Using quantitative analysis of external morphological characteristicsto distinguish larval instars of Parapoynx crisonalis(Walker) (Lepidoptera: Crambidae)
Author of the article:CHEN Qi1, 2 XIA Xi-Ju1 HUANG Jian-Bin1 MA Li1, 2 WANG Xing1, 2**
Author's Workplace：1. College of Plant Protection, Hunan Agricultural University, Changsha 410128, China; 2. Hunan Provincial Key Laboratory forBiology and Control of Plant Diseases and Insect Pests, Hunan Agricultural University, Changsha 410128, China
Key Words：Parapoynx crisonalis (Walker), larval instar, head capsule width, Crosby ratio
Abstract：
[Objectives] To establish a method to determine the larval instars of Parapoynx crisonalis (Walker, 1859) and thereby improve the accuracy of monitoring this pest in the field. [Methods] Four larval morphological variables, including head capsule width, distance between the compound eyes, body width, and body length, were measured. Based on frequency analysis and frequency histograms, Crosby’s law and linear regression analysis were used to choose the best morphological criteria for distinguishing larval instars. [Results] Head capsule width was the optimal criterion for distinguishing instars, since its mean coefficient of variation and Crosby ratio were the smaller than all other indices. On this basis, P. crisonalis has five instars. The average head capsule widths of 1st to 5th instar larvae were (0.2493±0.0053), (0.3454±0.0018), (0.5079± 0.0031), (0.7419±0.0190) and (1.1287±0.0369) mm, respectively. [Conclusion] Head capsule width is the best morphological criterion for distinguishing the instars of P. crisonalis, followed by the distance between the compound eyes.