A Comparison of Three Approaches for Larval Instar Separation in Insects—A Case Study of <i>Dendrolimus pini</i>

• Main Author: Lidia Sukovata
• Format: Article
• Language: English
• Published: MDPI AG 2019-11-01
• Series: Insects
• Subjects: brooks-dyar’s rule; clustering; crosby’s growth ratio; <i>dendrolimus pini</i>; frequency distribution; head capsule width; instar; kernel density
• Online Access: https://www.mdpi.com/2075-4450/10/11/384

The number of larval instars is important from both theoretical and practical perspectives. Three previous studies based on head capsule widths (HCWs) have suggested that <i>Dendrolimus pini</i> larvae pass through seven instars, but the estimated HCW means differed greatly. Various methods are available for determining the number of instars; however, these methods have not been compared on the same dataset. Therefore, the main goal of this study was to compare three approaches for instar separation in <i>D. pini</i> larvae: visual approach followed by non-linear least squares (NLLS) estimation, kernel density estimation (KDE) followed by NLLS, and model-based clustering. Two criteria were used to assess whether the resulting instar separations adhered to Brooks-Dyar&#8217;s rule: Crosby&#8217;s growth rule and a coefficient of determination indicating the goodness of fit of a straight line to the ln-transformed mean HCW of the respective instars. Our results showing that <i>D. pini</i> larvae pass through eight instars differed greatly from reports in the literature. The best results were obtained by KDE followed by NLLS. For proper instar separation, both criteria of Brooks-Dyar&#8217;s rule must be met.