John A. Byers

Chemical ecology of bark beetles

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 semiochemicals. 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 compounds 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 individuals avoidance of intra- and interspecific competition. Finally, a brief summary of topics where our understanding of the chemical ecology of bark beetles and their associates is poorly known is presented.

Olfactory Recognition of Host-Tree Susceptibility by Pine Shoot Beetles

Storm-fallen Scots pines with broken roots and trees with severed tops exude wound oleoresin. These trees are susceptible to bark beetles due to an injured vascular system that can not provide adequate oleoresin in order to resist new attacks by beetles [I]. Once a tree is attacked, most bark beetles use pheromone attractants to locate mates and often to overcome tree resistance through a mass attack [I, 21. It would be clearly advantageous for bark beetles to have evolved sensory systems for efficiently locating their host and in recognizing whether a particular host was less resistant than most healthy trees. However, little is known of how bark beetles select their host from among other plant and tree species [I], or what may attract the first individuals to a susceptible host. We have investigated the semiochemical basis of the mass aggregation of pine shoot beetles, Tomicus piniperda, on storm-injured Scots pine. We found that the beetle can recognize while still in flight a host tree and whether it is susceptible by means of olfaction of three plant monoterpenes evaporating from wound oleoresin. Scots pine logs infested with males and females of T. piniperda were tested in the forest of southern Sweden (Table 1, 1982) to determine their attractiveness to naturally swarming populations in the spring. The resulting catches of beetles indicate that there was a strong attraction to volatiles released from logs

Attraction of bark beetles,Tomicus piniperda,Hylurgops palliatus, andTrypodendron domesticum and other insects to short-chain alcohols and monoterpenes

Several Scandinavian forest insects,Hylurgops palliatus, Tomicus piniperda, andTrypodendron domesticum (Coleoptera: Scolytidae),Rhizophagus ferrugineus (Coleoptera: Rhizophagidae) andPollenia spp. (Diptera: Calliphoridae) were attracted to window traps baited with ethanol and placed on Scots pine trees (Pinus sylvestris) in May–June, 1986. Release of ethanol at increasing relative rates of 0, 0.01, 0.1 and 1.0 (800 mg/day) from the window traps on trees in 1987 causedH. palliatus, T. domesticum, andR. ferrugineus to be increasingly attracted, whileT. piniperda was equally attracted at both 0.1 and 1.0 rates. The attraction ofT. piniperda to ethanol was weak compared to attraction to a monoterpene mix, (±)-α-pinene, (+)-3-carene, terpinolene. The terpene mix plus ethanol was significantly more attractive toH. palliatus than ethanol alone, but terpenes significantly reduced the attraction ofT. domesticum to ethanol. Baiting of pipe traps with a series of short-chain alcohols (methanol to hexanol) each alone showed that ethanol was greatly preferred byH. palliatus, T. domesticum, andR. ferrugineus over alcohols of one more or one less carbon, while longer-chain alcohols were not attractive. However,Glischrochilus hortensis (Col.: Nitidulidae) was attracted only to propanol. A series of 10-fold increasing release rates of ethanol (0.0001-1.0, where 1.0=800 mg/day) with either a “low” or “high” release of the terpene mix had various effects on the sexes during their attraction to pipe traps and subsequent entering of holes. Release of (−)-verbenone at 0.25 mg/day had no significant effect onH. palliatus orR. ferrugineus attraction to ethanol, but the response ofT. domesticum to ethanol was reduced. Several theories on olfactory mechanisms of host selection byT. piniperda are integrated and placed in ecological perspective.

Effective attraction radius : A method for comparing species attractants and determining densities of flying insects.

The catches of bark beetles (Coleoptera: Scolytidae) were compared between attractive traps releasing semiochemicals and passive traps (cylindrical sticky screens hung, at 10 heights of 0.7–11.5 m, on poles). A central attractive-trap pole was surrounded by three passive-trap poles spaced 50 or 100 m away at the apices of an equilateral triangle. The catches ofTomicus piniperda and other scolytid species on the attractive-trap pole baited with host monoterpenes, or the catches ofIps typographus attracted to synthetic pheromone, were compared to passive trap catches in a Scots pine forest or in a Norway spruce clear-cut, respectively. Information about flight height distributions of the above scolytid species, andHylurgops palliatus, Cryphalus abietis, Pityogenes chalcographus, P. quadridens, P. bidentatus, andTrypodendron domesticum were obtained on the passive and attractive trap poles. A new method is presented for determining the densities of flying insects based on the passive traps dimensions and catch, duration of test, and speed of insect. Also, a novel concept, the effective attraction radius (EAR), is presented for comparing attractants of species, which is independent of insect density, locality, or duration of test. The EAR is obtained by the ratio of attractive and passive trap catches and the dimensions of the passive trap, and thus should correlate positively with the strength of the attractant and the distance of attraction. EARs are determined from catch data ofT. piniperda andI. typographus as well as from the data of previous investigations on the same or other bark beetles.

