Journal of Ecology and Environment

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Published online December 19, 2023
https://doi.org/10.5141/jee.23.050

Journal of Ecology and Environment (2023) 47:24

Comparison of terrestrial insect communities associated with the crabgrass (Digitaria ciliaris) community, Korea

Jeong Ho Hwang* and Jong-Hak Yun

Wetland Center, National Institute of Ecology, Changnyeong 50303, Republic of Korea

Correspondence to:Jeong Ho Hwang
E-mail chsh123@naver.com

Received: August 29, 2023; Revised: November 22, 2023; Accepted: December 8, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Background: Crabgrass (Digitaria ciliaris, Poaceae) is a globally distributed weed, including in Afro-Eurasia, America, and Australia. As a highly gregarious plant, crabgrass is an important habitat for a diverse array of insects, and a potential habitat for agricultural pests. To compare the insect communities associated with the crabgrass community, insects were sampled using sweep sampling (100 sweeps per sample) at five sites, including Daejeon (Daejeon and Gap rivers), Anseong, Namhae, and Inje, with a focus on the Daejeon River.
Results: A total of 5,888 individual insects belonging to eight orders, 42 families, and 115 species were collected from the five sites. Both the number of species and individuals of Hemiptera were the highest at all of the sites. In the present study, 73% of the insect population fed on D. ciliaris as a host plant. The dominant species in the D. ciliaris community was Laodelphax striatellus (Delphacidae), being ubiquitous at all the sites which showed a high abundance of rice pests in the communities and the suitability of D. ciliaris as an alternative host plant for them. The Shannon–Wiener diversity index was highest in Inje on 17 September (2.88), and the Chao1-bc diversity index was highest in the Gap River on 5 September (80). The sampling efficiency of 100 sweep samples (sample coverage) was calculated to be as high as 90%. The results of the samples taken from September to November in the Daejeon River showed that the number of species and individuals decreased gradually over time, and the number of dominant species decreased sharply between September and October. Similarity analysis indicated that sampling dates that were closer together yielded sampled assemblages with higher faunal similarity. In addition, in each sampling, the difference in the minimum temperature during the two-week period prior to sampling and faunal similarities were negatively correlated.
Conclusions: This study provides foundational data that could enhance our understanding of insect diversity in D. ciliaris. The data can facilitate ecological conservation and management of Korean grasslands generally, as well as identification of potential pests that may disperse from D. ciliaris communities to nearby farmland.

Keywords: assemblages, grassland, insect diversity, sampling, seasonality, similarity, sweep-netting

Crabgrass (Digitaria ciliaris, Poaceae) is a globally distributed weed, including in Afro-Eurasia, America, and Australia (GBIF 2023; Rojas-Sandoval and Acevedo-Rodríguez 2022). It is also a common weed found in grasslands and farmlands such as paddy banks and other crop field margins in Korea. In particular, as a characteristic C4 plant, D. ciliaris increases its biomass and proportional abundance in the community from late August after the rainy season, when growth conditions are the most suitable (Kim et al. 2012; Lee and Lee 1999). As a widespread and extensively gregarious plant, D. ciliaris is a potential habitat for various insects, including agricultural pests. Such pests may directly attack crops and reduce their quality, or act as vectors transmitting viruses to crops (Luisoni and Conti 1970).

Insects using plants as their main habitat or food source often form a distinct association with a specific dominant plant community, so plant communities with a single dominant plant species generally have similar association trends with insects under the same climatic conditions (Farrell et al. 1992; Hwang et al. 2022). Recent studies on insect communities associated with a specific plant community have mainly been related to cultivated plants, whereas studies on insect communities in a specific wild plant community have been rather limited (Frenzel and Brandl 2003; Hwang et al. 2022; Lee et al. 2022).

Because of its ecological and agricultural importance, many studies on vegetation associated with D. ciliaris have been conducted. However, to the best of our knowledge, no study has specifically evaluated the insect communities inhabiting D. ciliaris (Ahn and Kim 2005; Lee et al. 2018; Shim et al. 2013). This study focused on analyzing the characteristics and similarities of D. ciliaris insect communities at different spatiotemporal scales in Korea, comparing insect community composition through quantitative sweep sampling and evaluating the optimal sampling size based on the sampling coverage.

