Discrimination of Odors Associated With Conspecific and Heterospecific Frass by Sibling Species Dendroctonus frontalis (Coleoptera: Curculionidae: Scolytinae) and Dendroctonus mesoamericanus (Coleoptera: Curculionidae: Scolytinae).

In the Central American region, the aggressive, sibling bark beetles Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae: Scolytinae) and Dendroctonus mesoamericanus Armendáriz-Toledano & Sullivan (Coleoptera: Curculionidae: Scolytinae) commonly colonize pines concurrently, and in nature they avoid heterospecific pairing, although it can be produced in the lab. We performed walking arrestment bioassays in the lab to examine the capacity of both sexes of both species to discriminate odors from frass expelled from gallery entrances of either solitary females or conspecific pairs of either species. Males of both species strongly preferred odors of frass from solitary, conspecific females over those of heterospecific females or pairs of either species. Female D. frontalis did not discriminate among these frass categories, whereas female D. mesoamericanus preferred frass of conspecific females. In gas chromatography-electroantennographic detection and gas chromatography-mass spectrometry analyses, we determined that males of both species could sense a nearly identical spectrum of approximately 16 host- and beetle-produced compounds present in frass of females of one or both species. Only two of these compounds, endo-brevicomin and ipsdienol, which were present in frass of female D. mesoamericanus and pairs of either species but absent in frass of solitary D. frontalis females, qualitatively distinguished these categories. Several known attractants and synergists for either species declined in concentration postpairing. Our results complement earlier research and indicate how semiochemical composition and concentration in frass might mediate male discrimination of attack sites of conspecific, unpaired females. Furthermore, our data indicate that semiochemical responses for walking females differ from those of males and between species.

Responses by Dendroctonus frontalis and Dendroctonus mesoamericanus (Coleoptera: Curculionidae) to Semiochemical Lures in Chiapas, Mexico: Possible Roles of Pheromones During Joint Host Attacks.

In southern Mexico and Central America, the southern pine beetle Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae: Scolytinae) commonly colonizes host trees simultaneously with Dendroctonus mesoamericanus Armendáriz-Toledano and Sullivan, a recently described sibling species. We hypothesized that cross-species pheromone responses by host-seeking beetles might mediate joint mass attack, bole partitioning, and reproductive isolation between the species. Previous studies had indicated that D. frontalis females produce frontalin and that female D. mesoamericanus produce frontalin, endo-brevicomin, and ipsdienol (males of both species produce endo-brevicomin and possibly ipsdienol). In field trapping trials in the Mexican state of Chiapas, D. frontalis was attracted to the lure combination of turpentine and racemic frontalin; racemic endo-brevicomin enhanced this response. In a single test, D. mesoamericanus was attracted in low numbers to the combination of turpentine, racemic frontalin, and racemic endo-brevicomin after the addition of racemic ipsdienol; in contrast, racemic ipsdienol reduced responses of D. frontalis. Inhibition of D. frontalis was generated in both sexes by (+)- and racemic ipsdienol, but by (−)-ipsdienol only in females. Logs infested with D. mesoamericanus females (the pioneer sex in Dendroctonus) attracted both species in greater numbers than either D. frontalis female-infested or uninfested logs. Our data imply that D. frontalis may be more attracted to pioneer attacks of D. mesoamericanus females, and that this could be owing to the presence of endo-brevicomin in the latter. Possible intra- and inter-specific functions of semiochemicals investigated in our experiments are discussed.

Pheromone-Mediated Mate Location and Discrimination by Two Syntopic Sibling Species of Dendroctonus Bark Beetles in Chiapas, Mexico.

Where their geographic and host ranges overlap, sibling species of tree-killing bark beetles may simultaneously attack and reproduce on the same hosts. However, sustainability of these potentially mutually beneficial associations demands effective prezygotic reproductive isolation mechanisms between the interacting species. The pine bark beetle, Dendroctonus frontalis Zimmermann, is syntopic in the Central American region with a recently described sibling species, Dendroctonus mesoamericanus Armendáriz-Toledano and Sullivan, but mechanisms for their reproductive isolation are uncertain. We investigated whether semiochemicals mediate species discrimination by mate-seeking males of both species. In olfactometer bioassays, walking males of both species strongly preferred odors from gallery entrances of conspecific females. Coupled gas chromatography-electroantennographic detection and gas chromatography-mass spectrometry isolated 16 olfactory stimulants for males in these odors, but only two, ipsdienol and endo-brevicomin (both from D. mesoamericanus females), differed in quantity in female-associated odors between the species. In olfactometer bioassays, with 10, 1, or 0.1 female entrance equivalents of synthetic semiochemicals, the combination of ipsdienol and endo-brevicomin inhibited responses of male D. frontalis and enhanced responses of male D. mesoamericanus to two compounds associated with female entrances of both species (the pheromone component frontalin and host odor α-pinene). We conclude that ipsdienol and endo-brevicomin, pheromone components produced by females of just one of the two species (D. mesoamericanus), mediate interspecific mate discrimination by males of both species and provide an apparently symmetrical reproductive isolation mechanism.