5.1.3 Genetic-based variation of host plants and bark beetles

5.1.3 Genetic-based variation of host plants and bark beetles

Evolution of plant chemicals that increase the tree's resistance to colonization by a bark beetle population requires that (1) the plant chemicals are detrimental to the beetle, (2) the host chemistry is genetically determined, (3) populational variation in genotypes of these trees exists, and (4) the bark beetle exerts selection pressure on the tree (by killing or reducing fertility). The beetle population should coevolve, if possible, by shifting their genotype frequencies to those that offer more protection against the plant chemicals. The disadvantage for the tree in this "arms race" is that the beetle may undergo between 25 and hundreds of reproductive cycles compared to one generation for the tree, thus the chances for beneficial genetic recombinations and mutations are greater for the insect. Mutant bark beetles of greater endurance would in the beginning have help in killing "resistant" trees from "normal" beetles (that would die more frequently), until gradually mutant beetles would become the dominant, or only, genotype.

Many chemicals in the tree that affect colonization by bark beetles may not be under selection pressure from the insect. Chemicals that are required in physiological processes by the tree may not be readily dispensed with in an evolutionary response to evade their secondary use by insects. For example, various sugars are transported by the phloem and required by the tree for growth; the same sugars may be feeding stimulants for the beetle. If the tree could dispense with the sugars it would become undesirable as food. However, this is unlikely since all trees use sugars (photosynthate) in many biosynthetic pathways (e.g., cellulose).

A theory accounting for the evolution of bark beetle races of D. ponderosae each adapted to feeding in ponderosa, lodgepole or limber pines has been presented by Sturgeon and Mitton (1986). The three species of trees occur together in Colorado and are colonized by D. ponderosae. Five enzymes each varying in several isozymes that migrate differently in electrophoresis gels were investigated among beetles taken from the three host tree species. The isozyme frequencies, which represent different alleles at a polymorphic loci, were different among the beetles from the three hosts. The beetles from limber pines were less heterozygous than beetles from the other two hosts. Furthermore, heterozygous beetles were less numerous than expected, suggesting that selection had occurred against these beetles because they were not well adapted to any of these three hosts. If mating between host populations was restricted, for example by different emergence times due to differing development times in each host, then host races could develop. However, no host-related differences in isozyme variation were found for D. frontalis from shortleaf or loblolly pines (Namkoong et al., 1979). Langor et al. (1990) naturally reared D. ponderosae from limber and lodgepole pines and cross-bred them in each species again. They found small reductions in egg production and hatching when pairs were mated from different pine sources compared to the same host source, although beetles from all possible crosses could reproduce - indicating the "host-races" did not appear reproductively isolated (at least under epidemic conditions) thus precluding speciation.

Host trees also vary in monoterpenes which are undoubtedly genetically regulated. Tree monoterpenes appear to affect colonization of bark beetles in a variety of ways (discussed in parts 5.2 to 5.6). Monoterpenes (examples shown in Fig. 4) vary little within a tree, moderately between trees of the same species within a habitat, and greatly between geographic regions; the largest differences are evident among conifer species (Mirov, 1961; Smith, 1964, 1967, 1968, 1969; Sturgeon 1979; Byers and Birgersson, 1990). Genetic differences among beetles over large geographic areas may, in part, reflect the variation in the monoterpene composition of their host. For example, bark beetle populations of D. ponderosae, D. frontalis, D. terebrans, and I. calligraphus from different regions when analyzed for certain enzymes by electrophoresis were found to vary genetically within a species (Stock et al., 1979; Namkoong et al., 1979; Stock and Amman, 1980; Anderson et al., 1979, 1983).

There is also semiochemical evidence that bark beetles vary genetically over geographic regions. I. pini varies geographically in their production of and response to pheromone enantiomers of ipsdienol (Lanier et al., 1972, 1980; Miller et al., 1989). Two populations of D. pseudotsugae from Idaho (inland) and coastal Oregon, USA, were found to differ in isozyme frequencies (Stock et al., 1979). These two populations also have a number of possible genetic-based differences in behavioral responses to semiochemicals: (1) ethanol is much more attractive to inland beetles (Pitman et al., 1975; Rudinsky et al., 1972), (2) trans- verbenol inhibits pheromonal response in inland beetles but not in coastal beetles (Rudinsky et al., 1972), (3) the inhibitor 3- methylcyclohex-2-en-1-one (3,2-MCH, Fig. 5) is produced by coastal females but not in inland females (Pitman and Vit‚, 1974; Rudinsky et al., 1976). Borden et al. (1982) found Trypodendron lineatum response to host-released ethanol and alpha-pinene differed between continents. Western North American beetles responded weakly to ethanol plus alpha-pinene and these compounds did not enhance a strong attraction to the aggregation pheromone lineatin; whereas beetles in England were similarly attracted to lineatin or to the two host volatiles, and their combination was synergistically active.

Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.