Mating Systems of the Tree-Killing Bark Beetle Polygraphus proximus (Coleoptera: Curculionidae: Scolytinae)

Colonization success of a tree-killing bark beetle: Geographic variation and mismatch with host preference

The preference-performance hypothesis (PPH) predicts that female insects maximize their fitness by ovipositing on hosts where their offspring perform the best. The preference-performance relationships in bark beetles are complex because before offspring development can occur in the phloem, adult bark beetles must first successfully invade host trees, and then construct galleries beneath the bark. Therefore, a positive correlation between host preference and successful colonization is necessary for the PPH in bark beetles to hold (i.e., the preference-colonization hypothesis in bark beetles). In this study, through field choice experiments, I investigated the successful colonization of the bark beetle, Polygraphus proximus, within four allopatrically distributed Abies species across a distinct biogeographic boundary in Japan. The results of this study showed that the biogeographic boundary did not limit the successful colonization by P. proximus. I observed that successful colonization was low in A. firma, despite it being an exotic species in the study sites and the most preferred at the study sites, indicating a mismatch between preference and colonization success. Additionally, I observed that A. sachalinensis had a high colonization success rate, even though it was the least preferred species at the study sites.

Identification of sex-specific compounds in the invasive four-eyed fir bark beetle Polygraphus proximus

Polygraphus proximus, a four-eyed fir bark beetle, is an invasive bark beetle species which has caused extensive damage to forests of Abies sibirica in southern and western Siberia and to Abies species in the European part of Russia. There is a high risk that the pest insect will spread to areas where it is currently not considered present, such as the European Union. In these areas, it threatens to attack conifer forests of various species which may result in major environmental and economic impact. The aim of this study was to identify pheromone components of P. proximus that can be used as pheromone baits. Males and females of P. proximus were allowed to bore into the bark of stem sections of Abies sibirica at the laboratory, and volatiles were collected with solid-phase microextraction (SPME). Analyses of these extracts with gas chromatography-mass spectrometry (GC–MS) revealed several sex-specific compounds. In total, twelve male-specific compounds and one female-specific compound were identified. The major male-specific compound determined by GC peak area was (Z)‐2‐(3,3‐dimethylcyclohexylidene)‐ethanol [(Z)-DMCHE] and the minor male-specific compounds were 3-methyl-3-buten-1-ol, 3-methyl-2-buten-1-ol, 3-methyl-2-butenal, benzyl alcohol, fragranol, 7-methyl-3-methylene-6-octen-1-ol, (Z)- and (E)-2-(3,3-dimethylcyclohexylidene)-acetaldehyde, geraniol, geranial and papayanol. The only female-specific compound was identified as 1-hexanol. Two of the male-specific compounds, (Z)‐DMCHE and 3-methyl-2-buten-1-ol were shown to attract males and females of P. proximus in field studies. Thus, we now for the first time can present the structures of two male-specific components that are biologically active parts of P. proximus aggregation pheromone. However, some chemical communication overlap between P. proximus and P. subopacus needs to be further investigated as (Z)‐DMCHE also attracted males and females of P. subopacus.

Distribution pattern of entry holes of the tree-killing bark beetle Polygraphus proximus

Bark beetles attack their hosts at uniform intervals to avoid intraspecific competition in the phloem. Bark texture and phloem thickness also affect bark beetle attacks, and the bark characteristics are not spatially homogeneous; therefore, the distribution patterns of entry holes can demonstrate an aggregated distribution. Polygraphus proximus Blandford (Coleoptera: Scolytinae) is a non-aggressive phloephagous bark beetle that feeds on Far Eastern firs. They have caused mass mortality in Russia and Japan. However, the distribution pattern of entry holes of P. proximus and spatial relationships with bark characteristics have not been studied. Thus, we investigated the distribution pattern of entry holes of P. proximus. The distribution of entry holes was significantly uniform in most cases. As the attack density increased, an aggregated distribution pattern within a short distance (< 4.0 cm) was observed. The rough bark had a significantly higher number of entry holes than the remaining bark. The distribution pattern of entry holes demonstrated a significantly aggregated spatial association with rough bark. Finally, rough bark around knots had significantly thicker phloem than the remaining barks. These suggest that P. proximus may preferentially attack rough bark to reproduce in the thicker phloem under a rough bark surface.