Reviews Experientia 45 (1989), Birkhfiuser Verlag, CH-4010 Basel/Switzerland 271 Chemical ecology of bark beetles J. A. Byers Department of Ecology, Animal Ecology, University of Lund, S-223 62 Lund (Sweden) Summary. The purview of chemical ecology and the recent criticisms of improper application of theory to bark beetle phenomena is briefly discussed. Seven levels of research in chemical ecology are presented as well as their relationship to research on bark beetles. The biology and chemical ecology of several pest bark beetles from North America and Europe are discussed in regard to host tree selection theories of random landing on trees or attraction to semiochem- icals. The diversity and similarities of pheromone components among species are presented in relation to their biosynthesis from host tree precursors and in relation to the ecological implications of de novo or precursor syntheses. Individual variation in biosynthesis of, response to, and release of pheromones is discussed. Olfactory perception of semiochemicals at both the electrophysiological and behavioral levels is presented. Orientation to semiochemicals during walking and flying is discussed with reference to the significance of dose-response curves for determining a compound's functionality in short- or long-range communication. The regulation of attack density, termination of the aggregation, mechanisms of attack spacing, and recognition of host suitability are presented in the context of an individual's avoidance of intra- and interspecific competition. Finally, a brief summary of topics where our under- standing of the chemical ecology of bark beetles and their associates is poorly known is presented. Key words'. Scolytidae; semiochemical; pheromone; allomone; kairomone. Chemical ecology and ecology mone. The first step in the research is to (1) observe the biological phenomenon - e.g. demonstrate that Ecology is the science of the relations of an organism to D. brevicomis are attracted to odor from the infested host both its biotic and abiotic environment which influence tree s9. Once the biology is partially understood, one can the organisms' distribution and abundance w. The biotic design bioassays which are used to (2) isolate and identify factors are included in the disciplines of physiology, be- at least one semiochemical component e.g. exo-brevi- havior, genetics and evolution; ecology is especially con- comin was isolated from female frass by solvent extrac- cerned with the interface of these areas. Chemical ecolo- tion, concentration, gas-liquid chromatographic (GC) gy then concerns any aspect of ecology but involves the fractionation (collecting into discrete fractions the con- external chemicals which mediate the interactions. This tinuous successive elution of chemicals from the GC) and definition is actually more comprehensive than many bioassay (testing each of the several fractions for attrac- would accept, for instance, it could include macrophage tive activity)129 Once a relevant compound is isolated, antibody microbe interactions, nutrient cycling, and enough must be obtained for structure elucidation via much of biology. Traditionally, chemical ecology has GC-mass spectrometry (GC-MS), and often other spec- been restricted to studies of the chemicals (semiochemi- trographic methods 127- 129. The third step involves (3) cals) which mediate interactions between individuals of a isolating and identifying all participating semiochemical species (pheromones) or between co-evolved species (al- components. For example, D. brevicomis males were lelochemicals, such as kairomones and allomones). shown to produce frontalin 73 which was synergistic with Alcock 1 criticized several bark beetle (Scolytidae) re- exo-brevicomin (i.e. neither compound alone is very at- searchers for what he thought was their inadequate appli- tractive but together the blend is highly attractive), and cation of ecological theory in the explanation of bark the host tree monoterpene, myrcene, further enhanced beetle phenomena. His primary concern was that bark the attraction 11, 12 beetle mass-attack and colonization of host trees was Once one chemical part of the ecological system, in this often misunderstood in terms of 'species-selection' when case the aggregation pheromone, is described, then (4) in fact 'individual selection' was now the dominant theo- other pheromones, which may also act as kairomones or ry. Today most realize that a deeper knowledge of bark allomones, can be identified with respect to the biological beetle biology can be obtained if both the proximate and phenomena. For example, intraspecific pheromone in- ultimate causes for the phenomena are considered. hibitors of attraction such as verbenone 13,109 and trans- verbenol ~3, 32 