Attraction to pheromone sources of different quantity, quality, and spacing: Density-regulation mechanisms in bark beetleIps typographus

The density of bark-beetle colonization of a tree could be regulated by a quantitative effect of the pheromone signal from beetles in the tree (cessation of release of attractive pheromone) or by a qualitative effect (production of pheromone components inhibiting attraction). The quantitative hypothesis was tested onIps typographus by varying the release rate of the two known attractive compounds, 2-methyl-3-buten-2-ol (MB) and (4S)-cis-verbenol (cV). The highest number of beetles were captured at traps with the highest release rates. The catch was nearly proportional to the release of MB and cV at a distance between traps of 12 m or more. At 6-, 3-, and 1.5-m distances between traps deployed in a triangular arrangement there was still a good discrimination between release rates, but relatively more beetles, especially males, were caught on the blank. The lower release rates caught an equal sex ratio while the highest release rate caught only about 30% males. The qualitative hypothesis was tested by releasing the suspected inhibitors ipsdienol (Id) and ipsenol (Ie), from traps in the same amounts as cV. Only small effects were noted forI. typographus. However, the competitorI. duplicatus was attracted to Id and inhibited by Ie, while the predatorThanasimus formicarius was attracted to both compounds. On the other hand, when the ratio of Id or Ie to cV was 10∶1 or 0.1∶1 rather than 1∶1, they affected the numbers ofI. typographus attracted. A small amount of Id combined with the attractants increased trap catch, while large amounts of Id or Ie decreased attraction, especially when combined. Attack density regulation is modeled as an effect of both quantitative and qualitative mechanisms acting in sequence.

Synergistic pheromones and monoterpenes enable aggregation and host recognition by a bark beetle

cause the method applied did not allow its quantitative determination. The use of modified methods suitable for also detecting low-boiling organic compounds made it possible to obtain vertical concentration profiles for this class of compounds, too. The close relationship shown by the seasonal and spatial distribution of both compounds , benzene and toluene, points to a common biogenic source. While different aromatic compounds introduced into the water body by pollution or runoff from the lithosphere (Cz-and C3-benzenes) exhibited fairly closely related concentrations in both the oxygen containing and oxygen-depleted water layers, the concentrations of ben-zene and toluene increased significantly towards the sediment. After the turnover of the lake water, a homogeneous distribution of both compounds was observed in the lake. As soon as an an-oxic hypolimnion developed after strat-ification of the lake, the benzene and toluene concentrations again increased in the hypolimnion. Intense degradation of organic matter and protein as indicated by the high concentrations of ammonium ions present was taking place. Nitrate as a potential precursor for ammonium formation can be excluded since the input of oxidized nitrogen into the lake is rather low. Further evidence of the close relationship between protein degradation and the formation of benzene and toluene can be found in the concomitant occurrence of high concentrations of p/m-cresol (which isomers were not separated but have been shown to be p-cresol in an investigation the year before [7]) that is restricted to the zone near to the bottom of the lake. The accumulation of benzene and toluene in the anoxic hypolimnion also suggests that degradation processes of aromatic compounds which occur under denitri-fying conditions [8] or which have been shown for methanogenic consortia [9] could not compete with the production. Reports on a biogenic formation of benzene from carbon compounds such as phenylalanine are lacking and may have been overlooked. The data of Lake Schleinsee, however, indicate that such processes may occur in anoxic environments. I am greatly indebted to the Deutsche Forschungsgemeinschaft for financial support. The excellent assistance, provided by G. Riedl and A. Giinther is gratefully acknowledged. We report the strong synergism between two pheromone components in causing the attraction of the six-spined spruce bark beetle or Kupferstecher, Pityogenes chalcographus L. , in the field. Our field results also suggest that several monoterpenes of Norway spruce, Picea abies (L.) Karst, when presented with the pheromone components , play a role in host recognition and …

