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Long-term Biological Control of Japanese Beetle? A Winsome Fly Update

Bill Hutchison, Extension Entomologist, Ellie Meys, Undergraduate Student, & Erin Buchholz, Plant Health Specialist, Landscape Arboretum, University of Minnesota

Although Japanese beetle (JB) adults continue to be active throughout August to early September, we shared good news last week that beetle numbers on most host plants/crops are beginning to decline. The gradual reduction in adult populations is due to several factors, including the end of local emergence “curve” this time of year, but also reflects the impact of several natural mortality factors. One of these factors is biological control, specifically the Winsome fly (WF) (Istocheta aldrichi: Family Tachinidae), which lays its distinctive white eggs quite conspicuously just behind the head of JB adults (i.e., the pronotum; see photo, 2020). As noted in the photo, the fly can lay one, two, three, or more eggs on one beetle; however, only one parasitic larva will survive to kill the beetle. Within a few days of egg-lay on JB, the parasitic larva will hatch, and with “serrated” mouthparts bore through the host cuticle and begin feeding. Although, the biology of the fly has not been well-studied, most reports indicate JB adults will die within 6-7 days – not quite as fast acting as an insecticide, but still impressive.

(JB adult with 2 Winsome fly eggs on pronotum, Mark Holle, Shoreview (Ramsey Co) MN, 2000)

In addition, previous reports and recent field findings indicated that WF females prefer to lay their eggs on JB females. From a population ecology and IPM perspective, this is a highly valuable trait. Females dying within 6-7 days will not likely be able to lay their full complement of eggs, thus providing population suppression within the same summer, and potentially contribute to longer-term suppression the following year. 

Winsome fly in Minnesota: From 1998 to 2006, several releases of WF were made in Minnesota, by the USDA and MDA. The majority of releases were conducted in Dakota, Hennepin and Ramsey counties. In addition, until recently, only small percentages of JB adults were found to be parasitized by WF, including 1-10% in 2017-2018, respectively, in apple orchards (Dakota, Washington counties; see Shanovich et al. 2021). By contrast, during the past two summers, we have observed a significant and consistent increase in parasitism rates by WF. Recent data from the Landscape Arboretum (Carver Co.), in ornamental garden settings, as well as WF parasitism data from two commercial vineyards and one research vineyard, are summarized in Table 1. Moreover, we verified (as with other studies) that female JB were preferred for WF egg-lay, compared to males, with preference for females ranging from 55-81%.   

Table 1. Percentage parasitism by Winsome Fly on Japanese beetle adults, for selected MN locations, 2022-2023

Year                      Location*                                        Percentage (%) Parasitism by Winsome Fly

                                                                                              Avg.       Peak      Preference: Female vs M (%)

2022                     Arb-Sensory Garden Trap                              8           14                          ----

                              Arb-Woodland Azalea Trap                         13           29                          ----

2023                     Arb-Sensory Garden Trap                            23           33                          ----

                              Arb-Woodland Azalea Trap                         22          59                          ----

                              Alexis-Bailly Vineyard Trap                        7            15                         81

                              Hort. Res. Center Vineyard Trap                28           39                         55          

                              Sovereign Estate Vineyard Trap                 11           18                         73

*Arb sites = Landscape Arboretum, University of Minnesota (Carver Co.); Cooperating wine-grape growers: Alexis-Bailly (Dakota Co.); Hort. Res. Center (HRC), Chanhassen and Sovereign Estate, Waconia (Carver Co.)


Although the results shown in Table 1 are based on JB collected in commercial JB traps, we have also observed similar parasitism rates for JB collected directly from plants (wild grape). To date, WF activity, as measured by the presence of eggs on JB, has been limited to reports from Hennepin, Ramsey, Dakota, Washington and now Carver counties. More work is underway and planned for 2024, to extend our survey within and beyond the Metro region, to further understand the impact of the parasitoid. For more details on JB management in fruit crops, growers should review the JB Pest Profile on the FruitEdge web page.

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