First Report of Halyomorpha halys (Hemiptera: Pentatomidae) in Montana, USA

Here, we document the first record for Montana of Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), a major agricultural and nuisance pest in many parts of the world. The insect was found overwintering in a private residence in January 2021 in Billings (Yellowstone Co.). A preliminary survey of sites in Billings from May to October 2021 confirmed the presence of H. halys within a radius of at least 11 km from the January detection. Putatively overwintering H. halys and adult bugs found across the city of Billings in the summer suggest the insect may have been present in the area prior to 2021. Surveys conducted in various regions of the state between 2013 and 2020 reported no detections of H. halys . Yellowstone Co. was previously surveyed in 2017, indicating the species likely arrived between 2018 and 2020 or was present before 2018 at densities too low to be detected. The summary of presence and absence records we provide herein is valuable for future monitoring and management efforts of this economically important species.

The brown marmorated stink bug, Halyomorpha halys (Stål) (Fig.  1A), is native to eastern Asia, including Japan, the Korean peninsula, and eastern China. It has become a globally distributed invasive insect, with the first introduced populations to cause significant agricultural damage occurring in the United States in 2010 Nielsen 2018, Ludwick et al. 2020). In North America, H. halys has a broad host range of over 170 plant species, including many horticultural and agricultural crops (Leskey and Nielsen 2018). It can also be a significant nuisance pest due to its propensity to aggregate and overwinter in human structures (Hancock et al. 2019

Surveys and Detections in 2021 and Early 2022
To estimate a preliminary extent of H. halys presence within Billings, five pheromone-baited traps (Fig. 1B) were deployed across the city between 14 and 20 May 2021. Four traps were on private residential properties, including the site of the initial detection, and one was on the property of a commercial garden center and nursery (Fig. 2). Trap design and protocol followed the recommendations of Acebes-Doria et al. (2018). In brief, transparent, double-sided sticky cards (PHEROCON stinkbug STKY dual panel adhesive trap, Trécé, Inc.) were secured to a stake or branch that held the card ~1 m above ground. Each card was baited with a dual lure of the two-component H. halys aggregation pheromone and the synergist, methyl (E,E,Z)-2,4,6-decatrienoate (PHEROCON stinkbug, Trécé, Inc.), a combination known to be attractive to H. halys adults and nymphs throughout a growing season. Traps were visually checked approximately weekly and the number of H. halys was recorded. Sticky cards were replaced every 4 wk and lures were replaced once after 12 wk. Traps were removed on October 25 after 5 d with minimum temperatures of 0°C recorded for the city (https://www. ncei.noaa.gov/). At least one adult bug was captured at each of the five Billings sites during the trapping period, with the first and last bug caught on 14 May and 5 October, respectively (Fig. 3). No nymphs were reported. The furthest capture from the January detection site was 10.9 km to the west.
Montana State University's Western Agriculture Research Center (WARC) also monitored for H. halys in commercial and backyard  orchards in 2021. Following the same trap design as described above (Fig. 1B), sites across seven counties in western Montana (Fig. 2) were sampled approximately weekly, with the first trap deployed on 4 May 2021 (Ravalli Co.) and the last trap removed on 7 November 2021 (Flathead Co.). No H. halys were detected by these traps. However, an adult H. halys was found inside a car in Flathead Co. on 20 May 2021, highlighting the role of human-mediated spread in the dispersal of this insect (Wallner et al. 2014). Most recent to the time of this publication, a third county (Ravalli Co.) had a positive detection after a live adult H. halys was found inside the residence of one of this study's co-authors on 4 March 2022 in the city of Hamilton.

Surveys and Reports Prior to 2021
Coordinated efforts to look for H. halys in Montana have been recorded since 2013 (Fig. 2)  ) baited with the synergist methyl (E,E,Z)-2,4,6-decatrienoate (HALHAL lures, Alpha Scents, Inc.) were placed at six sites in Ravalli Co. and ran from 18 May through 18 November 2016 and from 8 June through 1 November 2017. Traps were checked biweekly. In 2019, monitoring occurred in commercial and backyard orchards in Corvallis and Hamilton (Ravalli Co.) using the aforementioned pyramid traps. Monitoring was expanded in 2020 to include Flathead, Gallatin, Missoula, and Sweet Grass counties, but utilized the clear sticky trap and dual pheromone lure protocol described previously. No H. halys were found during any of the above efforts.
A tentative adult H. halys in Billings (Yellowstone Co.) was reported by a community member in October 2020. Though the species identification was not confirmed, the same community member submitted a subsequent specimen collected from their residence in May 2021 (approximately 1.5 km from the January 2021 detection; Fig. 2) and noted they had continued to see additional insects of the same appearance in their house since their October report. The May-collected specimen was confirmed to be H. halys, suggesting the insects observed in the fall of 2020 were also H. halys.
Given the number and extent of bugs caught during the summer of 2021, it is likely H. halys has been present in Billings for more than a year prior. Historical monitoring data suggest that the insect was absent from much of the state through at least 2017 or was below a population density detectable by the trapping protocols (Fig.  2). Yellowstone Co. was included in the 2017 CAPS survey, with two sampling locations occurring in Billings near locations with positive captures in 2021 (Supp Table S1 [online only]); however, no bugs were detected in 2017.

H. halys Threat to Montana
Numerous valued plants are potentially at risk were H. halys to become established in Montana. Given where H. halys has currently  been detected (Fig. 2), threats to specialty fruit crops in the state are of particular concern, including apples, pears, grapes, cherries, and peaches (Fig. 1C). Commercial fruit production mostly occurs within river valleys of western Montana (e.g., Bitterroot, Flathead, Clarks Fork, and Gallatin Valleys), with some scattered locations further east (e.g., areas in and around the city of Billings) (Fig. 4). Many of the other major commodity crops known to be damaged by H. halys (e.g., soybean, sweet corn, field corn) (Leskey and Nielsen 2018) occur largely in the eastern regions of the state and have comparatively small commercial acreage and value to other areas of the country (USDA-NASS 2022). Though H. halys has caused severe damage to fruit and other crops elsewhere, particularly in the eastern United States, the severity and extent of impact in Montana agroecosystems remains uncertain.
Climate may also influence the impact of H. halys in the state. Pest risk maps forecasting the potential establishment of H. halys in the United States have estimated relatively low levels of climatic suitability in Montana. Though variable, most forecasts do show some suitability in southcentral and/or western areas that roughly include Yellowstone, Ravalli, and Flathead counties where H. halys has now been confirmed (Zhu et al. 2012, Haye et al. 2015, Kistner 2017, Kriticos et al. 2017. Forecasts incorporating climate change suggest similar suitability (Kistner 2017). The data presented here and from future monitoring in the state provide a useful opportunity for model validation in a U.S. region generally forecasted to have limited suitability for H. halys.

Supplementary Material
Supplementary data are available at Journal of Integrated Pest Management online.
Supplementary Table S1: List of survey and detection sites for H. halys in Montana from 2013 to March 2022. Site-specific georeferenced coordinates and date of sampling or occurrence are provided where possible. A visual summary of the data is provided in Figure 2 of the main text.