# Determination of the Morphometric Characteristics of Larval Instars in the Sap Beetle Urophorus humeralis (Coleoptera: Nitidulidae)

## TL;DR

This study identifies the first infestation of pear fruits by Urophorus humeralis and establishes a reliable method to distinguish larval stages using head capsule width measurements.

## Contribution

The study introduces the first morphometric framework for Urophorus humeralis larval staging and confirms its infestation of pear fruits.

## Key findings

- Head capsule width is the most reliable morphological indicator for distinguishing larval instars.
- Frequency distribution and Dyar’s rule validation confirmed three distinct larval instars.
- The study provides baseline data for Urophorus humeralis biology and pest management strategies.

## Abstract

This study provides the first report of Urophorus humeralis infesting pear fruits and establishes detailed morphometric criteria for the larval instars of this sap beetle. By measuring the larval three indicators—head capsule width (HCW), inter-antennal distance (IAD), and inter-caudal distance (ICD)—head capsule width was confirmed as the most reliable and stable morphological characteristic for distinguishing larval instars. Frequency distribution analysis and verification of Dyar’s rule (describes the phenomenon where the width of the clypeus in insects of adjacent age classes exhibits a geometric relationship) based on linear regression confirmed the existence of three distinct larval instars. It not only offers a valuable reference framework for the developmental classification of U. humeralis, but also enhances our understanding of its biological characteristics and life history (a comprehensive description of the series of developmental stages an individual goes through from birth to death), thereby supporting the refinement of pest monitoring systems and optimization of management measures at each developmental stage. The findings contribute to the development of more efficient and sustainable control strategies for sap-feeding beetles in pear orchard production systems.

Effective integrated pest management (IPM) relies on precise knowledge of pest developmental biology, particularly the identification of larval instars, which is fundamental for predicting population dynamics and timing control interventions. This study established a morphometric framework for the larval staging of a sap beetle pest infesting pear orchards. Specimens were collected and reared under laboratory conditions, with their identity confirmed as Urophorus humeralis through integrated morphological and molecular (COI barcoding) analysis. To determine the number of larval instars, head capsule width (HCW), inter-antennal distance (IAD), and inter-caudal distance (ICD) were measured. Frequency distribution analysis and validation using Dyar’s rule via linear regression revealed three distinct larval instars. Head capsule width was identified as the most reliable and consistent morphological character for instar discrimination. This study reports for the first time the infestation of pear fruits by U. humeralis and provides detailed morphometric criteria for larval staging, delivering essential baseline data for the biology of Nitidulidae and a scientific basis for developing stage-specific pest management strategies.