Pheromone production in a bark beetle independent of myrcene precursor in host pine species

Abstract

The California five-spined engraver, Ips paraconfusus (Coleoptera: Scolytidae), is an important pest of young pine forests in California and Oregon, where nearly all pine species within the range of this bark beetle are attacked [I]. The aggregation pheromone produced by males has been identified as a synergistic blend of three components (5')-( )-ipsenol, (5')-( + )-ipsdienol, and (45')-cis-verbenol 121. Ipsenol and ipsdienol accumulate only in males exposed to vapors of the host tree monoterpene, myrcene, in a logarithmic relationship 131. Recently emerged control males and females contain no volatile terpene compounds, neither pheromone components nor myrcene, while only myrcene is present in vapor-exposed females [3]. Hendry et al. [4] used deuterium-labeled myrcene to prove that myrcene could be converted by hydroxylation to ipsenol and ipsdienol in males exposed to vapors. Another host tree monoterpene, (-)-a-pinene, is converted in the vapor phase to cis-verbenol in both sexes 15, 61. A paradigm has been established that I. paraconfusus, and probably most other bark beetles of the genus Ips, use myrcene and a-pinene in their host tree as precursors to the aggregation pheromone components, ipsenol/ipsdienol and cis-verbenol, respectively [3 81. Host selection and host suitability may then depend, in part, on the concentration of myrcene and a-pinene in the tree 191. Pine trees exhibit a wide variation within and between species in their composition of myrcene and a-pinene, among other monoterpenes 110, 111. Thus, it has also been hypothesized that tree genotypes could have evolved to have lower titers of aggregation pheromone precursors as part of a resistance mechanism to bark beetles 181. We compared the relative attractiveness of five pine host species, ponderosa (Pinus ponderosa), sugar (P. lambertiana), Jeffrey (P. jeffreyl), digger (P. sabiniana), and lodgepole (P. contorta), that were infested with I. paraconfusus in order to detect possible differences in pheromone release and host suitability. Beetles were reared from naturally infested ponderosa pine collected from the Sierra National Forest, California. Fifty males were introduced (18:OO Aug. 29, 1985) to holes drilled in logs of each of the five host pines cut 2 days previously. These logs were wrapped with window screen and placed in sticky traps. A trap consisted of 6-mm mesh screen cylinders (19 cm, diam., 30.5 cm high) coated with Stikem Specialm at 1.2 m height. Traps were separated 10 m apart in a line (Sierra National Forest, California). Collections of flying beetles at each of the five infested logs were similar except for an approximate doubling of catch on the Jeffrey pine log (Fig. 1A). In all cases, attraction rates were significant since one to two or more females could have joined a male in his nuptial chamber if not intercepted by the traps, a natural pairing ratio in I. paraconfusus 1121. Another measure of the strength of a pheromone signal is the sex ratio of catch; a higher femalebiased ratio in I. paraconfusus indicates a higher release of pheromone, because high release rates cause inhibition of male response 1121. The collected sex ratios were similar to previous reports 1121, though the sex ratio on Jeffrey pine (a : Q = 1 : 15.6) was significantly higher than on the other pine species (Fig. lA), again indicating that a somewhat more potent attractant was released. Five days after introducing the males, the logs were dissected and no significant differences were noted in survival or in general appearance of nuptical chambers (43 47 per log). To determine the quantities of the pheromone components ipsenol, ipsdienol, and cis-verbenol in the feeding males, they were removed from nuptial chambers and their hindguts extracted in groups of eight in 150 p1 diethyl ether with 10 ng heptyl acetate per pl as an internal quantification standard. Monoterpenes, including myrcene and a-pinene, were extracted similarly from three samples of uninfested phloem (15-25 mg dry weight) from each of the infested logs using 250 p1 ether per sample, as well as from oleoresin (Table 1). Pheromone components and ipsenone in the males, and monoterpene hydrocarbons from the host were quantified by gas chromatography and mass spectrometry (GC-MS) using fused silica capillary columns (Fig. 1) and interpreted with respect to synthetic chemical standards (from Borregaard, Sarpsborg, Norway, and Aldrich Chemical Co., Milwaukee,