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Preparing for the emergence of Japanese beetles in fruit crops

Japanese beetles feeding  and mating on apples leaves, July, 2020. Photo: Annie Klodd

 

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production

Reviewed by Marissa Schuh, Extension Educator - Integrated Pest Management

 

Japanese beetles (JB) will begin emerging from the soil in Minnesota around the 3rd or 4th week of June. At this time, fruit growers should prepare their management strategies for JB and make any necessary purchases to control them. This involves understanding when to spray for this insect pest versus when it is alright to let some feeding occur. This article outlines cultural management strategies and best practices for using insecticides for Japanese beetles.

Cultural and non-chemical management practices for Japanese beetles:

Researchers have conducted, and are currently conducting, numerous studies on how cultural management practices can impact JB populations. These practices were discussed in-depth in the Cold Climate Grapes Webinar Series hosted by UMN and UW on June 2, 2021:

 

I have summarized some key findings here, but please refer to the webinar for citations and details.

Monitoring: Traps can be used to monitor for Japanese beetles, but as researchers have clearly found, they attract more beetles than they kill. The traps fill up quickly, and beetles that cannot fit in the traps are attracted to the area and then feed on the plants nearby. Therefore, they should only be used temporarily for early-season monitoring if desired, not as an attempt to control JB. Many fruit growers forego traps and "monitor" for JB simply by watching for the first feeding damage on the plants.

Irrigation: Withholding irrigation between rows will reduce larval density, as JB adults prefer to lay eggs in moist areas. Since 2021 is a drier-than-normal growing season, perhaps this will help reduce JB egg-laying.

Groundcover management: Your groundcover management practices - mowing, mulching, and tilling - can influence Japanese beetle pressure on the farm. The adult beetles need easy access to the soil to lay eggs.

  • Raise mowing height to at least 3 inches to reduce larval density in the soil. JB adults are discouraged from laying eggs when grass is taller.
  • One study found that areas treated with hardwood mulch or rubber mulch had lower larval density than areas with bare soil or bark mulch. It is logical that fields with landscape fabric between rows, such as in some blueberry operations, may also have lower JB larval density. See photo below.
  • While tillage is not a feasible option in most orchard and vineyard settings, tilled soil has been found to have lower larval density because it discourages soil egg-laying by adult JB compared to when groundcovers are used between rows. Therefore, day neutral strawberry fields with tilled interrows may have lower JB egg laying than fields with cover crops between rows. Additionally, it may discourage the presence of adult JB as well. One study in blueberries found that there were fewer adult JB around the field perimeter when the perimeter was tilled rather than cover cropped. However, this was less effective in the field interior when the same treatments were compared. 
  • This does not mean that we are recommending tilling between rows in an long-term perennial systems like orchards and vineyards. Groundcovers are important and have numerous benefits. It is simply a factor that should be considered when developing your groundcover management plan.

Landscape fabric and woodchip mulch in a blueberry field near Stillwater, MN. Photo: Annie Klodd.

 

Biological agents: Despite marketing claims, no biological agents (milky spore, fungal pathogens, parasitoids) have been found to significantly reduce Japanese beetle populations in controlled trials. While some level of biological control may occur, the overall impact is minimal compared to the massive population density of JB in the environment. In other words, just because we find parasitic eggs on a few JB, that does not mean that it's enough to impact the JB population in an economically meaningful way. Therefore it is not economically beneficial to rely on biological methods at this time. As more research on best practices for using biological controls becomes available, these recommendations may evolve.

How much does leaf defoliation matter, really? Good news - researchers have found that significant leaf defoliation by JB is required in order for this insect to impact grapevine health and growth. Similar research has been done at UMN on apples, which you can read about here. While the leaf loss can look severe, it may not be causing as much damage than we think. But you don't have to take my word for it: Watch this webinar recording, starting at minute 12:00:00 to hear a detailed, 3-minute discussion of this. 

Three different levels of Japanese beetle defoliation on grape leaves. Left to right: 2.10%, 6.79%, and 10.02% defoliation. Photos: Dominique Ebbenga.

 

When to apply insecticides: 

In the webinar above, Dr. Christelle Guedot at University of Wisconsin-Madison recommends applying insecticides for JB only once 30% defoliation has been reached. University of Minnesota has not yet released an official recommendation on the economic or action threshold for JB, because research is currently ongoing to help answer this question.

Tips for selecting insecticides: 

Based on conversations with growers, here are three tips that I think will be helpful at this time:

First, be sure that the insecticides you are applying for JB have strong effectiveness on this species. For example, did you know that Admire Pro is only considered "somewhat effective" on Japanese beetles, while Assail is rated "good?" Organic growers have fewer highly effective options for JB control, but Surround has relatively strong efficacy while Neemix has lower efficacy. Please make use of the Midwest Fruit Pest Management Guide (pages 39-41 in 2021 version) to find out which products are most effective; this guide does include organic pesticides in addition to the conventional options. 

Secondly, resist the temptation to base your insecticide choices on what you already have on the shelf rather than finding what works best. 

Third, incorporate products in your rotation that have some residual activity. In other words, some products remain active for 2-3 weeks, while others such as carbaryl (e.g. Sevin) work quickly but only knock the JB population back for a few days. Incorporating insecticides with residual activity will reduce the number of times you have to spray. These products include:


 



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