Monochamus galloprovincialis (Olivier, 1795)

Gonçalves, Elsa, Figueiredo, A. Cristina, Barroso, José G., Henriques, Joana, Sousa, Edmundo & Bonifácio, Luís, 2020, Effect of Monochamus galloprovincialis feeding on Pinus pinaster and Pinus pinea, oleoresin and insect volatiles, Phytochemistry (112159) 169, pp. 1-12 : 6-7

2.5. Volatiles collected from Monochamus galloprovincialis after feeding on Pinus pinaster

To determine which compounds from the blend of tree volatiles can be found in the insect after feeding, the insect volatiles were also determined. Although the SPME wipe sampling of the insect cuticle has been used for determination of insect cuticular hydrocarbons ( Ginzel and Blomquist, 2016), headspace SPME is not the most effective method of collection of trace amounts of volatiles adsorbed to the cuticle. Thus, whole-body extraction with pentane was chosen to evaluate the volatiles from M. galloprovincialis as a whole. The insects were extracted together, without separation of sexes or by the chemotype of tree they had fed upon. The resulting pentane extracts were complex mixtures in which 48 components were identified, representing ≥72% of the total volatiles. The identified components are listed in Table 5 in order of their elution on the DB-1 column, arranged according to the lowest and the highest percentages found for each component. α-Pinene (1–8%), β-pinene (1–7%), and abietic acid (traces-17%) were the phytochemicals detected in the extracts of beetles which had fed on the host pine trees.

The straight-chain n -alkanes heptacosane (n -C27), nonacosane (n - C29) (both 4–12%), and pentacosane (n -C25, 4–9%), along with the methyl-branched hydrocarbons 3-methylnonacosane (3-meC29, 5–8%), 2-methyloctacosane (2-meC28, 4–8%) and 3-methylheptacosane (3- meC27, 3–8%) were among the main components in the extracts. A similar profile was found in parallel studies of extracts of M. galloprovincialis at different stages of development ( Gonçalves et al., 2019). These compounds are typical insect cuticular hydrocarbons, which have dual roles in preventing desiccation while also being used as contact pheromones involved in recognition of species and sex ( Ginzel and Blomquist, 2016).

3. Conclusions

To the best of our knowledge, this is the first report of how the profile of volatiles emitted by P. pinaster changes after feeding by M. galloprovincialis , and the first report of the volatiles emitted from oleoresin exuded from feeding wounds. We also characterized the profile of volatiles associated with the beetles after they had fed on their host trees. For comparison purposes, P. pinea was used in parallel control experiments, because unlike P. pinaster , P. pinea is not affected by PWD.

Whereas most studies on Pinus spp. volatiles and oleoresin have evaluated the constitutive composition using different isolated plant parts (branches, needles, among others), or in lab induced injury, this study aimed to bring together the host and the insect vector of PWD, to evaluate in situ the volatiles induced by insect injury.

P. pinaster emitted constitutive volatiles showed the presence of chemotypes and the evaluation of volatiles emitted by P. pinaster during feeding by M. galloprovincialis revealed different patterns of responses according to the tree chemotypes, with the different chemotypes responding in different ways to insect damage. The main compounds involved in the host response should now be tested in bioassays individually and in blends, to determine which compounds or groups of compounds may mediate the attraction of the insect to its host plants. Further studies of tree-insect relationships and their combined relation to genetic expression are also required. Moreover, considering that the chiral components occur in specific enantiomeric ratios in the host plant, the influence of each enantiomer should also be assessed.

The studies reported here on the volatiles emitted by host trees attacked by M. galloprovincialis may contribute to the development of new methods of protecting pine forests against PWD, for example, by development of efficient lures for trapping the insect vector, and particularly, the newly emerged insects which are not yet responsive to the M. galloprovincialis aggregation pheromone. Such methods may help to prevent the infection and death of thousands of pines in southern Europe.