Ethanol and primary attraction of red turpentine beetle in fire stressed ponderosa pine

Abstract

Trees injured and weakened by wild or prescribed fires are susceptible to bark beetle attacks. Nonaggressive Dendroctonus valens in North American forests is often the first bark beetle to colonize fire injured pine. Although not an important contributor to post-fire mortality it does vector pathogens, and surviving diseased trees may serve as pathogen redistribution centers. Traps baited with both ethanol and monoterpene lures are known to attract D. valens, and fire injured pine are known to accumulate ethanol in tissues containing monoterpenes. The primary objective of this study was to quantify ethanol concentrations in fire injured ponderosa pine tissue near pioneering D. valens gallery entrances and compare them with levels from the unattacked side of the same tree, or their unattacked neighbors. The secondary objective was to quantify α-pinene in the same samples. Two separate but related studies were conducted, one in a June wildfire, the other in an October prescribed fire, near Bend, Oregon, USA where phloem and sapwood cores were collected for analysis by headspace gas chromatography. Wildfire damaged trees attacked by D. valens were sampled above beetle gallery entrance holes and on the opposite bole side without an entrance hole. An adjacent unattacked tree of similar size and injury was sampled at the same aspect and height. Prescribed fire damaged trees were sampled in groups of three characterized by: (1) one or more D. valens attacks with 100% crown scorch; (2) unattacked with 100% crown scorch; and (3) unattacked with ≤95% crown scorch. They were sampled above a beetle gallery entrance or equivalent positions on unattacked trees. Ethanol and α-pinene concentrations at both fires were higher in tissues above beetle entrance holes than in corresponding tissues from unattacked sides of the same tree, or adjacent trees. Ethanol concentrations in wildfire damaged trees were three or more orders of magnitude greater than in prescribed fire trees, likely a consequence of wildfire trees experiencing higher temperatures and greater heat stress indicated by injury measurements. Ethanol concentrations in stem bark char, analyzed only in the prescribed fire were two to four orders of magnitude greater than in the underlying phloem, a result of it being adsorbed and concentrated during outward diffusion. Ethanol synthesis, accumulation, and atmospheric release in combination with host monoterpenes, is proposed as the critical physiological process contributing to initial D. valens host tree and bole position selection of fire stressed ponderosa pine.