Study site characteristics

Insect community samplings in D. ciliaris communities were conducted in autumn (September to November) in Korea, when D. ciliaris dominates at the highest density (Kim et al. 2012; Lee and Lee 1999). We sampled insects on six different dates to compare their community composition over a time series in the Daejeon River (36.3414° N, 127.4141° E), located in Daejeon city, in 2020. For comparison with other D. ciliaris communities, samples were also collected twice at the Gap River, another river in Daejeon (36.370249° N, 127.372875° E), twice in Anseong (37. 117019° N, 127.432771° E), and once each in Inje (38. 108407° N, 128.179852° E) and Namhae (34.811487° N, 127. 835146° E) (Fig. 1, Table S1). The four administrative sites investigated in this study have different climates, variations in latitude, and contrasting average temperatures (KMA 2023). Also, as the study progressed, seasonal temperatures decreased, so we assumed that a site at a higher latitude sampled at an earlier date site would have a similar insect community to a site at a lower latitude sampled at a later date. To test for correlations between insect community similarity and the regional climate, the minimum temperature during the two-week period prior to the sampling date was obtained from the meteorological station closest to each site (KMA 2023). The Daejeon and Gap rivers, located in Daejeon city, are urban rivers with waterside facilities, such as riverside roads, sidewalks, and bicycle roads, whereas the other three sites were in rural environments, with cultivated land, including rice paddies or other crop fields, around them (Fig. 1).

Figure 1. (A) A map of sampling sites, (B) a panoramic photograph of the Digitaria ciliaris grassland in Daejeon River, and (C) a sampling design to collect insects in D. ciliaris grassland. In (A), sampling sites are A: Daejeon River; B: Gap River; C: Anseong; D: Namhae; and E: Inje.

Sampling methods

Terrestrial insect communities associated with the D. ciliaris plant community were sampled at the Daejeon River site on 9 September, 9, 18, and 24 October, and 7 and 21 November 2020, between 12:00 and 17:00. We also sampled other regions for comparing insect communities: the Gap River (5 September and 8 November), Anseong (11 and 30 October), Namhae (10 October), and Inje (17 September). According to the National Natural Environment Survey guidelines, D. ciliaris communities are determined based on the plants that dominate the uppermost layer by more than 70%, and D. ciliaris dominated all the communities in the sites by more than 80% (NIE 2019).

The quantitative sweep sampling method reflects the reality that it is not possible to collect all the species in a studied area. Therefore, it has been used as an effective method for rapidly sampling large numbers of insects to effectively compare the biodiversity of different regions (Spafford and Lortie 2013; Yi et al. 2012). Furthermore, the efficiency of sweep sampling in various habitats has been evaluated (Doxon et al. 2011; Hwang et al. 2022; Spafford and Lortie 2013; Swart et al. 2017).

For quantitative statistics at each site, 100 sweeps (50 m transects) were performed in each investigated date (Fig. 1). Insect nets with a total length of 1,200 mm, net depth of 700 mm, and diameter of approximately 300 mm were used. Before sorting, the terrestrial insects collected with 100 sweeps were shaken off into a zipper bag (25 × 30 cm). Afterward, sampled insects were placed in 94% ethanol and sorted in the laboratory before being classified under a stereomicroscope (Olympus SZ1145; Olympus, Tokyo, Japan). Various faunal atlases and papers were referred to for identification (Ahn et al. 2018; An 2011; An and Kim 2020; Hong et al. 2011, 2012; Park et al. 2012; Takizawa 2005), and experts were consulted for species that were difficult to classify. In addition, host plants were searched with various encyclopedias and papers to confirm insect species, and by implication, the percentage of sampled insects, that fed on Poaceae (including D. ciliaris) as host plants (Ahn et al. 2018; Bieńkowski and Orlova-Bienkowskaja 2018; Danthanarayana 1967; David et al. 2020; Denno and Perfect 2012; Doopedia 2023a, 2023b, 2023c; Greber 1990; Information of Korean Alien Species 2023; Kim and Kil 2014; Hossain and Kwon 2019; Korea National Arboretum 2023; Knight 1987; Lee 2012; Li et al. 2012; Lodos and Kalkandelen 1983; NIBR 2023a, 2023b, 2023c, 2023d; Park et al. 2018; Takeda 1998; Susumu and Hiroyuki 2010; Teakle et al. 1991; Zhou et al. 2013). The classification system was based on the 2020 National Species List (NIBR 2022). The collected insects were prepared as immersion specimens in 94% ethanol, or dried specimens, and stored at the Natural History Laboratory of the National Science Museum in Daejeon, Korea.

