Byers, J.A. Chemical Ecology of Bark Beetles. Experientia 45:271-283.


Attack density regulation and termination of aggregation/attack mediated by semiochemicals. In response to the pheromone from the pioneer beetles resident in the tree, flying beetles orient to the tree, some joining established galleries while others begin excavations in the bark. The release of pheromone from these many new attacks increases perhaps as a logistic function such that the resinous defenses of the tree are overwhelmed45,68, 108,152. Once the tree has been rendered effectively defenseless, the functional cooperation among beetles ends and competition for the two-dimensional phloem becomes progressively more severe as the aggregation and attack continues2,14,34 .

This intense competition has provided strong selection pressures for the evolution of individuals adept at avoiding or reducing the degree of competitive interactions with conspecifics and with individuals of competing species34,38. As mentioned previously, I. paraconfusus males can determine the likelihood of competition based on the strength of the pheromone signal. They avoid orientating to the highest release sources while females prefer these areas of the tree33. At the peak of the female landing period on a felled tree, the male landing rate actually declined indicating that males may have been attracted (as shown in long-range orientation through a grid of traps) but did not land and then chose to fly elsewhere33. These behaviors would serve to spread the colonization area in Ips33,120, 121 and P. chalcographus44 as well as by a more simple "spill-over" or imprecision in orientation to pheromone. This latter mechanism may also account primarily for the switch of attack focus to surrounding trees by D. frontalis and other pest species3,52,1 19.

Bark beetles appear to have behavioral mechanisms which space the attacks and gallery systems in order to reduce competition to tolerable levels. The mechanisms by which beetles choose whether to attack near previously attacked sites and at what distance are poorly known. Spacing of attacks has been shown in D. ponderosae124,11 8, D. frontalis88, D. pseudotsugae67, T. piniperda96 and I. typographus34. A computer method of comparing simulated attack densities at various minimal spacings between nearest neighbors to the natural attack patterns has been developed34. It showed that I. typographus attacks would have the observed average nearest neighbor distances if the beetles initiated entrance holes at least a minimum allowed distance (MAD) of 2.5 cm from other holes. This distance was apparently the same regardless of the natural attack density and thus appears to be an inherent biological distance characteristic of the species34.

The possible mechanisms that account for these spaced attack distributions and the MAD are (1) olfactory communication34, (2) stridulation/acoustic communication67,112 -116, (3) selection of bark structures57,118, and (4) visual inspection of attack sites or beetles, or various combinations of these. Olfactory communication between Ips males has been discussed above and could also function even more strongly at close-range. In D. pseudotsugae, an arriving male stridulates at the female's entrance hole and stimulates her to release MCH113,115. The male also releases MCH83,106 which has an inhibitory effect at long-range and apparently also at close- range112-115. Verbenone, trans-verbenol and (+)-ipsdienol have been shown to inhibit attraction of D. brevicomis to pheromone components at long- range13,30 and (-)-trans- verbenol at close-range32. Female stridulation alone has been implicated in the spacing of D. brevicomis113. Bark structure does influence the selection of attack site57,118 as well as the density of attacks, but it may have little affect on the spacing patterns or MAD.

Termination of aggregation and attack may be the result of saturation of the available bark areas as constrained by the spacing mechanism (MAD)34. After the sexes pair, the production of pheromone declines as shown experimentally in I. paraconfusus28 and by observations of gut volatile contents in tree-colonizing I. typographus17 and D. brevicomis45. Thus, a spacing mechanism and cessation of attractant release after mating could explain the mechanism of termination of the aggregation. However, in T. piniperda termination may also be influenced by verbenone emanating from infested logs with only a small portion from the beetles themselves43,80. Similar studies with I. typographus have shown that verbenone from microbial activity in infested trees may play a major role in termination of the aggregation6,15,8 1,82,119 .

In Fig. 5, semiochemical mechanisms which mediate the aggregation, attack density and termination of aggregation are presented for four species of bark beetle.

Fig. 5. Theoretical mechanisms for regulation of aggregation and attack density (intraspecific competition) and termination of aggregation in four species of bark beetles. A. Ips paraconfusus colonization of ponderosa pine. The male beetle arrives first (1) and constructs a nuptial chamber in the phloem layer. His release of ipsenol (Ie), ipsdienol (Id) and cis-verbenol (cV) [2], attracts both sexes [3]126-128,150, but at higher concentrations near the source, males are inhibited in close-range orientation and thus land in adjacent areas of lower male attack density [4](Byers 1983)33. This process is repeated and serves to evenly spread the colonization and regulate attack density [5](Byers 1983)33. After males are joined by several females, their production of the pheromone components, Ie and Id, declines rapidly [6](Byers 1981)28 and the tree becomes unattractive to beetles [7]. However, it appears that an additional mechanism (verbenone, V, from microbes in the tree?) is needed to both regulate density and terminate attack initiation during the later stages of colonization [8]. V has been shown to inhibit attraction to natural pheromone(Byers and Wood 1980)39.

