Japanese beetle (Popillia japonica) is an invasive species first detected in Minnesota in 1968. In recent years, JB adult activity has been most active in the Twin Cities region, but the beetle is also found in southeast MN, and in selected counties in central MN. In recent years, Japanese beetle (JB) populations have been increasing significantly, primarily in the southeast, and particularly in the 7-country metro. Fruit growers have noticed heavy infestations on a variety of crops, including: raspberry, blueberry, apples, plums and wine grapes.
Japanese Beetle Emergence and Overwintering Survival
Beginning July 1st, the first JB adults began emerging (Rosemount, Hastings), with adult counts in raspberries, wine grapes and traps, increasing steadily this week. As many will recall, this past winter included another “polar vortex” event in February. Coincidently, this past winter also included our first JB “grub stage” overwintering survival study.
New research is underway by Dominique Ebbenga (PhD student) to better understand the ecology of JB in Minnesota, and develop new tools for managing the beetle in fruit crops. One of
the first research objectives is to better understand overwintering survival – and 2018-2019 was a good year to get started.
Overwintering survival: Despite the winter weather, we observed high levels of grub survival this year. Survival numbers do vary by location; however, at our primary Rosemount location, October 2018 turf samples Innear an established fall raspberry stand averaged 26.3 grubs/sq-ft; the average soil depth was 6.5 inches, with some as low as 12”. The high grub number is substantial, but not surprising given the high (to very high) JB adult pressure we experienced last summer. It is also well known that JB adult egg-lay (turf areas preferred) occurs throughout summer to fall, and egg and larval survival is highest when ample soil moisture is present.
During the first spring sample (April 9), grub numbers had declined to 9.50/sq.ft, reflecting 36% winter survival. Compared to most insect species that overwinter in MN, this is a very high survival rate. Insects that can overwinter 5-6” below the surface, and with snow cover, are well-insulated from sub-freezing air temperatures, and this was likely a contributing factor to the high survival this past winter. Our most recent sample, as of July 8th indicated an average of 4.25 JB/sq-ft, including grubs, pupae and adults (with adults “hanging out” in the turf in preparation for emergence).
Even with only 4.25 JB/sq-ft remaining from last fall (16% survival), this estimate projects to ~185,130 adults/acre emerging at this location (at 43,560 sq-ft/acre). This may still be a slight over-estimate, given that some adults may still die during emergence from soil; however, thus far all adults found in soil samples to date are healthy and ready to go.
In contrast, we found much lower grub numbers at Hastings in turf samples on sandy soil, and lower numbers in a newly established raspberry planting (2018, Rosemount), where much less turf was available. However, adult JB are very mobile and even if adults aren’t emerging on your land, there is still a risk of migration into your vineyard or orchard from nearby areas. This is one example of the factors contributing to high JB numbers, even where vineyard or farm size is only 1-5 acres. This information will be valuable in developing alternative production systems, and an early-warning forecasting, degree-day model for JB emergence.
How Much Damage Is Enough to Warrant Control?
In addition to population forecasts, we realize growers need new information on JB defoliation levels that will lead to yield losses for several crops, and the necessary economic (or action) thresholds to be used to prevent yield loss.
Our research for raspberries and wine grapes is well underway this summer. Although it is too early to have specific thresholds at this time, we offer a few observations to consider:
For established perennial crops, it is likely that for most crops and varieties, the plants can withstand significant defoliation (e.g., .20 to 25%) before yield losses occur. This is based, in part, on research in other states (e.g., Virginia) on wine grapes. This is also based on our preliminary estimates last summer that indicated that what we estimate with the human eye often results in an over-estimate of what the actual defoliation may be. For example, what initially appeared on wine grapes/raspberry to be 40-50% defoliation, actually was closer to 15-20% defoliation using a standard leaf-area meter.
That said, it is clear from recent years that 1st-year wine grape stands and 1st year apple saplings should be watched more closely for JB feeding, due to the very low leaf area and a high level of JB feeding activity. Younger plants will likely require some insecticide use to minimize the risk of short-term and long-term injury and to preserve vineyard and orchard sustainability.
Management Recommendations for Japanese Beetle
Ongoing UMN Research on Fruit Variety Selection by Japanese Beetle
In 2018, we evaluated JB feeding damage on several MN wine grape varieties grown in Minnesota (Table 1). We found a wide range in beetle density and defoliation levels, but the higher defoliation levels were limited to 15-18%. These studies aimed to quantify mean density of JB adults present (meter-row samples), and the corresponding mean percent defoliation (per leaf basis). We are repeating and expanding upon the research this summer.
While these data do not demonstrate a high potential for immediate yield loss, the results can be useful when selecting future varieties for new plantings. The “new normal” for fruit pest management in Minnesota, is that invasive species are likely here to stay, and that more long-term planning and research will be essential to build more effective, sustainable IPM programs. Additional results will be published in future articles to assist growers in understanding the biology of JB, and how this information, along with additional pest management tactics can be useful for building sustainable IPM programs.