Statistical analysis

For the diversity index, the Shannon–Wiener function (H’) derived from the information theory of Margalef (1958) and modified by Lloyd and Ghelardi (Pielou 1966) was used. The dominance index was calculated using McNaughton’s dominance index (DI) (McNaughton 1967), the abundance index was calculated using Margalef’s (1958) index, and the evenness index was calculated using Pielou’s (1975) formula. The Chao1 bias-corrected index (Chao1-bc), which is widely used together with the diversity index and evaluated to have excellent performance, was calculated using the SPADE program (Chao and Shen 2010; Głowacki 2011).

Sørensen’s similarity index, an incidence-based similarity measure, and the Bray–Curtis similarity index, an abundance-based measure, which are both generally used to assess community composition and are resistant to sampling errors, were calculated using species prediction and diversity estimation (SPADE) (Beals 1984; Chao and Shen 2010; Giovas 2021; Schroeder and Jenkins 2018; Sørensen 1948). The calculated similarity indices were used to create a hierarchical clustering dendrogram using the unweighted pair group method with arithmetic means using SPSS version 21 (IBM Co., Armonk, NY, USA). Furthermore, to determine the relationship between insect communities and temperature, the minimum temperature during the two-week period prior to the sampling date and the Bray–Curtis similarity index were analyzed using Spearman’s correlation analysis, calculated in SPSS. In addition, on three sampling dates (Daejeon River on 9 September and 9 October, and Gap River on 5 September) the species richness estimation curves and sample coverage were built based on 10 sweep sampling units of 10 sweeps each using the R software package iNEXT in R version 3.6.0 (Hsieh et al. 2016). We assumed that sample coverage values in same dominant species are similar compared to different dominant species. Interpolation and extrapolation of the species richness estimation curves were extended to only twice the sampling units to maintain a reliability of 95% using the default setting.

Community analysis

In total, eight orders, 42 families, 115 species, and 5,888 individuals of insects were collected. In the samples from Daejeon River, the number of species and individuals were the highest in early September. In the present study, approximately 73% of the insect population fed on D. ciliaris as a host plant (Tables 1, S2, S3) (An 2011; An and Kim 2020; Ahn et al. 2018; Bieńkowski and Orlova-Bienkowskaja 2018; Danthanarayana 1967; David et al. 2020; Doopedia 2023a, 2023b, 2023c; Denno and Perfect 2012; Greber 1990; Hong et al. 2011, 2012; Hossain and Kwon 2019; Information of Korean Alien Species 2023; Kim et al. 2016; Knight 1987; Korea National Arboretum 2023; Lee 2012; Li et al. 2012; Lodos and Kalkandelen 1983; NIBR 2023b, 2023c, 2023d; Susumu and Hiroyuki 2010; Takeda 1998; Zhou et al. 2013). Most of them were Hemiptera species. Both the number of species and individuals of Hemiptera were the highest at all sites, and most of them accounted for more than half the proportion (Table 2).

Table 1 . Summary of the result on insect communities associated with Digitaria ciliaris plant communities in Korea, sampled by sweep-netting in September–November 2020.

