Solid-phase microextraction (SPME) fibre (50/30 µm Divinylbenzene / Carboxen / Polydimethylsiloxane) was purchased from Supleco, USA. Alkane standard solution (C8-C20; ⁓40 mg/L each in hexane) was procured from Sigma-Aldrich (Switzerland) for the determination of the Kováts retention index (KRI) of compounds. The nonanal (NL), lauric acid (LA) and stearic acid (SA) analytical standards were purchased from Sigma-Aldrich (USA) to confirm compounds secreted from beetles.
Insect cultureStored product insect pests cultures were maintained in the insectary unit of the Food Protectants and Infestation Control (FPIC) Department, CSIR-Central Food Technological Research Institute, Mysuru, under the controlled temperature (30 ± 2°C), relative humidity (75 ± 5%) and light-dark (13:11 h) conditions. From the stock cultures, sub-cultures of S. oryzae, C. maculatus and T. castaneum were prepared and maintained in the 1 L glass jars with wheat grain, green gram and wheat flour, respectively. After a week, beetles were removed from the culture medium using sieves and the culture was regularly observed. Newly emerged fresh adults were used for experiments. The used adult beetle species were taxonomically identified and authenticated as C. maculatus (SPIP-1), S. oryzae (SPIP-2) and T. castaneum (SPIP-3) by insect taxonomist Dr. Kolla Sreedevi (Principal Scientist, Division of Germplasm Collection and Characterization, ICAR-National Bureau of Agricultural Insect Resources, Bengaluru, India). Photographs and voucher specimens (SPIP-1, SPIP-2, SPIP-3) of the used beetle species were deposited at the FPIC Department, CSIR-CFTRI, Mysuru, India.
Sampling of chemicals from beetlesVolatile compounds (VCS) were extracted from live insects using the solvent-free HS-SPME method, as followed by Niu et al. (2016) andCai et al. (2022), with slight modifications. Insects were temporarily immobilised by ice-chilling for 3 minutes and immediately weighed (Eisemann et al., 1984). Two grams of active adults of each species (S. oryzae, C. maculatus and T. castaneum) were released into the headspace (HS) vials (20 mL), and the crimped neck was sealed with a Teflon cap fitted with a rubber septum. Then, SPME fibres (50/30 µm Divinylbenzene / Carboxen / Polydimethylsiloxane; Supleco, USA) were inserted into the vials for adsorption of emitted VCS from the beetles (Figure S1). After 15 min of sampling, the