B. Ips typographus colonization of Norway spruce. The male arrives first [1] and releases 2-methyl-3-buten-2-ol (MB) and cis-verbenol (cV) [2] which attracts both sexes from a distance [3]8, but as males approach high concentrations of pheromone they become less precise than females in close-range orientation and so land in adjacent areas [4]120,121. These sex-specific behaviors function to spread the attack and limit attack density in a way similar to I. paraconfusus. Males of I. typographus also reduce their release of pheromone components after "mating" [5]17 but in addition they release small quantities of ipsenol (Ie) and ipsdienol (Id)15,17, which inhibit response of both sexes probably only at close range [6]6,119. These compounds could function to both regulate attack density and terminate aggregation (along with a decline in attractants). However, verbenone (V) from microorganisms in the bark beetle galleries [7] could be the more general inhibitor responsible for termination of aggregation [8]6,81,82,119.

C. Dendroctonus brevicomis colonization of ponderosa pine. The female beetle arrives first [1] and bores into the trunk and after feeding produces exo-brevicomin (eB) [2]129, which primarily attracts males [3]35,37. Males, upon locating a female gallery, soon release frontalin (F) [4]74, which synergizes with eB to elicit a mass aggregation [5]11,12. However, at the same time females and males produce trans- verbenol (tV)32 and males produce verbenone (V) and (+)-ipsdienol (Id)30,45. At close range these compounds apparently inhibit the attraction of beetles to eB and F (tV appears to primarily affect females while Id and V affect both sexes, which would regulate the attack density13,30,32. After several days the production and release of eB and F diminishes to levels that are unattractive at long range45. The few females attracted during this latter period may be inhibited from attacking by the still significant, although reduced levels of tV [7]45. The few males would not find any unpaired females and so would continue searching elsewhere. Verbenone may also increase in release rate as the tree decays and so inhibit beetles [8], although measurements have not been done.

D. Tomicus piniperda colonization of Scots pine. Both sexes arrive simultaneously in mass on the first April day above 13o C in response to host monoterpenes, terpinolene, (± )-alpha-pinene and (+)-3-carene, released from resinous wounds incurred during winter storms [1]47,80. Females and males pair up under bark flakes and the females tunnel into the bark followed immediately behind by the male. Trees that have fallen earlier in the winter and are beginning to decay probably release ethanol which in relatively small amounts may enhance the attraction to monoterpenes [2]76 but at higher amounts is inhibitory76. Both sexes contain verbenone (V) in the largest amounts shortly after entering the tree and amounts decline thereafter [3]80 while V released from the tree continues to increase [4]43 possibly due to microbial activity in the tissues surrounding the galleries. Ethanol may increase in release rate due to fermentative processes (unproven) while monoterpene release gradually decreases43. V released in traps at rates comparable to rates released from infested sections of a tree was able to inhibit attraction of both sexes to the attractive monoterpenes also released at natural rates43. V may serve both to space attacks (when released from individuals) and to terminate the aggregation (when released generally by microbial decay).

Interspecific interactions mediated by semiochemicals. Semiochemicals function in interactions between bark beetles in primarily two ways: (1) as allomones for interspecific communication of resource use in order to avoid competition and (2) as kairomones for use in locating weakened hosts colonized by another species. In California and other regions of the western United States, four pest bark beetles compete more or less for ponderosa pine (Fig. 6).

Fig. 6. Inhibition of the attraction response to conspecific pheromone by allomones produced by four sympatric bark beetles, Dendroctonus brevicomis, D. ponderosae, Ips paraconfusus, and I. pini which appear to function in reducing interspecific competition for ponderosa pine in California. Response inhibition by the pheromones trans-verbenol, verbenone, and (+)- ipsdienol may also reduce intraspecific competition in D. brevicomis. The asterisks indicate pheromone attractants and black squares indicate pheromone inhibitors of attraction.

