MONOTERPENE VARIATION IN PONDEROSA PINE XYLEM RESIN RELATED TO WESTERN PINE BEETLE PREDATION

Pinus ponderosa Laws. is one of the most widely distributed pines in North America, extending from southern British Columbia to Durango, Mexico, and from South Dakota and Nebraska to the Pacific coast (Fowells, 1965). Two varieties which are distinguished from the Pacific coast form by differences in morphology and growth characteristics are recognized, P. ponderosa var. scopulorum Engelm., which grows in the Rocky Mountain region, and P. ponderosa var. arizonica (Engelm.) Shaw which is found in southern Arizona, New Mexico, and in northern Mexico (Critchfield and Little, 1971). Recently, as a result of an investigation of the chemical composition of its xylem resin, the range of ponderosa pine was divided into five chemical regions separated by transition zones (Fig. 1) (Smith, 1977). Variation in the proportions of the major monoterpenes, alpha-pinene, betapinene, delta-3-carene, myrcene, and limonene, was used to establish the regional and zonal classification for the species. The individual monoterpene components present are a specific property of a plant species although considerable quantitative variation may occur between individuals and between populations of one species (Harborne, 1973). Both the kinds of monoterpene present and the relative proportions in which they are found are genetically controlled (Squillace, 1976). In a series of papers Smith (1964a, 1964b, 1964c, 1968, 1977, Smith et al., 1969) showed that the monoterpene composition of individual ponderosa pines does not vary with position within a tree, with age of the tree or with environmental conditions such as season or site. In addition, he showed that populations of ponderosa pine are chemically highly polymorphic; he identified 115 different types of trees, two of which account for 30% of nearly 5,000 trees sampled. These chemical regions and zones coincide with the ranges and areas of greatest abundance of many species of bark beetles of the genus Dendroctonus. Bark beetles are major predators of ponderosa pine throughout its range (Hopkins, 1911; Smith et al., 1969; Smith, 1977) and, except for a short flight period, they complete their entire life cycle within the bark of the pine (Rudinsky, 1962). In the western United States the western pine beetle, Dendroctonus brevicomis Le Conte, is the most destructive insect attacking ponderosa pine (Keen, 1952). D. brevicomis has been particularly devastating to forests in northern California and southern Oregon (Miller and Keen, 1960) where, coincidentally, two chemical regions converge (Smith, 1977). The association of chemical regions of Pinus ponderosa with members of the genus Dendroctonus suggests that these groups of organisms may have coevolved (Smith, 1977). Monoterpenes have been shown to play a role in the selection and colonization of pines by bark beetles (Wood, 1973). For example, the monoterpene myrcene enhances the response of aggregating beetles to the female D. brevicomis pheromone (Bedard et al., 1969; Silverstein, 1970). On the other hand, limonene was shown to be toxic or inhibitory to D. brevicomis (Smith, 1965, 1966, 1975). In related bark beetle species, D. frontalis, D. terebrans and Ips paraconfusus, trans-verbenol and verbenone are synthesized from alpha-pinene by microorganisms symbiotically associated with the insects (Brand et al., 1975, 1976; Hughes, 1975). These compounds are also produced by the western