Sampling site
and date
AbbreviationOrdersFamiliesSpeciesIndividualsPercentage of individuals that feed on D. ciliaris
Daejeon River
9 September
A-ES (early September)518421,82990.3
Daejeon River
9 October
A-EO (early October)4163642756.7
Daejeon River
18 October
A-MO (mid-October)5152327065.9
Daejeon River
24 October
A-LO (late October)47118779.3
Daejeon River
7 November
A-EN (early November)28145947.5
Daejeon River
21 November
A-LN (late November)1342090.0
Gap River
5 September
B-ES (early September)624451,69358.1
Gap River
8 October
B-EO (early October)6111840087.3
Anseong
11 October
C-MO (mid-October)5152616055.0
Anseong
30 October
C-LO (late October)4792222.7
Namhae
10 October
D-MO (mid-October)5173370479.1
Inje
17 September
E-MS (mid-September)6204121758.5
Total8421155,88873.0

Site abbreviations refer to localities in Fig. 1.


Table 2 . Species richness and abundance (in parenthesis) by insect orders sampled by sweep-netting in Digitaria ciliaris grassland in Korea in September–November 2020.

OrderSpecies richness (abundance)
Sampling sites
A-ESA-EOA-MOA-LOA-ENA-LNB-ESB-EOC-MOC-LOD-MOE-MS
Hemiptera24 (1,711)23 (382)15 (242)7 (78)10 (55)4 (20)21 (1,214)8 (350)17 (134)5 (16)22 (643)24 (171)
Coleoptera7 (49)6 (25)2 (3)4 (4)12 (396)4 (29)1 (1)2 (4)10 (15)
Diptera8 (60)5 (10)4 (9)2 (2)6 (63)3 (18)4 (13)2 (4)4 (33)2 (7)
Orthoptera2 (8)2 (10)1 (2)3 (13)1 (1)3 (11)1 (1)3 (13)1 (2)
Hymenoptera1 (1)1 (14)1 (6)2 (6)1 (1)2 (11)3 (21)
Odonata1 (1)1 (1)1 (1)1 (1)
Lepidoptera1 (1)
Mantodea1 (1)

Abbreviations of sampling sites were given in Table 1.



In this study, there were three cases of sampling (Gap River on early September, Daejeon River on early September and early October) where the 100 sweep samples were comprised of ten subsamples of ten sweeps each, to make it possible to estimate the number of species per ten sweeps. The numbers of species were similar between the Daejeon and Gap rivers in September, but when estimating the richness based on 200 sweeps (two extrapolations), the number of species in Dajeon River on early September tended to converge with that of the Daejeon River on early October. All sample coverages were calculated to be as high as 90% (Fig. 2).

Figure 2. Species richness (A) and sample coverage (B) of insect communities sampled from Digitaria ciliaris grassland in three particular sample events, divided into 10 subsamples of 10 sweeps each.

In the Daejeon River samples, community analysis showed that the Shannon–Wiener diversity index was the highest in early October (2.71), and the Chao1-bc diversity index was the highest in early September (57.6). The most frequently dominant and sub-dominant species at each site were Laodelphax striatellus (Delphacidae), Psammotettix striata, and Balclutha rubrinervis (both Cicadellidae) (Table 3).

Table 3 . Community structure of insects associated with Digitaria ciliatus in Korea.

Abbreviation of sampling sitesCommunity analysis indicesDominant (subdominant) species
H’DIRIEIChao1-bc
Daejeon River
9 September
A-ES2.160.555.460.4057.60Laodelphax striatellusStenotus rubrovittatus
Daejeon River
9 October
A-EO2.710.405.780.5254.33Balclutha rubrinervisNabis stenoferus
Daejeon River
18 October
A-MO2.000.603.930.4425.50B. rubrinervis
N. stenoferus
Daejeon River
24 October
A-LO1.370.722.240.4026.00B. rubrinervisPsammotettix striata
Daejeon River
7 November
A-EN1.880.643.190.4932.00P. striata
N. stenoferus
Daejeon River
21 November
A-LN0.590.901.000.297.00P. striata
L. striatellus
Gap River
5 September
B-ES2.710.385.920.4980.00Trigonotylus caelestialium
L. striatellus
Gap River
8 October
B-EO1.900.532.840.4625.00P. striata
B. rubrinervis
Anseong
11 October
C-MO2.360.544.930.5035.00S. rubrovittatus
Nephotettix cincticeps
Anseong
30 October
C-LO1.680.642.590.5316.50N. cincticeps
Sepedon aenescens
Namhae
10 October
D-MO2.270.494.880.456.40B. rubrinervis
S. rubrovittatus
Inje
17 September
E-MS2.880.377.440.5460.00P. striata
L. striatellus
Total2.990.3113.130.44164.40L. striatellusS. rubrovittatus

Abbreviations of sampling sites were given in Table 1. H’: diversity index; DI: dominance index; RI: richness index; EI: evenness index.



