The Morphometry of Male Genitalia as a Reliable Tool for Identifying Forest Pests Dendrolimus sibiricus, D. pini (Lepidoptera: Lasiocampidae), and Their Hybrids in Eurasia

The Siberian moth, <i>Dendrolimus sibiricus</i> Tschetverikov, is one of the most destructive conifer pests in Northern Asia, causing severe ecological and economic losses. In Russia, its range overlaps with that of the closely related pine-tree lappet <i>Dendrolimus pini</i> (L.), and this raises the potential for hybridization and complicates accurate identification, particularly in the context of the potential westward expansion of <i>D. sibiricus.</i> Here, we present the first comprehensive morphometric analysis of male genitalia aimed at distinguishing these two major forest pests and their hybrids. The study was based on <i>D. sibiricus</i> and <i>D. pini</i> specimens collected during the last 130 years (1894-2024) across Europe and Asia, including their hybrids reared indoors by crossing <i>D. pini</i> females with <i>D. sibiricus</i> males in 1956 and preserved in the collection of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg, Russia). Overall, 70 permanent genitalia slides were prepared (33 <i>D. sibiricus</i>, 33 <i>D. pini</i>, and 4 hybrids), and the following genital structures were measured: valva and harpe length, aedeagus width and length, and cornuti length. <i>Dendrolimus sibiricus</i> had significantly larger genital structures compared to <i>D. pini</i>: 74% longer harpe, 32% longer valva, and a 28% wider and longer aedeagus. In contrast, in <i>D. sibiricus</i> cornuti were 21% shorter than in <i>D. pini.</i> Hybrids displayed intermediate values for valva, harpe, and aedeagus lengths, and for these parameters, they significantly differed from both parental species. The following diagnostic indices were suggested to distinguish between the two species and their hybrids: Harpe Length/Valva Length Index (HL/VL) and Cornuti Length/Aedeagus Length Index (CL/AL). Decision-tree analysis identified HL/VL as the strongest predictor for separating the parental species and the Combined Genital Proportion Index (CGPI), which integrates harpe, valva, aedeagus, and cornuti lengths, as the strongest predictor for identifying hybrids. The morphometric criteria developed here have practical applications for monitoring programs and quarantine diagnostics, particularly in sympatric zones and regions at risk of <i>D. sibiricus</i> expansion.