Attraction of Scolytus unispinosus bark beetles to ethanol in water-stressed Douglas-fir branches
Introduction
The Douglas-fir engraver, Scolytus unispinosus LeConte (Scolytidae), is a non-aggressive or ‘secondary’ bark beetle that typically attacks weakened, dying, or recently dead Douglas-fir and occasionally other species of conifers (McMullen and Atkins, 1962, Furniss and Carolin, 1977). They usually colonize the stem and branches of crowns where the bark is thin, preferring crowns exposed to sunlight over crowns in the shade (Stoszek, 1973). Live trees weakened by drought stress may be attacked by S. unispinosus, depending on the tissue water potentials (Stoszek, 1973). These beetles were attracted to Douglas-fir stems with water potentials of −2.58 MPa, and colonization began at a threshold level of −2.94 MPa. Advanced egg galleries were observed in trees with water potentials exceeding −3.44 MPa. When the water status of drought stressed trees improved after rainfall, the S. unispinosus were trapped or ‘pitched out’ in their galleries by oleoresin flow (Stoszek, 1973).
In July of 1998, while setting up an unrelated experiment, the top 2–3 m from several Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) crowns were cut and left on the ground with full exposure to the sun. One week later we noted these crowns were being attacked by S. unispinosus. We analyzed a few tissue samples from the drying crowns and found high ethanol concentrations. Various species of secondary scolytids are attracted to ethanol, or ethanol plus host monoterpenes (Cade et al., 1970, Moeck, 1970, Klimetzek et al., 1986, Phillips et al., 1988, Chenier and Philogene, 1989, Liu and McLean, 1989, Schroeder and Lindelow, 1989, Lindelow et al., 1993; Kelsey, 1994a, Kelsey, 1994b; Kelsey and Joseph, 1997, Kelsey and Joseph, 1999a). Therefore, we hypothesized that ethanol might play a role in host selection by S. unispinosus. We designed the present experiment to examine the synthesis and accumulation of ethanol in rapidly drying Douglas-fir tissues and to determine whether ethanol is a primary host attractant for S. unispinosus.
Section snippets
Study site
This study was conducted in a Douglas-fir plantation (44°48′N, 123°17′30′′W), approximately 3.4 km south of Monmouth, Oregon. The site was planted in 1985 with 2-0 stock seedlings germinated from seed collected at various populations in western Oregon and Washington. The trees were planted in rows running north–south and east–west. When selected for this experiment they were 3.1 m apart and had a mean diameter of 17.7 cm±0.7 S.E. at breast height.
Treatments
On 31 July 1998, seven trees were selected from the
Ethanol concentrations
The woody tissues from stressed branches contained two and three orders of magnitude more ethanol than defoliated or control branches (both p≤0.002), respectively, whereas ethanol concentrations in the defoliated and control branches did not differ (p=0.382) (Fig. 1). In needles, ethanol was affected by treatment and needle age (Fig. 1). Current- and previous-year needles on stressed branches contained 182 and 24 times more ethanol (p<0.001 and 0.044), respectively, than control branches. On
Discussion
S. unispinosus attacked and colonized water-stressed Douglas-fir branches only if they contained high ethanol concentrations, thus providing strong evidence that ethanol functions as a primary attractant for this bark beetle. On 19 August, there were two stressed branches without beetle attacks, and they contained the lowest ethanol concentrations within this treatment. Five days later these two branches were attacked and the ethanol concentrations in their needles and woody tissues had
Acknowledgements
We thank Dr. R. Silen, PNW Research Station, for allowing us access to the Douglas-fir plantation. We thank Dr. K. Cromack and Dr. D. Ross from Oregon State University for comments and review of the manuscript. The use of trade names is for the information and convenience of the reader and does not constitute endorsement or approval by the US Department of Agriculture.
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