©2023 International Society of Arboriculture 48 DISCUSSION As demonstrated in this study, although dieback, can- opy colour, defoliation, bark peeling, exit holes, and proximity of bark peeling and/or exit holes to trunk base were common symptoms used to assess X. globosa infestation, they had greatly different associations with the actual infestation. Canopy colour and defoli- ation were found to be weakly associated with infes- tation of X. globosa as seen from the small absolute values of rotations of these variables along PC1 in the 3D PCA plot (Table 4 and Figure 7). These symptoms often signify tree decline which is not specific to this pest in rain trees. In Singapore, there are multiple stress factors, both biotic and abiotic, that can cause such symptoms to manifest in rain trees. For instance, dieback in rain trees can be caused by fungal patho- gens such as Phomopsis sp. (Chareprasert et al. 2006; NParks 2019) and Botryodiplodia sp. (Boa and Lenné 1994; Hossain 2004). In Singapore, defoliation in rain trees has also been observed to be associated with the increase in larval populations of Hypopyra sp. (Lepidoptera: Erebidae) and Pandesma quenavadii (Guenée 1852)(Lepidoptera: Noctuidae) moths. The defoliation caused by the larvae of these moths is cyclical, with approximately 2 major outbreaks a year in rain trees, making the parameter insufficient alone in assessing infestation caused by X. globosa. In addi- tion, environmental conditions such as water stress during extensive drought periods (King 2008) and root damage due to construction activities (Hauer et al. 1994) can also cause rapid decline in rain trees and symptoms of leaf yellowing, defoliation, and dieback. Meanwhile, exit holes, bark peeling, and proxim- ity of observed bark peeling and/or exit holes to trunk Sensitivity to Temporal Progression of Infestation The BIS value distribution of 48 selected trees along ECP was summarized in Figure 10. In July 2020, minimum, first-quartile, medium, third-quartile, and maximum BIS scores for these trees were 0.1088, 0.9511, 1.6970, 2.0609, and 3.2732. In October 2020, minimum, first-quartile, medium, third-quartile, and maximum BIS scores for the same trees were 0.5178, 1.5088, 2.0146, 2.7470, and 4.4780. Analysis of variance indicated significant difference (F = 13.42; df = 1.94; P-value = 0.0004) between the mean July BIS value (1.5662) and the mean October BIS value (2.2293). Table 6. Summary of number of infested and noninfested trees and the average Borer Infestation Score (BIS) values by surveyed areas. Developed BIS formula was applied to calculate BIS values for all trees during July 2020 survey. Average BIS values for trees for each respective area were obtained and compared against the general feedback of infestation level by managers. Serial number Location No. of infested trees No. of noninfested trees Average BIS score Feedback 1 Bedok South Avenue 1 (BSA) 35 32 +0.0862 High 2 East Coast Park Service Road (ECP) 86 58 +0.2932 High 3 Geylang East Central (GEC) 1 83 –1.4218 Low 4 Penjuru Road (PR) 15 41 –0.9846 Low 5 Siglap Link (SL) 16 21 –0.5218 Moderate Total 153 235 - - Figure 10. Violin plots of BIS values for selected 48 trees along ECP in July and October 2020. The BIS formula developed from PCA analysis was applied to survey data from July and October 2020 surveys of the selected 48 trees to calculate BIS values for these trees. Distribution of BIS values for the selected 48 trees in July and October 2020 were plotted using ggplot function from ggbiplot package in R (The R Foundation for Statistical Computing, Vienna, Austria). Nguyen Hoang Danh et al: Visual Assessment Method for Lebbek Borer
January 2023
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