Aggregation in Bark Beetles

Bark beetles belong to the family Scolytidae, many species of which aggregate on their host trees and then breed in the host tissues. Aggregation is mediated by pheromones in combination with other stimuli. Some species colonize living trees, some, weakened or dying trees, and others, recently fallen timber. 

Chemical Ecology of Insects. Edited by William J. Bell and Ring T. Card6 1984 Chapman and Hall Ltd. 

The mechanisms by which a tree or dead limb is first located by a bark beetle and then determined to be suitable or unsuitable for colonization are little 

Primary attraction has been demonstrated in several species. The clearest example is that of the ambrosia beetle Trypodendron lineatum which is attracted to ethanol produced under anaerobic conditions in dying trees (Moeck, 1970). Using host material cut or damaged to expose phloem and xylem tissues, primary attraction has also been demonstrated in four other ambrosia beetles, two Scolytus, two Dendroctonus, Ips typographus and five other species (Moeck, 1981). 

In none of his experiments could Moeck demonstrate that primary attraction is used in host selection by the three principal species in the area Dendroctonus brevicomis (the western pine beetle), D. ponderosae (the mountain pine beetle) and Ips paraconfusus (the California five-spined engraver beetle). What Moeck s results did show, however, was that beetles landed indiscriminantly on healthy and stressed trees at about one beetle a day on each tree. Theoretically only one beetle is needed to initiate mass attack, since as soon as it releases pheromone, landing rates would increase markedly on that tree. In Moeck s study, landing rates on trees that became attacked by D. brevicomis increased to up to 800 beetles a day. 

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Birch, M.C. (1984). Aggregation in bark beetles. In Chemical Ecology of Insects, W.J. Bell, R.T. Carde (Eds.), pp. 331-353, Chapman and Hall, ISBN 041223260X, New York, NY... 

Borden J. H. (1982) Aggregation pheromones. In Bark Beetles in North American Conifers, eds J. B. Mitton and K. B. Sturgeon, pp. 74—139. University of Texas Press, Austin. 

First identified in Asian elephants during a headspace analysis of volatiles collected from secretions of the musth temporal gland of adult males,156 frontalin (52) is a bicyclic ketal, which is structurally reminiscent of the male mouse priming pheromone component 3,4-dehydro-ara-brevicomin (37). Frontalin (52), Z-7-dodecen-l-yl acetate (51), was already known because of its chemosensory role in the insect world it is an aggregation pheromone in bark beetles.157 Interestingly, the ratio of the two enantiomers of frontalin (52) changes with age and stage of musth and elicits different behavioral responses.158... 

Terpenes are known to play important ecological roles not only in the defense of the tree, but also in bark beetle chemical ecology. Two chapters in this volume (see chapters by Raffa, et al, and Tittiger, et al.) review aspects of terpenoids as chemicals in conifer-bark beetle interactions and isoprenoid formation in bark beetle chemical ecology. While some species of bark beetles may be able to produce monoterpene pheromones de novo, bark beetles can also use host-derived monoterpenes as precursors to their own sex and aggregation pheromones. Bark beetle monoterpene pheromones are often used in a stereospecific manner such that often only one enantiomer is able to serve as a sex or aggregation pheromone. Since both enantiomeric host-derived monoterpene precursors are accepted and chemically modified by the beetle, the exact monoterpene enantiomeric mixture of conifer resin is important to both the beetle s ability to produce the correct pheromone and to the trees ability to escape an attack. ... 

Conifer resin, which is a mixture of monoterpenes and diterpenes is an important protective compound against bark beetles and other conifer herbivores. The volatile monoterpenes emanating from a specific tree is often the cue for bark beetles to find a tree where the tree defenses could be compromised from abiotic or biotic stresses. The synchronized mass attack is the strategy bark beetles use to reduce the effects of resin-based defenses in conifers. Aggregation hormones released by bark beetles are oxidized monoterpenoids such as ipsdienol, ipsenol and verbenol. It is believed that these compounds can be oxidation products of host plant monoterpenes such as myrcene. Recently it has been observed that most of the monoterpenoid aggregation pheromone components are biosynthesized de novo in bark beetles [7]. 

Claisen rearrangement of glycolates. Two laboratories 2 have reported that allylic glycolate esters undergo Claisen-Ireland rearrangement (6, 276-277) with useful diastereoselectivity. This rearrangement was used in a synthesis of 1, the aggregation pheromone of the European elm bark beetle.1... 

Separation of terpinen-4-ol enantiomers performed by a chiral GC columnand a chiral lanthanide shift reagent Eu(hfc)3, showed that the enantiomeric composition of an isolated compound from sweet marjoram oil was 73% (5)(- -) 27% R) —). The (4f )(—)-enantiomer was found in the oil of Eucalyptus dives Terpinen-4-ol was also found in several bark beetle species and is the main component in the aggregation pheromone of Polygraphuspoligraphus ... 

Attempts to investigate boll weevil (Anthonomus grandis) pheromone biosynthesis have identified isomerization, dehydration, and oxidation of the pheromone alcohols, and anticipated allylic oxidation of myrcene and limonene, but no evidence for the cyclization of acyclic precursors. The aggregation pheromones of bark beetles have been reviewed. Ips calligraphus responds to ipsdienol only in the presence of the c/5-verbenol (32) large additional concentrations of the enantiomer (l/ ,4i ,5/ )-(32) reduce beetle response. 5-(-)-Ipsenol, the pheromone of Ips grandicollis, increases the response of /. avulsus to its own pheromone ipsdienol. ... 

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