Key structural features in cis‐carane, (+)‐3‐carene, cis‐pinane, (+)‐α‐pinene, and (−)‐β‐pinene influencing red turpentine beetle primary attraction when released with ethanol

Abstract

In US Pacific Northwest ponderosa pine forests the primary attraction order shown previously for red turpentine beetle, Dendroctonus valens (Coleoptera: Curculionidae: Scolytinae), is (−)-β-pinene+ethanol > (+)-3-carene+ethanol > (+)-α-pinene+ethanol. The monoterpenes are bicyclic C 10 H 16 isomers containing one 6-carbon ring with one double bond. Both pinenes have a 4-carbon second ring and differ only by their endocyclic or exocyclic double bond. The (+)-3-carene second ring has 3-carbons; its double bond is endocyclic like (+)-α-pinene. 2 Ring system and double bond influences on primary attraction were evaluated by hydrogenating (+)-3-carene and (+)-α-pinene to cis-carane and cis-pinane, respectively. Field test primary attraction strengths were (−)-β-pinene+ethanol > cis-carane+ethanol > cis-pinane+ethanol > ethanol. 3 In combination with ethanol (i) a double bond is not required in either ring system to attract D. valens, (ii) the cis-carane bicyclic 3, 6-carbon ring system provides stronger beetle attraction than the cis-pinane 4, 6-carbon bicyclic ring system, and likely structural basis for stronger (+)-3-carene attraction over (+)-α-pinene, (iii) adding an exocyclic double bond to the 4, 6-carbon ring system elevates attraction above the 3, 6-carbon ring system with no double bond, and (iv) the 4, 6-carbon ring system is a much stronger attractant with an exocyclic rather than endocyclic double bond.

Red turpentine beetle primary attraction to β‐pinene or 3‐carene (with and without ethanol) varies in western US pine forests

“…(+)‐3‐Carene (>90.0%), and (−)‐β‐pinene (≥97%) were purchased from Sigma Aldrich (St. Louis, MO) and dispensed from low‐density polyethylene (LDPE) bottles (15 mL, The Cary Company, Addison, IL) similar to those used in previous tests (Erbilgin et al, 2007; Kelsey & Westlind, 2020, 2021, 2022; Liu et al, 2013; Sun et al, 2004). They were filled in advance to the threaded neck with a small air bubble under the cap, then grouped by lure type in sealed plastic bags and stored at 5°C.…”

“…and screw cap (0.95 cm dia.) in an X pattern (Kelsey & Westlind, 2020, 2021). A screen inside the cap prevented insects from entering the top holes, and the outside jar wall was wrapped with aluminium foil to reduce solar heating.…”

Red turpentine beetle primary attraction increases linearly with (−)‐β‐pinene+ethanol dose regardless of component ratios, and no change in response with addition of high‐release frontalin

“…Lure mixing jars used here were smaller (9.3 × 9.7 cm, D × L cap on, 473 mL), with five equally spaced ventilation holes drilled into the bottom (0.79 cm diameter) and into the screw cap (0.95 cm diameter) in an X pattern (Kelsey & Westlind, 2020, 2021). A screen inside the cap prevented insects from entering the top holes, and the outside jar wall was wrapped with aluminium foil to reduce solar heating.…”

“…However, in Washington and Oregon, (−)‐β‐pinene+ethanol lures attracted 1.4–1.9 times more beetles than (+)‐3‐carene+ethanol, depending on the site, and (+)‐α‐pinene+ethanol or (±)‐α‐pinene+ethanol lures were least attractive at all sites where tested (Kelsey & Westlind, 2020). The exocyclic double bond in (−)‐β‐pinene is a key structural feature contributing to its stronger primary attraction over (+)‐3‐carene and (+)‐α‐pinene when ethanol is present (Kelsey & Westlind, 2021).…”

β‐Pinene + ethanol attracts more red turpentine beetles than carene+ ethanol, with or without traces of frontalin, at prescribed burn or thinned sites

“…To simulate mixing of ethanol and monoterpenes vapours from stressed conifer tissues (Gara et al, 1993), the ethanol pouches and weighed monoterpene bottles were combined in a wide mouth, high‐density polyethylene jar (9.3 × 9.7 cm, D × L cap on, 473 mL, Uline, Pleasant Prairie, WI) with five equally spaced ventilation holes in an X pattern drilled in the bottom (0.79 cm dia.) and screw cap (0.95 cm dia., Kelsey et al, 2023; Kelsey & Westlind, 2020, 2021). A screen inside the cap prevented insects from entering the top holes, and the outside jar wall was wrapped with aluminium foil to reduce solar heating.…”