Larval response to frass and guaiacol: detection of an attractant produced by bacteria from Spodoptera littoralis frass

Santosh V Revadi,Paul G Becher,Vito Antonio Giannuzzi,William B Walker,Ramesh R Vetukuri

doi:10.1007/s10340-021-01352-9

Santosh V Revadi, Paul G Becher + Show 3 more

Open Access

https://doi.org/10.1007/s10340-021-01352-9

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Abstract

Larval frass in herbivorous lepidopterans is mainly composed of plant-derived material and microbes from the gut. Despite the fact that frass from conspecific larvae repels female moths in Spodoptera littoralis from oviposition, the role of frass volatiles on larval foraging behavior is largely unknown. Here, we show that larvae of S. littoralis walk upwind to larval frass volatiles in a wind tunnel assay. We identified the frass volatile guaiacol (2-methoxyphenol) as key ligand for the S. littoralis odorant receptor (OR) SlitOr59 which we expressed heterologously. We isolated guaiacol-producing bacteria identified as Serratia marcescens from frass of larvae that were fed on cotton, and Enterobacter cloacae, E. ludwigii and Klebsiella sp. from frass derived from cabbage-fed larvae. In addition to guaiacol, we also identified volatiles acetoin, 3-methyl-1-butanol and dimethyl disulfide, in large proportions in headspace collections from the bacteria. A Y-tube olfactometer assay showed that fourth instar S. littoralis larvae are attracted to guaiacol. Moreover, cotton leaves treated with the insecticide Spinosad and guaiacol were highly attractive to the larvae. Our results provide a basis for management of the pest by directly targeting larvae, based on an attract-and-kill strategy. Further studies are needed to test the application of guaiacol for semiochemical-based pest management of Spodoptera pest species.

Highlights

Organic compounds are predominant cues for caterpillars to organize their foraging tactics to locate food resources such as plants (Carlsson et al 1999; Becher and Guerin 2009)
All three bacterial isolates from frass that derived from cotton-fed larvae were identified as Serratia marcescens; from cabbage, we identified Enterobacter cloacae, E. ludwigii and Klebsiella sp.; and from maize, all three isolates were identified as S. grimesii
We demonstrated that S. littoralis larvae are attracted to guaiacol

Summary

Introduction

Organic compounds are predominant cues for caterpillars to organize their foraging tactics to locate food resources such as plants (Carlsson et al 1999; Becher and Guerin 2009). Differences in larval and adult behavior to host plant volatiles have been shown in noctuid moths, with females being repelled by volatile organic compounds (VOCs) from herbivore-damaged plants (De Moraes et al 2001; Signoretti et al 2012), while larvae are attracted. To relocate to suitable host plants when crawling on the ground, larvae use reliable olfactory signals such as plant volatiles induced by conspecific feeding (von Mérey et al 2013). It is unclear whether larvae use other larval-derived cues to identify suitable host plants. Behavioral responses to insect frass have been shown in other insect species (Dillon et al 2000; Axelsson et al 2017; Molnár et al 2017; Zhang et al 2019)

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