The Winsome Fly, Istocheta aldrichi: A unique biocontrol agent for Japanese Beetle in Minnesota

W.D. Hutchison^1,2, E.K. Buchholz³, E.L. Meys⁴ and S. Wold-Burkness¹

Dept. of Entomology, University of Minnesota¹, UMN Extension Service², UMN Landscape Arboretum³, & University of Illinois, Champaign-Urbana IL⁴

The Winsome Fly, Istocheta aldrichi (Mesnil, Diptera: Tachinidae), is a parasitoid of Japanese beetle (JB), Popillia japonica), which in recent years has become a significant economic pest of several Minnesota fruit crops (Ebbenga et al. 2023). In addition, with adult feeding damage to >300 host plants, JB is an ongoing pest of concern within urban gardens and ornamental plants and turf within the urban, residential landscapes in Minnesota. The Winsome fly (WF) was introduced as a biological control agent in 1923 in New Jersey, just 6 years after the accidental introduction of Japanese beetle (Shanovich et al. 2021, Althoff & Rice 2022). Since 1923, there have been numerous detections of WF in several Northeastern states and it is well established in Quebec and Ontario, Canada. WF is considered one of the more effective biological control agents for JB due to its potential impact on JB populations and its ability to adapt to a variety of climatic conditions (Potter & Held 2002, Shanovich et al. 2021, Althoff & Rice 2022).

Description & Life Cycle

Winsome Fly adult — Fig. 1. Winsome fly adult. (photo: Univ. of Maine, Extension)

Adult flies are small (5 mm), gray to beige, and about the same size as a housefly. Winsome flies emerge about a week before the Japanese beetles and feed on nectar-producing plants. Female flies seek out Japanese beetles and typically lay one egg just behind the head (pronotum) of the beetle. However, multiple eggs can be laid on one beetle, particularly when beetle populations are low. Eggs are pearly white and are easily seen. Within a few days of egg-lay, the parasitic larva will hatch, and with “serrated” mouthparts bore through the host cuticle and begin feeding. As the larvae develop through three instars, the beetle becomes increasingly lethargic, stops feeding in 3-4 days, and dies within 5-10 days. WF larvae typically pupate within the JB cadaver, and remains in the cadaver to overwinter. By mid- to late-June, the first WF adults emerge from the pupal stage, feed on floral nectar, mate and begin seeking JB adults for oviposition. There is only one generation per year.

Winsome fly eggs on Japanese beetle — Fig. 2A. Winsome fly eggs laid just behind the head (pronotum) and hind wing (elytra)of Japanese beetle. The distinctive white eggs are easy to observe in the field. (photo: E.R.H. Cervantes) B. WF eggs on mating pair of Japanese beetles. One or more eggs are laid on each beetle; eggs are nearly always laid on the pronotum, but may be found on wings. Overall, WF prefers to lay its eggs on female JB. (photo: J.E. O'Hara, 2014)

Winsome fly larva and pupa — Fig. 3A. Winsome fly larva stage (fully grown last instar), which is nearly the size of the interior of a Japanese beetle abdomen. B. Winsome fly pupa, and overwintering stage of the parasitoid. Following completion of the 3rd larval instar, WF will usually pupate within the Japanese beetle cadaver. Occasionally, the last instar will “cut out” of the cadaver, then pupate in preparation for overwintering. (photos: Jacques Lasnier, Quebec)

History

During 1998 to 2006, several releases of WF (pupal stage) were made in Minnesota, by the USDA and Minn. Dept. of Agric. (MDA). The majority of WF were released at Theodore Wirth Park, Minneapolis. The first detections confirming establishment, by the MDA, were in 2004. Until recently, only low levels of WF parasitism had been reported in Minnesota, including 0.01-10% during 2017-2018 in Washington and Dakota county apple orchards (Shanovich et al. 2021). Although parasitism rates have been historically low in Minnesota, samples from the University of Minnesota’s Landscape Arboretum in 2022 confirmed a peak of 14-29% parasitism (E. Buchholz). In 2023, additional monitoring for WF found peak parasitism rates of 33-55% at the Arboretum, and 15-39% at 3 commercial vineyards (Hutchison et al. 2023). This recent experience generated new research in 2024 to further estimate the growing biocontrol impact of WF on JB populations in Minnesota. In addition, the results to date indicate that WF is adapting to Minnesota’s climate, including warmer winters, and that the fly's emergence phenology is better aligned with that of Japanese beetle emergence (also recently observed in Quebec, Canada; Gagnon et al. 2023).