Similarity analysis based on species presence and abundance

The results of the Sørensen/Bray–Curtis similarity analysis by sampling dates in the Daejeon River D. ciliaris community showed that sampling dates closer to each other had higher similarity (Table 4, Fig. 3). As such, the samples taken at the start and end of the study dates were the most dissimilar.

Table 4 . Comparison of the index of Sørensen and Bray–Curtis (in parenthesis) on insect communities sampled in different sampling times in the Daejeon River.

A-ESA-EOA-MOA-LOA-EN
A-EO0.74 (0.28)
A-MO0.43 (0.21)0.51 (0.55)
A-LO0.26 (0.08)0.30 (0.30)0.41 (0.47)
A-EN0.29 (0.05)0.32 (0.22)0.38 (0.24)0.24 (0.27)
A-LN0.13 (0.02)0.15 (0.09)0.22 (0.10)0.13 (0.15)0.33 (0.48)

Abbreviations of sampling sites were given in Table 1.


Figure 3. Comparison of the Sørensen (A) and Bray–Curtis (B) indices on insect communities sampled in different sampling times in the Daejeon River. Abbreviation of sampling sites were given in Table 1.

The Sørensen/Bray–Curtis similarity analysis results for all samples, including Daejeon and Gap River, Inje, and Namhae, also showed that regardless of the inter-site distance, the similarities between the samples with a large temporal difference between them were conspicuously low (Table 5, Fig. 4). Furthermore, Bray–Curtis similarities were more indicative of the dominant species in samples, showing high similarity between the samples with the same dominant species.

Table 5 . Comparison of the index of Sørensen and Bray–Curtis (in parenthesis) on insect communities from all sites.

A-ESA-EOA-MOA-LOA-ENA-LNB-ESB-EOC-MOC-LOD-MO
A-EO0.74 (0.28)
A-MO0.43 (0.21)0.51 (0.55)
A-LO0.26 (0.08)0.30
(0.30)
0.41 (0.47)
A-EN0.29 (0.05)0.32 (0.22)0.38 (0.24)0.24 (0.27)
A-LN0.13 (0.02)0.15 (0.09)0.22
(0.10)
0.13 (0.15)0.33 (0.48)
B-ES0.53 (0.53)0.54 (0.18)0.38 (0.09)0.21 (0.03)0.20
(0.04)
0.12 (0.02)
B-EO0.40
(0.33)
0.41 (0.44)0.44 (0.36)0.48
(0.30)
0.31 (0.14)0.18 (0.09)0.41 (0.27)
C-MO0.38
(0.10)
0.42 (0.32)0.49 (0.27)0.43 (0.24)0.15 (0.14)0.13 (0.11)0.31 (0.09)0.41 (0.29)
C-LO0.20
(0.01)
0.22 (0.03)0.38 (0.05)0.30
(0.07)
0.17 (0.05)0.15 (0.05)0.11
(0)
0.30
(0.02)
0.46 (0.23)
D-MO0.40
(0.29)
0.41 (0.31)0.39 (0.39)0.18 (0.17)0.17 (0.04)0.16 (0.03)0.38 (0.14)0.27 (0.24)0.37 (0.19)0.19 (0.01)
E-MS0.46 (0.15)0.49
(0.30)
0.44 (0.21)0.19 (0.11)0.29 (0.25)0.13 (0.16)0.47 (0.16)0.31 (0.34)0.33 (0.18)0.08 (0.02)0.43 (0.16)

Abbreviations of sampling sites were given in Table 1.


