Full length articleSpatio-temporal distribution of Ricania shantungensis (Hemiptera: Ricaniidae) in chestnut fields: Implications for site-specific management
Graphical abstract
Introduction
Chestnuts with distributions from middle subtropical zones to northern temperate zones (Choo et al., 2001) are mainly produced in China, Korea, Italy, and Tukey (Son et al., 2004a). In Korea, chestnut was one of the most important export items among forest products, accounting for 35% of forest product export (Son et al., 2004b). However, economically important chestnuts in Korea have been seriously damaged by R. shantungensis, an invasive insect (Kim et al., 2015).
After the first discovery at forest and orchard areas in Gongju and Yesan of Chungcheongnam-Do in 2010, R. shantungensis quickly spread out to almost all areas of Korea except for eastern parts, and it has been still spreading (Jo, 2014, Baek et al., 2019a). The occurrence of R. shantungensis in agricultural areas was increased by 109.6% in 2017 compared to that in 2016 (Hong et al., 2017). Moreover, damages caused by R. shantungensis are more serious in mid mountain areas (Baek et al., 2019a) where most chestnuts are grown. Thus, economic damages caused by R. shantungensis in chestnut fields are increasing continuously.
Management of R. shantungensis is difficult due to chestnut fields’ locations (mid mountain areas) and large field size. Although aerial application of insecticides is the control option that chestnut growers wish, its cost does not economically meet the benefit of this management without government subsidy. Moreover, all registered and applicable insecticides for R. shantungensis are toxic to honeybees (RDA, 2018) and thus this control tactic is hard to implement. In these circumstances, site-specific management for R. shantungensis could be a solution to decrease management cost, minimize side effects of insecticides, and increase management efficiency because control action is implemented in specific areas within a field control is needed. Site-specific management can be defined as a management of pests based on localized insect densities within a field rather than average densities throughout the area (Park et al., 2007) because it is theoretically possible that within-field distribution of most insects is aggregated (Davis, 1994). Moreover, this tactic should become effective when pest distributions are consistent because pest distributions can be predicted (Park and Tollefson, 2006).
For application of site-specific management, spatial distribution of R. shantungensis should be identified. Each insect species has its own, readily recognizable distribution pattern resulting from interactions between its selected environment and its behavior that has evolved for survival in such environment (Taylor, 1984). Insect distribution in any geographical areas can be divided into three spatial patterns: uniform, random, and aggregation (Krebs, 1999). There are multiple methods to characterize spatial distribution of insect species, including Taylor's power law (TPL) (Taylor, 1961), Iwao's patchiness regression indices (Iwao, 1972), Ripley’s K and L functions (Ripley, 1977, Diggle, 1983, O'Sullivan and Unwin, 2010), spatial analysis by distance indices (SADIE) (Perry, 1995, Moran’s I (O'Sullivan and Unwin, 2010), variogram (Issaks and Srivastava, 1989, Rossi et al., 1992), Kriging (Cressie, 1993), and so on. Application of site-specific management requires multiple information (e.g., aggregated locations and consistency of spatial pattern) with spatial distribution pattern. Considering these aspects, SADIE is a good method to characterize distribution patterns of R. shantungensis in chestnut fields. SADIE can be used to show relative aggregation degree in each sample point, to analyze spatial association between different sampling time, to provide statistical significance for analyses of spatial aggregation and association, and to visualize results of analyses (Perry, 1995, Perry et al., 1999, Perry et al., 2002).
Even though the distribution of R. shantungensis in chestnut fields is consistent from year to year, additional areas from aggregated spots should be applied because this pest could actively escape pesticide applied areas and aggregated spots should slightly change every year. To define the distance of additional application from aggregated areas of R. shantungensis in chestnut fields, semivariogram can be applied to determine spatial dependence of R. shantungensis by characterizing spatial variations among samples (Issaks and Srivastava, 1989, Rossi et al., 1992). The objective of the present study was to determine spatial distribution, association, dependency of R. shantungensis populations, and application possibility of site-specific management for R. shantungensis in chestnut fields.
Section snippets
Study site and sampling of R. shantungensis
Field studies were conducted in a chestnut field (N 36.203759, W 126.856220) located at Jangam-Myeon (JAM) from 2017 to 2019 and in a chestnut-walnut mixed field (N 36.195951, W 126.808179) located at Nam-Myeon (NM) in 2019 to monitor R. shantungensis populations. In the chestnut field of JAM, approximately 8,000 chestnut trees were grown from downhill to the ridge of hills in roughly 108,000 m2 area. There were rice fields in front of the experimental field (Fig. 1). Rice is not a host plant
Population dynamics of R. shantungensis
The number of nymphs was dramatically decreased compared to the number of spring egg mass (Table 2). The number of adults was generally lower than the number of nymphs except for the sample collected from JAM in 2017 (Table 2). However, the number of fall egg masses was higher than the number of adults except for the sample collected from JAM in 2018 (Table 2). Generally, fall egg densities were similar to spring egg densities regardless of sites or years (Table 2).
Characterization of spatial distribution
Spatial distribution of R.
Discussion
Site-specific management is required for a chestnut field due to its large size and pesticide application difficulty to the whole field. To apply the site-specific management, spatial aggregation of target insect pests is required (Park et al., 2007). R. shantungensis generally showed spatial aggregations and positive spatial associations during three years at experimental sites except for a few occasions. Thus, site-specific management could be used for R. shantungensis in these chestnut
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was carried out with the support of “Research Program for Forest Science & Technology Development (Project No. FE0100-1988-01 and FP0802-2017-02-2018)” funded by National Institute of Forest Science, Republic of Korea. This work was also partially supported by BK 21 Plus.
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Present address: Chungnam Agricultural Research and Extension Services, Yesan 32418, Republic of Korea.