Winsome Fly Attributes that Contribute to Biocontrol of Japanese Beetle

During the past 3 years, we have observed several WF traits that appear to be contributing to the ongoing success of this parasitoid in Minnesota, including:

The timing (phenology) of the emergence of WF is now aligned well with the phenology of JB adult emergence (e.g., from late June to early August, southern Minnesota)
Effective adult searching ability, as indicated by high egg-lay rates when JB host densities are low (e.g., early or late season), and 30-55% parasitism at higher host densities (July)
Although parasitized JB will die within 5-10 days, it’s important to note that due to the parasite’s larval growth and damage to the host’s internal organs, JB stop feeding within 3-4 days (lethargic behavior becomes obvious)
With feeding cessation after 3-4 days, much less foliar damage is done by parasitized JB adults
WF has a significant preference for laying eggs on JB females; when females are parasitized, they do not lay their full complement of eggs (normally ~80 eggs/female)
WF therefore has the ability to reduce the current year’s JB population (direct mortality), and reduce JB egg-lay during summer/fall, resulting in lower JB populations the following year

Summary

As we continue to learn more about the geographic distribution and biology of WF in Minnesota, we will update this profile. Given the recent range expansion of WF in Minnesota and western Wisconsin, it appears this parasitoid is well positioned to continue to have a beneficial, biocontrol impact of JB, not only in urban, residential landscapes but also in several agricultural crops. Some of the higher parasitism rates to date have been recorded in commercial vineyards and apple orchards. More research is needed to better understand how WF mortality, and its impact on reducing JB female fecundity, can have long-term benefits for pest suppression.

Funding: This research was funded in part by a USDA-MDA Specialty Crop Block Grant, from 2021-2024.

Selected References

Althoff, E.R. & K.B. Rice. 2022. Japanese Beetle (Coleoptera: Scarabaeidae) invasion of North America: history, ecology, and management. J. Integ. Pest Mgmt. 13(1): 2, https://doi.org/10.1093/jipm/pmab043

Ebbenga, D., S.J. Wold-Burkness, E.C. Burkness, and W.D. Hutchison. 2023. Japanese Beetle: an Emerging Pest of Fruit Crops. FruitEdge. https://fruitedge.umn.edu/japanese-beetle/japanese-beetle-emerging-pest-fruit-crops

Gagnon, M-E., J. Doyon, S. Legault & J. Brodeur. 2023. The establishment of the association between the Japanese beetle (Coleoptera: Scarabaeidae) and the parasitoid Istocheta aldrichi (Diptera: Tachinidae) in Québec, Canada. The Canadian Entomol. 155: e32, 1-11.

Hutchison, B., E. Meys, & E. Buchholz. 2023. Long-term biological control of Japanese beetle: A Winsome Fly update. Minnesota Fruit & Vegetable News. https://blog-fruit-vegetable-ipm.extension.umn.edu/2023/08/long-term-biological-control-of.html

Pelletier, M., Legault, S., Doyon & J. Brodeur. 2023. Where and why do females of the parasitic fly, Istocheta aldrichi, lay their eggs on the body of adult Japanese beetles? J. Insect Behav. 36: 308–317. https://doi.org/10.1007/s10905-023-09841-8

Potter, D.A. & D.W. Held. 2002. Biology and management of the Japanese beetle. Ann. Rev. of Entomol. 47:175-205.

Shanovich, H.N. et al. 2019. Biology and Management of Japanese Beetle (Coleoptera: Scarabaeidae) in Corn and Soybean. J. Integ. Pest Mgmt. 10(1):9; 1-14. https://academic.oup.com/jipm/article/10/1/9/5454734

Shanovich, H.N., A.V. Ribeiro & R.L. Koch. 2021. Seasonal abundance, defoliation, and parasitism of Japanese beetle (Coleoptera: Scarabaeidae) in two apple cultivars. J. Econ. Entomol. 114: 811-817. https://doi.org/10.1093/jee/toaa315

Published August, 2024

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