Mixed Messages Across Multiple Trophic Levels: the Ecology of Bark Beetle Chemical Communication Systems
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Utah State University DigitalCommons@USU Quinney Natural Resources Research Library, The Bark Beetles, Fuels, and Fire Bibliography S.J. and Jessie E. 2001 Mixed Messages Across Multiple Trophic Levels: The Ecology of Bark Beetle Chemical Communication Systems Kenneth F. Raffa Follow this and additional works at: https://digitalcommons.usu.edu/barkbeetles Part of the Ecology and Evolutionary Biology Commons, Entomology Commons, Forest Biology Commons, Forest Management Commons, and the Wood Science and Pulp, Paper Technology Commons Recommended Citation Raffa, K. (2001). Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems. Chemoecology, 11(2): 49-65. This Article is brought to you for free and open access by the Quinney Natural Resources Research Library, S.J. and Jessie E. at DigitalCommons@USU. It has been accepted for inclusion in The Bark Beetles, Fuels, and Fire Bibliography by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Chemoecology 11:49–65 (2001) 0937–7409/01/020049–17 $1.50+0.20 © Birkha¨user Verlag, Basel, 2001 Mini-review Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems* Kenneth F. Raffa Department of Entomology, University of Wisconsin – Madison, Madison, WI 53706, USA Summary. Chemical, physiological and behavioral differential exposure to some predators, the difficulty components of pheromone communication have been of locating signalers during extensive endemic periods, described for a number of bark beetle species, yet our and the low costs incurred during host assessment. understanding of how these signals function under However, the possibility that beetles employ flexible, natural conditions remains relatively limited. Develop- density – dependent strategies deserves heightened at- ment of ecologically based models is complicated by tention. the multiple functions and sources of variability inher- The ability of bark beetles to collectively exhaust ent in bark beetle semiochemistry. This discussion ad- host defenses poses a particular problem for plant dresses four ecological issues of chemical signaling in defense. It is argued here that the ideal defense should bark beetles: the effects of aggregation on individual include both direct resistance mechanisms against fitness, the possibility of cheating, how plants can de- invading beetles, and indirect mechanisms that in- fend themselves against herbivores that employ aggre- hibit chemical communication. Evidence for the gation pheromones, and the implications of variability latter mechanism is explored. The ability of predators in chemical communication systems to predator to efficiently exploit aggregation pheromones as avoidance. kairomones in prey finding poses significant risk to An analysis of published data from thirteen bark beetles. It is proposed that minor alterations in scolytid – conifer systems indicates that the net benefit pheromone components may provide colonizers with and optimal colonization density vary with host condi- partial escape from such natural enemies while main- tion and beetle species. When beetles attack live trees, taining intraspecific functionality. the benefit of cooperative host procurement exceeds Traditional interpretations emphasized the fidelity losses due to competition for the limited substrate, at and consistency of pheromones, but under natural least up to moderate densities. When beetles colonize conditions chemical signals are modified by unpre- dead tissue, however, the effect of subsequently arriv- dictable features of the biotic and abiotic environment. ing beetles on initial colonizers is almost entirely nega- Although we typically view variation in pheromonal tive. This suggests that aggregation originated as signals as experimental noise or simple deviations from exploitation of senders, but evolved into manipulation a population norm, such variation may reflect evolu- of receivers. It is also proposed that the optimal colo- tionary dynamics. Complex ecological interactions nization density which typifies each species or popula- may impose trade-offs between the clarity versus di- tion may offer a more objective and less value–laden versity of their signals. index of behavior than current labels such as ‘‘aggres- siveness’’. Beetles can maximize the relative benefits of Key words. Pheromones – Scolytidae – coevolution – group attack by incorporating instantaneous measures cooperation – competition – insecta – pinaceae of host resistance into their colonization behavior, and by adjusting oviposition with colonization density. This system may provide opportunities for cheating. Introduction – signaling in a complex and variable However