ANCESTRAL SEMIOCHEMICAL ATTRACTION PERSISTS FOR ADJOINING POPULATIONS OF SIBLING IPS BARK BEETLES (COLEOPTERA, SCOLYTIDAE)

Three species of Ips pine bark beetles in Hopping's group IX (S.L. Wood's grandicollis group), Ips confusus, I. lecontei, and I. paraconfusus, are parapatrically distributed in the American Southwest. They share post-Pleistocene altitudinal ecotones with their host pines. Adjacent to these areas of host overlap, we tested the hypothesis that aggregation pheromones produced by male beetles and/or host volatiles are sufficient to elicit the speciesspecific colonization behaviors typical of these three Ips species in nature. A more distantly related species, I. pini (Hopping's group IV, S.L. Wood's pini group) was used for outgroup comparison. Under the influence of pheromone, males of I. confusus and I. paraconfusus do not discern among unin-fested log bolts of host and nonhost pine prior to bark contact. Males responding to pheromones emanating from infested bolts are similarly undiscriminating. Females of I. confusus and I. lecontei olfactorily discern the combination of conspecific males in host pine from other possible beetlepine combinations; females of I. paraconfusus do not. Female I. pini discerned conspecific pheromone from that of I. lecontei. The bark beetle predator, Enoclerus lecontei, is attracted by, but does not discriminate among, the male-produced volatiles of these Group IX Ips species. These results support a hypothesis that divergence in pheromonal responses by these group IX Ips species has evolved following their speciation, having been manifested first in the female sex. Evolutionarily, the derived pheromonal messages have preceded their behavioral discrimination by these beetles. Additional speciesspecific cues may operate between the sexes in the field that may preclude heterospecific pairings. © 1990 Plenum Publishing Corporation.