A Comparison of Three Approaches for Larval Instar Separation in Insects-A Case Study of Dendrolimus pini.

Lidia Sukovata

doi:10.3390/insects10110384

Abstract

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 Dendrolimus pini 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 D. pini 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’s rule: Crosby’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 D. pini 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’s rule must be met.

Highlights

The pine-tree lappet moth, Dendrolimus pini L. (Lepidoptera: Lasiocampidae) is one of the most important defoliating insect species of Scots pine Pinus sylvestris L. in Europe and Asia [1]
All larvae from the first group and different portions of larvae from the two latter groups were reared in the laboratory, in different years, under the room conditions, in groups of up to 50 larvae at younger, i.e., first through fourth, instars, and up to 30 larvae at older instars on Scots pine twigs maintained in 45 × 30 × 23 cm or 60 × 30 × 21 cm glass aquaria with the two smaller opposite sides covered with a net to ensure air flow
It was determined that D. pini larvae pass through eight instars during their growth, the eighth instar might be a supernumerary instar as a result of larval development in unfavorable conditions

Summary

Introduction

The pine-tree lappet moth, Dendrolimus pini L. (Lepidoptera: Lasiocampidae) is one of the most important defoliating insect species of Scots pine Pinus sylvestris L. in Europe and Asia [1]. In Poland, outbreaks of this species have become more frequent and widespread in the last two decades than in the previous 60 years [2], and might have been caused by climate change. Climate change seems to be responsible for an increase in the D. pini populations in Scotland [3] and an isolated outbreak in the Stockholm archipelago [4]. Depending on the country and region, another pine moth species, Dendrolimus punctatus (Walker) may have up to five generations per year [8,9]. The probability of outbreaks increases with increasing voltinism, reproductive synchrony, synchrony with host plants, etc. It seems that instar ratios in the larval population could serve as an indicator of population status

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Conclusion