cause individuals to avoid colonizing in Levels of research in chemical ecology high attack density patches and thus function in termi- Research in chemical ecology of insects can be described nating the aggregation 45. To further understand the with seven hierarchial levels which to some extent are chemical ecology, which appears to grow ever more com- chronological. The following examples will refer to the plex, one must (5) quantify the rates of production and western pine beetle, Dendroctonus brevicomis, which ag- release of semiochemicals over the period of coloniza- gregates en masse in response to an aggregation phero- tion. Some studies have measured the release rates of 272 Experientia 45 (1989), Birkh~iuser Verlag, CH-4010 Basel/Switzerland Renews some semiochemicals from D. brevicomis over short peri- of the following discussion will best apply to those in the ods 27, and others have followed the production of pher- first group. A generalized life cycle of these bark beetles, omone components in the beetle's guts during the coloni- including many non-pest species, is depicted in figure 1. zation period 45. Before bark beetles begin their host tree and mate seeking Once the release rates of semiochemicals are known, the flight, there may be a period of required dispersal flight (6) mechanisms of pheromone production, olfactory per- before they become responsive to pheromone 4'51'65 ception, and the sites of biosynthesis and perception can However, several species appear responsive to phero- be investigated - these areas will be discussed later. How- mone or host attractants soon after emerging from brood ever, these topics become increasingly more peripheral to logs 40, 46 or overwintering sites 47. chemical ecology as they become more involved in phys- iology, behavior, genetics and evolution. Finally, one can Host selection. Little is known about dispersal and host (7) attempt a synthesis of the knowledge concerning selection except for the stages immediately preceding known semiochemicals, release rates, interspecific inter- landing on the tree. Host selection for several pest species actions and biological phenomena in order to construct of the western United States is thought to be a random a working theory on how the semiochemicals mediate the process in regard to landing on host and non-host interactions between individuals of various sexes and trees 92, 14-7. Ponderosa pines that were killed by freezing species. and screened to prohibit beetle attack, did not exhibit higher landing rates for I. paraconfusus, D. brevicomis or D. ponderosae, among others, than did healthy trees. Chemical ecology of bark beetles Landing rate differences also were not observed between Reviews of various aspects of bark beetle chemical ecol- healthy trees and trees with diseased roots. Other studies ogy since 1980 have concerned semiochemicals in host have shown that population densities of D. ponderosae 71 selection and colonization 147, host odorants and and Ips typographus 3, 42 are often high enough to allow pheromones 78, orientation 23, aggregation 18, aggrega- nearly every tree in a stand to be visited by at least one tion pheromones 22, 23 stridulation and pheromones 1x 7 beetle. If beetles test the defenses of host trees (although biosynthesis 59, and competition and semiochemicals 381 there is as yet no evidence of this) then in theory stronger These reviews cover the field in more detail than can be trees will repel beetles while weaker trees will allow pher- done here. Instead this paper will highlight the chemical omone production and mass colonization. It is well ecology of a few economically important systems: the known that conifers can be induced to exude copious pine beetles of the western and southern United States amounts of resin which can entrap beetles 45's6"89' 125 and the pine and spruce beetles of Europe in regard to and has some toxic properties as well 131. Healthy trees selected topics. are able to exude more resin at a higher pressure 141 while The diversity of bark beetles and their biology can be weaker trees, diseased or drought-stressed, are less able appreciated in the taxonomic compilation of Wood 152 to produce resin. In late summer when trees in California and in discussion of mating systems 74. However, pest are drought-stressed it is often observed that attacks of bark beetles are included in three categories: 1) 'aggres- D. brevicomis are not defended by resin flow and trees sive' phloem-feeding beetles which must attack en masse easily succumb 89. Thus, in many bark beetle species host living trees and kill them in order to reproduce, 2) vec- selection appears to occur after landing on the bark, and tors of tree diseases such as Dutch elm disease, and in L paraconfusus it was