Within-tree distribution of Ricania shantungensis (Hemiptera: Ricaniidae) eggs on chestnuts

Highlights

• Eggs were not different (p > 0.05) vertically and horizontally within a tree.

• Optimal sample unit was determined as 50 cm branches from their tip.

• Egg occurrence (%) relative to locations on a whole branch was well predicted.

Abstract

Within-tree distribution of an insect pest is useful to determine sample unit, sampling location, and required sample number for this pest within a tree. This study was conducted to characterize the within-tree distribution pattern and determine an efficient sample unit for eggs of Ricania shantungensis (Hemiptera: Ricaniidae) on a chestnut tree. A total of 60 chestnut trees were randomly selected from chestnut fields in Gongju-Si and Buyeo-Gun, Korea in 2019. For each tree, 12 branches were cut at three vertical heights (low, middle, and high) in four cardinal directions (east, west, south, and north). The cut branches were observed for counting the number of egg masses per 10 cm from the branch tip. Coefficient of variation (CV) and coefficient of determination (r²) were calculated and used to determine an optimal sample unit for eggs of R. shantungensis on chestnut trees. Numbers of R. shantungensis egg masses were not significantly different (P > 0.05) vertically and horizontally within a tree. Based on CV and r² values, the 50 cm branch tip would be the optimal sample unit for R. shantungensis eggs. Using this small sample unit for sampling of R. shantungensis eggs could be economical and efficient.

Introduction

An invasive species, Ricania shantungensis Chou & Lu (Hemiptera: Ricaniidae), which became an important pest in Korea since its occurrence in 2010, is suspected to originate from China (Rahman et al., 2012). In China, R. shantungensis is an important economic pest of various fruit plants and roadside trees in Zhejiang province (Chou et al., 1985). In Korea, host plants of R. shantungensis are known to be 138 species belonging to 62 families, including chestnut, peach, persimmon, apple, black locust, Japanese angelica tree, snowbell, pussy willow, paper mulberry, silk tree, tree of heaven, and Japanese cornlian cherry (Kim et al., 2015). Broad host range and absence of native natural enemies might allow R. shantungensis to spread out quickly since its invasion. Especially, the occurrence of R. shantungensis at agricultural areas in 2017 was increased by 109.6% compared to those in 2016 (Baek et al., 2019a, Baek et al., 2019b). Thus, most studies for R. shantungensis has been focused on developing insecticides (Jo, 2014, Jeon et al., 2016, Choi et al., 2018, Lee et al., 2018).

For integrated pest management (IPM) of R. shantungensis, sampling of R. shantungensis within a tree (i.e., count of R. shantungensis on sample unit) is necessary to estimate its population density and distribution, and determine control options. For density estimation, selected sampling units not only determine the nature of a population but also affect the components of sampling plans such as sampling techniques, sampling number, spatial pattern of samples, and timing of sampling (Pedigo and Buntin, 1994). In general, sample units are selected based on multiple criteria including stability and possibility of delineation in the field, consistency, the balance between cost (e.g., labor and time) and variation (e.g., precision), and suitable size (Morris, 1955).

Sampling of R. shantungensis for determination of its population size is targeting to eggs because its nymphs and adults are very active (Kang et al., 2013, Choi et al., 2016). Currently, there is no sample unit suggested for eggs of R. shantungensis. The vertical distribution of R. shantungensis eggs on blueberry trees was reported (Kim et al., 2016) as 56.6–60.2%, 23.8–28.1%, 11.7–19.7% in the upper, middle, lower crowns, respectively. No report was made for within-distribution of R. shantungensis eggs on chestnuts. Information of within-distribution of R. shantungensis eggs could be used not only to determine sample unit for R. shantungensis, but also to evaluate the efficiency of aerial application of insecticides. In Korea, chestnut fields are generally located in forests and most chestnut growers are aged. Thus, for control of R. shantungensis in chestnut fields, aerial application of insecticides by government agencies is commonly requested by farmers. If R. shantungensis eggs occur on the upper crowns in chestnuts like blueberries, the efficiency of aerial application would be higher.

Therefore, this study was conducted (1) to characterize within-tree distribution of R. shantungensis eggs, (2) to determine an optimal sample unit of R. shantungensis eggs, and (3) to find out the relationship of numbers of R. shantungensis eggs between a sample unit and a whole branch.