a number of factors may select against a environment fixed strategy of cheating, including the linkage be- tween tree allelochemistry and beetle semiochemistry, ‘‘The behaviour of scolytids – including the effects the reduced quality of substrate available to late arriv- and significance of secondary attraction – must be ers, the short adult lifespans of most bark beetles, expected to vary between and even within species … Without such behavior, these species could not utilize temporary habitats’’ Atkins 1966: Behavioural variation among Scolytids Correspondence to: Kenneth F. Raffa, e-mail: raffa@entomology. in relation to their habitat wisc.edu * From a presentation at the Plenary Session of the International Research during the last three decades has generated a Society of Chemical Ecology, Vancouver, BC; May, 1997. wealth of mechanistic information on the isolation, 50 K. F. Raffa CHEMOECOLOGY synthesis, molecular biology, and chemoreception of feedback, their variation in space and time, the herita- bark beetle (Coleoptera: Scolytidae; altern: Curculion- ble and environmentally induced plasticity among indi- idae: Scolytinae) pheromones (e.g., Borden 1982; Wood viduals, and the difficulty of communicating under 1982a; Byers & Birgersson 1990; Vanderwel 1994; Sey- natural conditions (Schlyter & Birgersson 1989; Beris- bold et al. 1995a, 2000; Cognato et al. 1999). Labora- ford et al. 1990; Miller et al. 1997). Beetles are con- tory and field assays have likewise provided a strong fronted with plant species compositions in their forest information base on the responses of numerous bark habitats that can range from highly diverse communi- beetle species to a complex array of pheromone signals ties to almost pure monocultures (both natural and (e.g., Lanier & Wood 1975; Mustaparta et al. 1980; managed). Superimposed on this mosaic is variation in Dickens et al. 1983; Byers et al. 1984; Borden et al. host plant physiology, canopy architecture, and 1986; Teale et al. 1991; Salom et al. 1993; Bertram & weather. Moreover, beetles must communicate against Paine 1994; Ross & Daterman 1998; Paine et al. 1999). an enormous background emission of plant and animal But despite a relatively strong knowledge base on the volatiles in natural ecosystems. For example, Litvak et chemical identity of bark beetle pheromones and how al. (1999) estimated conifer emissions of monoter- they affect landing behavior, we still know relatively penoids into the atmosphere at 120–480 Tg/yr. This little about how these signals actually function under background emission is complex, often similar to the natural conditions. chemicals used by bark beetles as pheromones, and In its simplest sense, bark beetle chemical communi- largely unpredictable (Byers 1983, 1995; Birgersson & cation is an input – output system. Plant compounds Bergstro¨m 1989; Wilson et al. 1996; De Groot & Mac- stimulate the biosynthesis of pheromones, either from Donald 1999). Even once effective communication with host precursors or de no6o pathways. This biosynthesis conspecifics can be established, a diverse guild of eaves- proceeds under the regulation of internal and external droppers, such as habitat-specialist predators, host-spe- stimuli. The resulting molecules leave the plant surface, progress through the atmosphere, reach insect anten- cific parasitoids, and competing subcortical herbivores, nae, and bind with chemoreceptors. Volatile stimuli, in can greatly reduce bark beetle reproductive success combination with visual and other cues, elicit landing. (Wood 1982a; Mizell et al. 1984; Ross & Daterman However, each of these steps is compounded in nature 1995; Poland & Borden 1997). From an ecological by multiple sources of biotic and abiotic variation. perspective, bark beetles not only need to communicate, Pheromone synthesis is known to vary among beetles they must whisper their message amidst the noisy ca- within a population, and within beetles as they age cophony of thousands of intra- and inter-specific (e.g., Miller et al. 1989a). Most bark beetles colonize conversations. several species of trees, which vary significantly in their An ecological framework is crucial both to a com- phytochemistry, thereby adding further complexity to prehensive understanding of bark beetle chemical com- the plume (Wood 1982a; Miller & Borden 1990; Erbil- munication, and to fully implementing these powerful gin & Raffa 2000a). Moreover, different trees within tools for pest management (Borden 1989; Payne & populations of each host species vary in their chemistry, Billings 1989). The challenge, as in much of biology, is and individual trees vary throughout the season, in to bridge the gap between pattern and process. Field response to environmental stress, and with age. Enor- experiments