Objective The carpenter moth Cossus orientalis is an important wood-boring pest of forest trees in China, causes severe damage to shelterbelt tree species such as Populus × xiaohei var. gansuensis and Salix matsudana in the Hexi Corridor region. Its larvae are concealed in the bored trunks, making instar classification in the field and sample collection highly difficult; there are differences among larvae of the same instar from different hosts, which poses a great obstacle to the accurate classification of instars.Clarifying the larval instar classification and identifying the optimal instar determination indicators are important prerequisites for achieving accurate prediction and scientific control.

Method A total of 2 139 larvae of the Cossus orientalis at different developmental stages were collected from sampling plots in Jiayuguan City, Gansu Province, from July 8, 2023 to September 18,2025.Through field sampling combined with indoor rearing, six morphological indicators of C. orientalis larvae were systematically measured, including head capsule width, mandibular length, mandibular width, body length, body width, and pronotum width. Frequency distribution analysis and Dyar's law were used to preliminarily determine the larval instars, and Crosby's growth rule together with regression models were applied to cross-validate and evaluate the instar determination performance of each indicator.

Result Among the six morphological indicators of C. orientalis larvae, the frequency distributions of head capsule width, mandibular length, mandibular width, and pronotum width each showed 19 aggregation peaks (distinct peaks), while body length and body width showed 18 and 20 distinct peaks, respectively. The coefficients of variation of head capsule width, mandibular length, and mandibular width were all below 20%, and the Crosby indices were all below 10%, which conform to Crosby's growth rule and can therefore be used as instar determination indicators. Based on these results, the larvae of C. orientalis were determined to have 19 instars. Regression analysis showed that the cubic model provided the best fit for larval instar determination, with head capsule width showing the highest goodness of fit with instar, the largest coefficient of determination (R² = 0.997 1), and the lowest Crosby index and coefficient of variation, indicating that it is the optimal indicator for larval instar determination.

Conclusion This study demonstrated that C. orientalis larvae in the Hexi Corridor region undergo 19 instars, and that head capsule width is the optimal indicator for larval instar determination. This article investigates the occurrence patterns and biological-ecological characteristics of the C. orientalis, aiming to develop control strategies and monitoring methods.