Abstract: In order to reveal the development of the carpenter moth Holcocerus vicarius (Walker) larva and forecast its occurrence time, the head capsule width, body length, body width, pronotum width, mandible length and mandible width were measured, and Crosby law and linear regression analysis were used to choose the optimal morphological index for determining the larval instars of H. vicarius. The results showed that the head capsule width provided the best index to divide instars, since its mean coefficient of variation and Crosby ratio were the least, while those of other indexes were greater. H. vicarius was determined to have twenty instars. Different instars conformed to the growth regularity of larval head capsule width proposed in Dyar’s law. The linear regression equation of the head capsule width against the instar number was: y=0.233+1.686x+0.127x²-0.005x³ (R²=0.996). Determination of the larval instar number of this moth could provide the basis for studying its occurrence and biological characteristics, and facilitate to develop comprehensive prevention and control.

Key words: Holcocerus vicarius, larval instar, Crosby ratio, Brooks ratio, index of instar separation