GC–MS Profiling of PGPR-Induced Volatiles Reveals Repellent Compounds Against Spodoptera exigua in Cotton

Plant growth-promoting rhizobacteria (PGPR) trigger induced systemic resistance (ISR) in plants, which result in the impairment of herbivore's host location, oviposition behavior, and development. Operating under the conventional wisdom that PGPR consortium (blend of strains) is often more effective than single strains, but its efficacy depends on crop cultivar, herbivore-crop system, and herbivore's degree of specialization, among other variables, the efficacy of five PGPR consortia was evaluated to screen the most promising consortia in controlling the polyphagous insect pest, Spodoptera exigua in two cotton cultivars (one susceptible and one resistant to pests). Gas chromatography-mass spectrometry (GC–MS) was used to evaluate the effects of the PGPR treatments on oviposition behavior, larval development and olfactory responses of S. exigua, and on cotton volatile emissions. A paper based on this research was published in Pest Management Science.¹

One of the economically important crops grown in many regions worldwide to produce fiber and oil, cotton crops are damaged by a wide range (approximately 200 species) of insect pests which cause annual yield losses of approximately 15% of the possible harvest.^2-4 The beet armyworm, Spodoptera exigua, is a highly destructive insect pest and one of the top 10 lepidopteran species that cause economic damage to cotton.⁵

The researchers found that Spodoptera exigua laid significantly fewer eggs on susceptible plants treated with consortia designated by the team as AU8, AU9a, TX1, and TX2a (compositions of which can be found in Table 1 of their paper¹), and on resistant plants treated with AU8, TX1, or TX3, compared to untreated plants. However, the difference in larval survival rates (81-83% in untreated plants versus 66-71% in PGPR-treated plants) was not significant. Y-tube olfactometer bioassays using the promising PGPR treatments revealed that headspace volatiles of susceptible plants treated with AU8 or TX1, and resistant plants treated with AU8, TX1, or TX3 were avoided by S. exigua when compared against blank air, indicating an inhibitory effect on S. exigua behavior. GC–MS analysis of headspace volatiles revealed that the most discriminant compounds between PGPR-treated and untreated plants were α-pinene, β-pinene, benzaldehyde, D-limonene, (Z)-3-hexenyl acetate, β-ocimene and DMNT. These compounds are known to modulate herbivore's host location and oviposition behavior.¹

“This study,” write the authors of the paper,¹ “identifies two promising PGPR consortia (AU8 and TX1) and potential repellent compounds for the protection of cotton cultivars against S. exigua and discusses their role as part of an integrated pest management (IPM) approach.” The authors go on to state that the role of these two PGPR consortia and volatile compounds as part of an IPM approach will be further explained in future studies.¹

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References

1. Ayelo, P. M.; Kafle, B. D.; Xiong, C. et al. Treatment of Cotton with Plant Growth-Promoting Rhizobacteria Consortium Alters Host Location and Oviposition of Spodoptera exigua. Pest Manag Sci. 2026.. DOI: 10.1002/ps.70789
2. Tokel, D.; Dogan, I.; Hocaoglu-Ozyigit, A. et al. Cotton Agriculture in Turkey and Worldwide Economic Impacts of Turkish Cotton. J Nat Fibers 2022, 19, 10648–10667. DOI: 10.1080/15440478.2021.2002759
3. Munir, H.; Rasul, F.; Ahmad, A. et al. Diverse Uses of Cotton: From Products to Byproducts, in Cotton Production and Uses; S. Ahmad and M. Hasanuzzaman, Eds.Springer, 2020. pp. 629–641.
4. Greenberg, S. M.; Parajulee, M. N. Integrated Pest Management in Cotton, in Integrated Pest Management in Tropical Regions; C. Rapisarda and G. E. M. Cocuzza, Eds. CABI International, 2017, pp. 134–174.
5. Razaq, M.; Mensah, R.; Athar, H, Insect Pest Management in Cotton, in Cotton Production; K. Jabran and B. S. Cauhan, Eds. John Wiley & Sons, 2019. pp. 85–107.