Figure 4. Comparison of the Sørensen (A) and Bray–Curtis (B) indices on insect communities sampled from all sites. Abbreviation of sampling sites were given in Table 1.

Analyses comparing sampling date and temperature with individuals and similarity

The population trends at Daejeon River showed that the abundance of most of the dominant species (L. striatellus, Stenotus rubrovittatus [Miridae], Trigonotylus caelestialium [Miridae], and P. striata) in the D. ciliaris community declined sharply between September and October (Fig. 5). However, B. rubrinervis showed a relatively gradual decline, whereas Nabis stenoferus (Nabidae), the only predator among the dominant species, increased slightly in early October, when other prey decreased, before declining steadily (Fig. 5).

Figure 5. Seasonal changes of six dominant insect species in the Digitaria ciliaris community at Daejeon River from September–November 2020.

Spearman’s correlation between the difference in minimum temperature during the two weeks before sampling and the Bray–Curtis similarity was significant for all the comparisons (p < 0.01), being very high (–0.798) at the Daejeon River site. In addition, when considering all samples excluding the Daejeon River, the correlation was –0.601. When considering all the samples in this study, the correlation was –0.59 (Tables S4, S5).

The results of the insect community sampling of the D. ciliaris plant community showed that the number of species was relatively high in the Daejeon and Gap Rivers, and Inje in September, with 42, 45, and 41 species, respectively. September was warmer than the other sampling dates, and the minimum temperatures for the two weeks prior to the sampling dates were 16.3°C, 20.6°C, and 14.8°C at Daejeon River, Gap River, and Inje, respectively. Therefore, a more conducive temperature for insect growth in September was assumed to have influenced the higher number of species in September than in October and November. Between 6–24 species were collected during the Shinan insect community study, which sampled the Phragmites japonica (Poaceae), Miscanthus sinensis (Poaceae), and Artemisia indica (Asteraceae) communities in September of the same season (Hwang et al. 2023). In contrast, 41–45 species were collected in the present study in September, representing far higher species richness. This indicates that the D. ciliaris community is an important habitat for insect diversity.

Sample coverage was calculated to evaluate the sampling efficiency of 100 sweeps in collecting insects in the D. ciliaris community, and it was consistently high at around 90%. Compared with other plant communities evaluated in Korea, i.e., the A. indica and Beckmannia syzigachne (Poaceae) communities, the sample coverage of the B. syzigachne community was more comparable to that of D. ciliaris (Hwang et al. 2022).

The dominant species in the D. ciliaris community were L. striatellus, P. striata, and B. rubrinervis, being ubiquitous at all the sites. All of them used Poaceae plants as hosts and thrived during the period when the D. ciliaris community dominated the grassland (Doopedia 2023b; Knight 1987; NIBR 2023b). Proper monitoring is necessary, because L. striatellus is a major rice pest that causes the rice stripe virus disease (Noh et al. 2013; Son et al. 2014). The carnivorous Nabis stenoferus, which is known to prey on planthoppers, was the most abundant on 9 October, showing typical predator-prey population dynamics (Crawley 1975; Paik 1974). In Australia, the genus Balclutha is common on Digitaria plants, and because the genus Nabis is a natural enemy, the situation is similar to plants in Korea (Narhardiyati and Bailey 2005).

Digitaria plants are C4 grasses that prosper after the rainy season, with the seeds ripening from early July to early September. It is expected that as the biomass and protein content increase, the populations of the dominant herbivorous species will be high in early September and decrease gradually as ambient temperatures decrease (Lee and Lee 1999; Shin and Kim 1983). Based on a results of the Daejeon River, the most dominant species tended to decrease after 9 September. The mortality rate of L. striatellus is over 90% when exposed to temperatures of 12.5°C, and the minimum temperature was confirmed to be below 13°C for six consecutive days prior to the sampling on 9 October; therefore, it can be deduced that the decrease in the population of the dominant Delphacidae and Cicadellidae speciesis closely related to the ambient temperature (Park et al. 2011).