Verbenone released by male D. brevicomis, in addition to its intraspecific effects, appears to inhibit the response of its competitor, I. paraconfusus, to pheromone39. Verbenone has also been found in I. pini males from Idaho that had fed in red pine logs, but no behavioral role was ascribed79. However, I suggest here that the observed inhibition of attraction of I. paraconfusus to its pheromone by logs infested with I. pini19 appears due in part to verbenone, a major volatile component in I. pini79,142. Earlier it was reported that only (-)-ipsdienol from I. pini was responsible for inhibition of I. paraconfusus84. Thus, the inhibitory effect of verbenone on I. paraconfusus may have resulted from selection pressures to reduce interspecific competition from both D. brevicomis and I. pini (Fig. 6). Similarly, the inhibitory effects of verbenone on D. brevicomis could be the result of selection pressures from both intra- and interspecific competition (from I. pini). Multiple selection pressures could also account for the inhibitory effects of trans-verbenol on D. brevicomis32 (Fig. 6) which is produced in significant amounts by D. brevicomis32, by I. pini79,142 and by D. ponderosae107. The (+)-ipsdienol produced by male I. paraconfusus126 and by male D. brevicomis in the early stages of colonization45 may function both to reduce intraspecific competition in both species30,33 and to reduce interspecific competition from I. pini in both species (Fig. 6)20,30.

A similar situation of competitive interactions exists for four sympatric bark beetles of southern pines (Fig. 7).

Fig. 7. Inhibition of the attraction response to conspecific pheromone by allomones produced by four sympatric bark beetles, Dendroctonus frontalis, Ips grandicollis, I. calligraphus, and I. avulsus which appear to function in reducing interspecific competition for southern pines in the southeastern United States21,136. Verbenone also acts as a pheromone in D. frontalis that reduces response to aggregation pheromone. The asterisks indicate pheromone attractant and the black square a pheromone inhibitor.

I. calligraphus produces (-)-ipsdienol as one of the attractive components142,14 4 and this also acts as an allomone to inhibit I. grandicollis attraction to its pheromone21,143. Allomonal volatiles from I. avulsus are inhibitory to I. calligraphus21. Verbenone, produced by D. frontalis106,10 9 acts as a pheromone that inhibits attraction as well as an allomone that inhibits attraction of I. grandicollis21. The use of allomones appears to reduce competitive interactions among the southern pine beetles just as they do in the western pine beetles.

The bark beetles attacking pines in the southern United States are different from their western counterparts18,19, 39, however, in that they are cross-attractive to each others pheromones (Fig. 8)21,136.

Fig. 8. Cross attraction of four sympatric bark beetles colonizing pines in the southeastern United States21,136. Thick lined arrows indicate strong attraction (65%-100% of conspecific attraction), thin lines indicate weaker attraction (5-12%) and dashed lines little attraction (2-5%).

The reason for the strongest interspecific attraction, I. avulsus attraction to pheromone of I. calligraphus, is probably that I. avulsus locates weakened hosts already infested by I. calligraphus and that the potential competition is tolerable since both species utilize somewhat different levels of the tree (Fig. 7)98,136.

Therefore, the above discussion indicates that olfactory systems evolve as a result of the relative benefits of multiple factors including avoidance of intra- and interspecific competition. Coevolution would favor the sharing of semiochemicals so that specific compounds could serve in communication both intra- and interspecifically. There would also be a selection pressure for use of biosynthetic pathways that produce the semiochemicals from host precursors, as long as the precursors were consistently available. Thus (+)-ipsdienol, from myrcene, and trans-verbenol (and possibly verbenone), from alpha-pinene, may have become pheromone components and allomones for a growing list of species. However, another hypothesis is that verbenone was already consistently present in decaying hosts due to microbial activity25,26,4 3,82, and then was subsequently used over evolutionary time by bark beetles as pheromones and allomones38.

The predators, parasites and other associated insects interacting with bark beetles comprise a complex guild of species. During the course of aggregation of D. brevicomis on ponderosa pine, over 100 species of insects were collected134. Associated insects of bark beetles locate the breeding habitat either by landing at random, or using kairomones from the beetle (often their pheromones)7,12,1 40,146,1 47 or other kairomones from the dying tree76 or possibly other associated insects. The primary requirements of a kairomone are that it be a reliable indicator of host condition or presence and that the host (or fermentative process) can not dispense with its use. Thus, pheromone components of bark beetles are the most likely volatiles to be utilized as kairomones by predators and parasitoids since the adaptive advantages for the emitter outweigh the disadvantages as a kairomone, and so no net selection pressure for changing to other compounds will occur.

The chemical ecology of adult and larval interactions under the bark (e.g. they appear to avoid intersecting galleries38), callow adult movements, emergence, and overwintering (Fig. 1, d-h) are practically unknown. Gregoire et al.66 indicated that larval D. micans maintained a feeding aggregation in response to cis-verbenol. Fungal interactions with bark beetles in regard to host tree resistance68,108, 125, nutrition of larvae10 and host unsuitability25,26, 43,81 are not well understood. Sexual recognition and species recognition are due in part to stridulation9 but probably more so by specific cuticular chemicals105 - a subject not yet investigated in bark beetles.

The field of chemical ecology of bark beetles encompasses much more than can be covered here, but it is hoped that one can appreciate that bark beetles and their associates are among the best of model systems for the study of chemical ecology.

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