Spreading of the Brown Marmorated Stink Bug Across West and Central Kentucky: Distribution and Trends

Stink bugs in Kentucky

Every year, soybean farmers in Kentucky usually face the attacks of two common stink bugs: the green stink bug (Chinavia hilaris) and the complex of brown stink bugs(Euschistus spp.–i.e., E. variolarius, E. servus), and since 2010 the brown marmorated stink bug (BMSB,  Halyomorpha halys) (Figures 1A-C). These species are known to damage soybeans during summer and fall. Their damage on pods can cause aborted seeds and reduced economic value. However, when looking at the attack rates by these hemipterans, the soybean planting periods (full season vs. double crop) can make a big difference on stink bug numbers (check KPN for more information).

Figure 1. Common stink bugs found in soybean fields from west and central Kentucky. Green (A), brown (B) and nymphs of brown marmorated stink bugs (C) (Photos: Armando Falcon-Brindis, UK).

Spreading context and scouting

Since our first report of the expansion of the brown marmorated stink bug toward western Kentucky in 2020 (KPN 9-22-20), farmers and county Extension agents have been reporting increasing numbers of BMSB in western counties of Kentucky.

We conducted standardized sampling (100 sweeps/field) in 34 commercial soybean fields from 21 counties in western and central Kentucky. In the western region, the counties sampled were Fulton, Hickman, Carlisle, Ballard, McCracken, Graves, Calloway, Livingston, Lyon, Caldwell, Christian, Trigg, McLean, and Henderson; whereas in the central region, we sampled in, Daviess, Hancock, Breckenridge, Hardin, Warren, Nelson, and Fayette counties (Figure 2).

We focused on two aspects with the information obtained. First, we compared the proportion of the three most common stink bugs on soybeans: green, brown, and BMSB. Then, we examined whether the planting strategies (double crop vs. full season) could help explain the number of stink bugs. Tallies were conducted using sweep nets during August and September in 2020 and 2021.

Figure 2. Numbers of stink bugs (Mean± SEM) across 34 commercial soybean fields in 21 counties . In Livingston County, we collected only one stink bug; the field might have been sprayed on days previous to when the tallies were conducted (Chart by Armando Falcon-Brindis, UK).

Stink bug populations and species in 2020 and 2021

We collected 1,112 stink bugs from western and central counties (Figures 2 and 3). The proportion between the green, brown, and BMSB was statistically different (χ2 = 265.2, df = 2, p<0.001). The total number of stink bugs varied according to the planting strategy (t = -3.01, df = 129.2, p = 0.003), where full season fields had more stink bugs than those planted as double crops (Figure 4). Interestingly, the proportion of these stink bug species from central and western counties is closely related (χ2 = 1.18, df = 3, p<0.552). Although the most abundant species is the green stink bug in double crop systems, the brown marmorated stink bug seems to be starting to dominate among full season fields (Figure 3).

Figure 3. Percentages of the three most common stink bug species in central and western Kentucky (donut charts). The species proportions by planting strategy are displayed in the small pie charts. (Figure by Armando Falcon-Brindis, UK).

The total proportion of stinkbugs did not vary between the western and central regions but between counties. The latter is probably the result of farmer management strategies and periods when we conducted the scouting (prior or post-application of insecticides). The highest number of stink bugs was found in Hancock County (Figure 2).

How fast is the BMSB spreading?

Comparing 2020 and 2021, we noticed that BMSB moved fast, apparently from east to west or north to south. The BMSB has been present in Kentucky since 2010; however, in the western region of the state and prior to 2020, finding were sporadic, and in most cases, hitchhickers –a well known behavior of this species. Moreover, we confirmed the presence of the BMSB in five new counties in the western region (Carlisle, Ballard, McCracken, and Livingston) and one central (Hancock) Kentucky county. In terms of the the geographic spread of BMSB, its proportion is rapidly increasing when compared with 2020 (Figure 4).

The expasion of BMSB may bring new problems for soybean and vegetable growers in western Kentucky. As invasive species, their numbers can increase as natural enemies are not as abundant in Kentucky as in native regions of Asia. In addition, BMSB is a nuisanse for human dwellings, as they move into barns and houses to overwinter, staining walls, producing a foul smell, and/or causing allergies. During the week of October 11 to 15, 2021, one of the authors of this report (Villanueva) was finding between 15 to 32 BMSB on the outdoor side of his screen room. This is significant as he did not observe this from 2016 to 2019.

Figure 4. Changes in the proportion of common stink bugs during 2020 and 2021 in western and central Kentucky. N = total stink bugs collected (Figure by Armando Falcon-Brindis, UK).

By Armando Falcon-Brindis, Entomology Research Analyst, and Raul T. Villanueva, Entomology Extension Specialist

Posted in Grains
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