In addition, in the community analysis related to temperature, the Bray–Curtis similarity tended to be high among the samples with similar minimum temperatures for the 2 weeks prior to sampling, which was clearly confirmed by Spearman’s correlation analysis. This is because the minimum temperature is related to the survival rate of insects and greatly affects the community, especially the abundance of the dominant species, such as Delphacidae and Cicadellidae (Park et al. 2011).

In the Daejeon River, the closer the sampling dates, the higher the insect community similarity (Fig. 3). This is assumed to reflect the characteristics of insects with short life cycles and high minimum temperatures required for survival. Also, when comparing the similarities of all samples, there is a tendency for the similarities to be high if the sampling dates are similar, even if the sites are different. In the case of the Sørensen’s similarity values, samples were divided into one group belonging to samples mainly collected in September, and another group mainly collected in October. Similarly, in the case of Bray–Curtis similarity, there was a tendency for similarities to be higher between samples with proximate sampling dates, except for Angeong, which had a dominant species (Nephotettix cincticeps) different to the other sites (Fig. 4). In addition, the Daejeon and Gap rivers showed relatively high similarity, which is assumed to be because these localities are proximate, with similar climate and habitat conditions (Fig. 4).

Sweeping is a collecting method that targets insects living on plants, and the sampling results can vary greatly depending on which vegetation is swept. However, if sampling is performed in a specific dominant plant community, it is possible to reduce the variability attributable to vegetation composition and obtain basic data to identify the insect community trends in a specific dominant plant community (Hwang et al. 2022). The insect community data for D. ciliaris in the present study cannot be generalized to D. ciliaris communities elsewhere because of the limited number of samples and the influence of climatic and other environmental factors across regions. A clearer statistical trend could be expected if the number of samples is increased. As a result of the D. ciliaris community sampling, even if there are differences between some sites related to distance, there are many cases where the similarity is relatively high, with at least Sørensen 0.4 and Bray–Curtis 0.3 or higher. Therefore, it is plausible that generalization of the insect communities is possible, to some extent.

As D. ciliaris communities are often close to cultivated fields, they are considered potential habitats for agricultural pests. In the present study, pests including the delphacids L. striatellus, Sogatella furcifera, Nilaparvata lugens, and Pachygrontha antennata (Pachygronthidae), were sampled (Paik et al. 2009; Tindall et al. 2005). Monitoring and management of the D. ciliaris community is important, because the pest species sampled can directly harm crops and transmit viruses.

Digitaria ciliaris is considered one of the most common and competitive weeds in farmlands worldwide and is also common throughout Korea (Jones et al. 2021; Lee and Lee 1999). Since there are correlations between plant communities with the same dominant plant species and their associated insect communities, it is important to understand and accumulate data on the insect community associated with D. ciliaris, which is a prominent component of grassland and agricultural ecosystems in Korea (Hwang et al. 2022). This study provides foundational data that could enhance our understanding of the relationship between D. ciliaris and insect communities in future, and the data can facilitate ecological conservation and management of Korean grasslands in general, as well as identification of potential pests that may disperse from D. ciliaris communities to nearby farmland. The latter aspect is of particular importance, as several rice pests were sampled in abundance from D. ciliaris in this study, indicating that it is a suitable host plant for these pests. This suggests that D. ciliaris could potentially serve as a population sink, allowing these pests to recolonize even well-managed crop fields and affect crop yields. Effective monitoring of grassland stands dominated by D. ciliaris is therefore necessary to improve pest management on a landscape scale. The data provided in the present study can be used as baseline data to inform pest management practices and promote ecological conservation of D. ciliaris communities.

Supplementary information accompanies this paper at https://doi.org/10.5141/jee.23.050.

Table S1. Annual temperature and precipitation in the sites. Table S2. Insect inventory in the Digitaria ciliaris communities. Table S3. Percentage of the number of individuals of Poaceae-feeding insects from each sampling. Table S4. The minimum temperature during the 2-week period prior to each sampling. Table S5. Pairwise comparisons of differences in minimum temperature during the 2-week period before the sampling dates across all sites.

JHH did conceptualization, data curation, investigation, and writing the original draft. JHY did methodology, supervision, writing the review and editing. All the authors approved the manuscript.

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