This study aims to establish accurate and efficient methods for identifying the external morphologicacharacteristics of all life stages of Streblote castanea, a pest of the mangrove plant Lumnitzera littorea, and to develop a larval instar determination system based on head capsule width. The findings are expected to provide a scientific basis for rapid pest identification, monitoring of infestation dynamics and damage severity, investigation of occurrence patterns prediction of occurrence trends, and development of control strategies. [Methods]  The external morphological features of each life stage (egg, larva, pupa, and adult) of S. castanea were systematically observed and recorded using visual inspection combined with high-definition digital photography. The head capsule width of larvae at each instar was measured with a vernier caliper. Multiple statistical methods, including frequency distribution analysis, Dyar’s rule, Brooks’ index, Crosby’s index, and regression analysis, were applied to analyze the larval head capsule width data. [Results]  The eggs are broadly oval with a smooth surface and dark brown patches. 1st-instar larvae possess a deep black, glossy head capsule and an orange-yellow dorsum on the mesothorax and metathorax. 2nd- and 3rd-instar larvae are morphologically similar, with head capsules covered by gray pubescence. 4th-instar larvae exhibit color differentiation, appearing in white, gray, and reddish-brown. 5th-instar larvae are generally similar in shape to fourth-instar larvae. 6th-instar larvae are distinguished by a dark “V”-shaped marking on the abdominal tergites. The pupae are encased in a cocoon covered with dark brown setae. Female adults are larger than males. Despite not conforming to Crosby’s growth rule, the head capsule width of 5th-instar larvae exhibited significant differences between adjacent instars (P < 0.05) and a high coefficient of determination (R²= 0.958 2). Combined with its strong sclerotization, distinct size variation boundaries, and low coefficient of variation, head capsule width serves as a reliable indicator for rapid larval instar discrimination. (1st instar: approximately 0.97 mm; 2nd instar: approximately 1.55 mm; 3rd instar: approximately 2.32 mm; 4th instar: approximately 3.39 mm; 5th instar: approximately 4.35 mm; 6th instar: approximately 5.39 mm). [Conclusion]  The external morphological characteristics described herein allow for rapid and accurate field identification of S. castanea at different life stages. Moreover, larval head capsule width measurements enable efficient and precise instar determination. This study provides essential baseline data and technical support for real-time monitoring, prediction of occurrence periods, and the development of targeted control measures against this pest.