Behavioural sequence in the attraction of the bark beetle Ips typographus to pheromone sources

ABSTRACT. The functions of the two synergistic pheromone components, (4S)‐cis‐verbenol (cV) and 2‐methyl‐3‐buten‐2‐01 (MB), and the role of ipsdienol in the attraction of Ips typographus (L.) (Scolytidae) to pheromone sources were studied in the field. Absolute and relative beetle catches were compared between several traps placed at and nearby a central pheromone source: a pipe trap containing the source, a surrounding sticky trap, a nearby window trap, and four distant window traps. A higher catch in the outer down‐wind distant traps indicated an up‐wind anemotaxis to the source. Increased MB release, with cV constant, increased the proportion caught in the central pipe trap, indicating MB as a landing stimulus. Release of MB alone gave a very small catch. Ipsdienol could not substitute for cV in the synergism with MB. An increase of cV, with MB constant, increased the number of beetles caught, but not the proportion caught in the pipe trap. The sex ratio was equal in the window traps, but fell to 30% males in both sticky and pipe traps, showing that a large proportion of the males attracted to the source did not land. The proportion of males in the pipe trap was reduced at the highest cV dose. The results support the idea of each pheromone component having a different relative importance in releasing different steps in the behavioural chain.

Field response of spruce bark beetle, Ips typographus, to aggregation pheromone candidates

Six compounds previously identified from hindguts of unmated maleIps typographus (L.) during host colonization: 2-methyl-3-buten-2-ol (MB),cis-verbenol (cV),trans-verbenol (tV), myrtenol (Mt),trans-myrtanol (tM), and 2-phenylethanol (PE), were tested for their attractivity in the field with a subtractive method. The amounts of MB and cV released from a pipe trap were similar to those given off from the commercial bait Ipslure as well as that from a Norway spruce tree,Picea abies (L.) Karst., under mass attack. The blend of the compounds became nonattractive when either MB or cV was subtracted, while subtraction of any of the other four compounds had no effect. Addition of ipsdienol (Id) to the blend did not significantly increase the attraction. In a second comparative test, the addition of three compounds as a group (tV + Mt + PE) to MB + cV again had no effect on the attraction, but the addition of Id increased the catch somewhat. Addition of host logs to a bait releasing MB + cV at a rate lower than in previous experiments did not influence the attraction to pipe traps. Sticky traps containing natural pheromone sources (50 males in a log), which released 1–5 mg/day of MB as determined by aerations with deuterated MB as internal standard, were less attractive than a synthetic source releasing similar amounts of MB.

Isolation of pheromone synergists of bark beetle, Pityogenes chalcographus, from complex insect-plant odors by fractionation and subtractive-combination bioassay.

Capillary gas chromatography with columns of different polarity and two-dimensional fractionation of effluents were used with novel subtrac-tive-combination bioassays to rigorously isolate host- and insect-produced pheromone synergists of the bark beetlePityogenes chalcographus (Coleop-tera: Scolytidae). Methyl (E,Z)-2,4-decadienoate (E,Z-MD) and the previously identified chalcogran were found to be synergistically attractive to both sexes.E,Z-MD was produced sex-specifically in males, and only when they had fed on host-plant tissue. A Norway spruce monoterpene fraction (including α-pinene, β-pinene, and camphene) increased the attractive response to the pheromone components. Dose-response curves forE,Z-MD and chalcogran in the laboratory bioassay indicated the two components are highly synergistic. The isolation methods are important for further progress in identifying certain semiochemical synergists found in trace amounts in complex chemical mixtures, such as when insects must feed in host plants in order to produce pheromone.

E-Myrcenol in Ips duplicatus: an aggregation pheromone component new for bark beetles.

Males of the Eurasian bark beetleIps duplicatus, when feeding in host Norway spruce (Picea abies (L.) Karst.), produced and released ipsdienol andE-myrcenol, which we show to be aggregation pheromone components. Bioassays using walking beetles indicated thatE-myrcenol in synergistic combination with ipsdienol is essential for attraction. Synergism ofE-myrcenol and ipsdienol released at natural rates in the forest was also demonstrated with a new technique using mechanical slow-rotation of sticky traps.