil that we have to use if we want to have a crop."
Beetles do have a preference for certain cultivars
Guédot says Japanese beetles do prefer certain plant species over others, including grape varieties and certain apple varieties. And while growers select their fruit varieties for many reasons — taste, hardiness, consumer preference and pest resistance — there are some varieties among raspberry, blueberry, grape and apple cultivars prized by the pests.
Fruit and vegetable grower Bill Zeleske says Japanese beetles have shown a penchant for certain fruit varieties on his small farm Fondy Fruits & More.
"A couple of years ago the beetles made lace of all the leaves on my table grapes and they like to decimate the leaves on my Honeycrisp apple trees," Zeleske said. "I guess they have their preferences, too."
The Fond du Lac-based grower has also planted more disease-resistant fruit crops, including new apple tree varieties. He does, however, spray his fruit for pests.
"We have a lot of trouble with wasps eating fruit, especially the grapes and blackberries. I sometimes see them in the apple and pear trees," he said.
A new pest is on the horizon
Rabe says newer pests that burrow into fruit are working their way towards Wisconsin. The spotted wing drosophila has been spotted in northern Illinois and Michigan. It's a pest that could devastate grape crops in Wisconsin.
According to the University of Michigan, the spotted wing drosophila can cause significant damage to grape crops. The invasive fly lays eggs in healthy, ripening fruit, unlike most other fruit flies that attack damaged or overripe fruit. The larvae then feed inside the fruit, causing it to soften and rot, leading to significant losses for grape growers.
"Growing fruit in Wisconsin and the Midwest is very challenging," Rabe said. "It's really having boots on the ground and eyes open and having the resources like folks in the Wisconsin Winery Association to help. They can help growers get a handle on issues before they have to deal with them.
"It's pretty sad after growing your fruit for a whole year only to lose it because you didn't go out and scout and recognize the issue and take care of it ahead of time."
This article originally appeared on Wisconsin State Farmer: Japanese beetles are a formidable foe for Wisconsin grape growers
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