©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(1): January 2023 49 calculation, we could see significant increase in infes- tation level if trees were left untreated. From an operational perspective, although manag- ers knew that symptoms such as exit holes, bark peel- ing, defoliation, and dieback were associated with borer infestation, the lack of standardized and sys- tematic approaches in recording and computing these variables hindered their ability to accurately assess the progress of infestation and its severity. Conse- quently, managers could not ascertain whether their attempts at treatment including pruning of infested branches, chemical soil drenching, or chemical tree injection were effective due to lack of reference points for infestation severity. Assessment using BIS calculation filled the gap by providing a quantitative score to estimate infestation severity based on visual symptoms which could be quickly observed and recorded on site. A nonpositive change in BIS values over time after treatment would indicate effectiveness of the treatment in controlling X. globosa infestation. Since symptoms such as exit holes, bark peeling, and their proximity to trunk base have very high weights in BIS formula, pruning to remove infested branches would cause a temporary decrease in BIS values. Therefore, for accurate interpretation, it is important to take into consideration the pruning schedule of trees and multiple BIS values of trees over an extended period. A temporary drop in BIS values after pruning followed by an increase in BIS value indicates that the larvae inside tree trunks and branches are still alive and have emerged to create new exit holes and bark peeling. Our survey took approximately 3 to 10 minutes to complete the assessment of one tree to calculate BIS values. Therefore, BIS assessment system can be read- ily integrated into surveillance routines without major additional daily workload. CONCLUSIONS We demonstrated that the newly developed BIS sys- tem provides a nondestructive method of estimating the infestation severity of X. globosa in individual rain trees and trees in an area. Future research can investigate the capacity of BIS in assessing the effi- cacy of X. globosa control measures such as chemical treatments via soil drenching or trunk injection. Since the BIS assessment method generates quantitative data, further study can be done to impose appropriate action thresholds based on BIS values to devise base were highly indicative of X. globosa infestation as seen from PCA. The absolute values of rotations for these variables were at least 6- to 10-fold larger than those of dieback, canopy colour, and defoliation. Such results aligned with the biology of the pest observed in Singapore and as described by Matsumoto and Irianto (1998). Based on internal pest survey and observational data, X. globosa was the only borer spe- cies found infesting rain trees. Appearance of holes in rain trees is unique to X. globosa infestation because the adult borers create these holes when they emerge. In this study, the high absolute value of rotation of lowest position of bark peeling and/or exit holes along PC1 provided the empirical evidence to downward- infestation behaviour of X. globosa in rain trees. It could be hypothesized that younger branches located at higher positions in the canopy are more frequently pruned as part of routine maintenance operation, cre- ating open wounds and bark cracks that allow easier oviposition by females. As larvae feed and mature, they would progress downward to find more food in bigger and more mature branches lower in the canopy. The developed BIS formula gave values that were positively correlated with X. globosa infestation. Noninfested trees had typical BIS values of –1.42 or lower, while infested trees had BIS values of –1.01 or higher. Despite their low level of associations with X. globosa infestation in rain trees, symptoms such as dieback, canopy colour, and defoliation still con- tained valuable information which can help to obtain a more accurate assessment of infestation level. Therefore, weights for these variables were balanced in such a way that these variables still contributed to overall calculation but did not cause noninfested trees to have high BIS values. As shown from the confu- sion matrix for infestation status of the 98 trees in the test data set (Table 5), there was only one noninfested tree with a BIS value that was within the range of those infested. This may be due to an assessment error in recording bark peeling. Nonetheless, the error rate was found to be low (1.02%). Thus, the BIS for- mula provided a clear separation between infested and noninfested trees. Furthermore, when applied to calculate BIS for all trees within an area, average BIS for each area reflected the infestation level of said area based on feedback from managers. In terms of temporal progression of infestation, we demonstrated that by tracking infestation symptoms and using BIS
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