Electrophysiological and Behavioral Responses of the Bark Beetle Dendroctonus rhizophagus to Volatiles from Host Pines and Conspecifics

Abstract

The bark beetle Dendroctonus rhizophagus is endemic to northwestern Mexico where it kills immature pines < 3 m tall. We report the first investigation of the chemical ecology of this pest of forest regeneration. We used GC-EAD to assess olfactory sensitivity of this species to volatile compounds from: resin of a major host, Pinus arizonica; mid/hindguts of single, gallery-initiating females; and mate-paired males within galleries of attacked host trees in the field. Antennae of both sexes responded to monoterpenes α-pinene, β-pinene and 3-carene as well as to the beetle-derived oxygenated monoterpenes fenchyl alcohol, myrtenal, cis-verbenol, trans-verbenol, verbenone, and myrtenol. These monoterpenes were quantified from pre-emerged D. rhizophagus adults forced to attack host tissue in the laboratory, and from individuals dissected from naturally-attacked hosts at different stages of colonization. In both bioassays, myrtenol and trans-verbenol were the most abundant volatiles, and trans-verbenol was the only one produced in significantly greater quantities by females than males in a naturally-colonized host. Two field experiments were performed to evaluate behavioral responses of D. rhizophagus to antennally-active monoterpenes. Results show that 3-carene was significantly attractive either alone or in a ternary (1:1:1) combination with α-pinene and β-pinene, whereas neither α-pinene nor β-pinene alone were attractive. None of the beetle-associated oxygenated monoterpenes enhanced the attractiveness of the ternary mixture of monoterpenes, while verbenone either alone or combined with the other five oxygenated terpenes reduced D. rhizophagus attraction to the ternary mixture. The results suggest that attraction of D. rhizophagus to the host tree P. arizonica is mediated especially by 3-carene. There was no conclusive evidence for an aggregation or sex attractant pheromone.

Comparison of orthologous cytochrome P450 genes relative expression patterns in the bark beetles Dendroctonus rhizophagus and Dendroctonus valens (Curculionidae: Scolytinae) during host colonization

“…In the absence of functional data, our results suggest that the induction of CYP6BW5v1 , CYP6DJ2v1 and CYP9Z20v1 genes in D. rhizophagus could be related to monoterpene metabolism, and consequently to production of oxygenated monoterpenes. The elevated expression levels of these genes observed in females and males forced to attack trees in the field coincides with the increase of oxygenated‐monoterpenes ( trans ‐verbenol, cis ‐verbenol, myrtenal, myrtenol and verbenone) recorded in the midgut and hindgut of solitary females and males and pairs of this species under both laboratory and natural conditions (Cano‐Ramírez et al ., ). The expression of the genes CYP6DG1v3 and CYP6DJ2v3 in the gut of both sexes of D. valens also suggests a similar function, because this induction level also coincides with high levels of oxygenated monoterpenes recorded in the midgut and hindgut of females and males, whether solitary or in pairs (Shi & Sun, ).…”

“…Nevertheless, our results show that the overexpression of these genes declines as the colonization attack proceeds. The overexpression of P450 genes before oviposition (T1 and T2 stages) in D. rhizophagus coincides with the production of significant amounts of certain oxygenated monoterpenes in both sexes during the same stages (Cano‐Ramírez et al ., ). Some of these compounds are produced in similar quantities by both sexes; however, the oxygenated monoterpene trans ‐verbenol is produced in higher quantities by females (Cano‐Ramírez et al ., ).…”

“…Expression patterns of cytochrome P450 genes of the families 6 and 9 suggested that they may be involved in the detoxification of these compounds. Moreover, it was recorded that during the first stages of natural tree colonization (solitary females boring into the bark, and paired males and females mining a gallery prior to oviposition), both sexes of D. rhizophagus had high concentrations of oxygenated monoterpenes in the posterior midgut and hindgut, and these concentrations decreased considerably after oviposition (Cano‐Ramírez et al ., ). A similar physiological response was found in the hindgut of both sexes, alone or paired, in D. valens fed on phloem (Shi & Sun, ).…”

“…It has been demonstrated that the common pine monoterpenes (+)‐3‐carene, (‐)‐β‐pinene and (+)‐α‐pinene alone or mixed (1:1:1 ratio) are attractive to these species (Hobson et al ., ; Sun et al ., ; Zhang & Sun, ; Cano‐Ramírez et al ., ), with (+)‐3‐carene typically exhibiting greater activity than the other compounds (Erbilgin et al ., ; Cano‐Ramírez et al ., ). Additionally, both species produce oxygenated monoterpenes in the posterior midgut and hindgut after they are fed or stimulated with vapours of these monoterpenes (Shi & Sun, ; Cano‐Ramírez et al ., ). Some of the oxygenated‐monoterpenes (eg cis ‐verbenol, trans ‐verbenol, myrtenol, myrtenal, fenchyl alcohol and verbenone) share the same carbon backbone of pinenes (particularly α‐pinene) and are presumably generated through direct oxidation of them.…”

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

“…The GC-EAD apparatus and antennal preparation procedures were identical to those in Cano-Ramirez et al (2012). The GC was fitted with an HP-INNOwax capillary column (Agilent Technologies, Wilmington, DE, USA; polyethylene glycol phase; 30 m long, 0.25 mm diam, 25 μm film thickness) and used a temperature program of 50°C for 1 min, 16°C.min −1 to 80°, 7°C.min −1 to 200°C, and then held for 10 min.…”