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Dendroctonus frontalis Zimmermann is considered one of the most important economic and ecological forest pests in the United States, Mexico, and Central America. Recently, two apparent morphological variants of this species were discovered occurring syntopically in Central America and southern Mexico. Morphotype A beetles lack a series of fine parallel ridges on the episternal area of the prothorax that are present on morphotype B. The goal of the present work was to clarify the taxonomic status of the morphotypes of the D. frontalis species complex. Geometric morphometric analyses of seminal rod and spermatheca shape together with the characterization of 16 attributes of external morphology revealed differences in quantitative and qualitative characters that distinguished adults of the two morphotypes from each other as well as from the closely related species Dendroctonus vitei Wood and Dendroctonus mexicanus Hopkins. Karyotype analysis of morphotype B revealed a chromosomal formula (5AA + Xyp) distinct from that found in morphotype A previously reported for D. frontalis (7AA + Xyp). In the laboratory, forced intermorphotype crosses produced F1 progeny but at lower frequency than intramorphotype pairings, and dissections of spermatheca revealed a lower frequency of insemination at least one type of heterotypic cross. Phylogenetic analysis of the D. frontalis species complex based on 786 bp of the cytochrome oxidase I gene indicated that morphotypes B and A are two independent groups with 98% nodal support within D. frontalis. These data provide compelling evidence that the two syntopic morphotypes represent two distinct sibling species.

Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae: Scolytinae) is a major economic pest of pines in the United States, Mexico, and Central America. We report biochemical investigations relevant to the taxonomic status and semiochemistry of two distinct morphotypes of D.frontalis recently detected in the Central American region. Morphotype A beetles (pre-episternal area of prothorax of both sexes smooth, bulging callus on anterolateral margin of prothorax of females) and morphotype B beetles (pre-episternal area of prothorax of both sexes with fine ridges, reduced female callus) collected in infestations in Chiapas, Mexico differed significantly in their production of 10 behaviorally-active compounds occurring in the genus Dendroctonus, including the major pheromone components for D. frontalis. Notably, host-attacking morphotype B females produced hundreds of nanograms of both endo-brevicomin and frontalin, whereas morphotype A females produced similar amounts of frontalin but subnanogram quantities of endo-brevicomin. Reanalysis of a published D. frontalis trapping study in Chiapas indicated that both morphotypes responded in greatest numbers when frontalin and endo-brevicomin baits were both present. In addition, we quantified 18 different cuticular hydrocarbons (the methyl-branched alkane components) from both morphotypes collected in Belize and Chiapas as well as morphotype A beetles from the southeastern United States, and principal component analysis revealed nonoverlapping clusters associated with either morphotype. This evidence of two distinct, complex phenotypes coexisting in the same sites and host trees supports the hypothesis that the D. frontalis morphotypes represent separate species and consequently indicates that the taxonomy of D. frontalis should be re-evaluated in the Central American region.