Influence of molecular descriptors of plant volatilomics on fumigant action against the three major stored product beetle pests.

Plant volatilomics such as essential oils (EOs) and volatile phytochemicals (PCs) are known as potential natural sources for the development of biofumigants as an alternative to conventional fumigant pesticides. This present work was aimed to evaluate the fumigant toxic effect of five selected EOs (cinnamon, garlic, lemon, orange, and peppermint) and PCs (citronellol, limonene, linalool, piperitone, and terpineol) against the Callosobruchus maculatus, Sitophilus oryzae, and Tribolium castaneum adults. Furthermore, for the estimation of the relationship between molecular descriptors and fumigant toxicity of plant volatiles, quantitative structural activity relationship (QSAR) models were developed using principal component analysis and multiple linear regression. Amongst the tested EOs, garlic EO was found to be the most toxic fumigant. The PCs toxicity analysis revealed that terpineol, limonene, linalool, and piperitone as potential fumigants to C. maculatus (< 20 µL/L air of LC50), limonene and piperitone as potential fumigants to T. castaneum (14.35 and 154.11 µL/L air of LC50, respectively), and linalool and piperitone as potential fumigants to S. oryzae (192.27 and 69.10 µL/L air of LC50, respectively). QSAR analysis demonstrated the role of various molecular descriptors of EOs and PCs on the fumigant toxicity in insect pest species. In specific, dipole and Randic index influence the toxicity in C. maculatus, molecular weight and maximal projection area influence the toxicity in S. oryzae, and boiling point and Dreiding energy influence the toxicity in T. castaneum. The present findings may provide insight of a new strategy to select effective EOs and/or PCs against stored product insect pests.

Profiling of semiochemicals from three stored product beetles by headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry.

This work focuses on the profiling of semiochemicals (SCS) from Callosobruchus maculatus, Sitophilus oryzae and Tribolium castaneum by headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) technique. Totally, 6, 9 and 8 volatile compounds (VCS) were detected from C. maculatus, S. oryzae and T. castaneum, respectively. As a result of pherobase analysis and preference bioassay, stearic acid (C. maculatus); nonanal, lauric acid and stearic acid (S. oryzae); stearic acid (T. castaneum) were identified as new SCS that could be useful for integrated pest management (IPM) practices.

Eugenol nanoemulsion as bio-fumigant: enhanced insecticidal activity against the rice weevil, Sitophilus oryzae adults.

Nanoemulsion is a promising delivery system for delivering the plant bioactive molecules against insect pests. In this study, we aimed to prepare eugenol based nanoemulsions (EL-NE) by ultrasonication method to investigate its fumigant toxicity against Sitophilus oryzae adults and to analyse the residual characteristics of eugenol bioactive on the treated grains and beetles. In EL-NE preparations, 1:1 ratio of eugenol: Tween 80 combination with 5 min of ultrasonication at frequency of 10 kHz and 12 W power output was determined as optimal. In the optimized nanoemulsions, 19.21 to 42.82 d.nm range of mean droplet size, 0.50 to 0.77 range of polydispersity index and -21.80 to -29.83 mV range of zeta potential values were observed with respect to 2.5 to 10.0% of eugenol concentrations. After 72 h of fumigation, enhanced fumigant toxicities (3.5-11.2 fold) were observed against S. oryzae adults for the optimized EL-NEs compared to eugenol alone. Fumigant toxicity results revealed 14.40 µl/L air of least LC50 value for the 10.0% EL-NE. Persistence of eugenol was more (12.46%) in EL-NE treated wheat grains compared to eugenol alone treatments based on Gas Chromatography-Mass Spectroscopy analysis, which indicates the improved fumigation. This study results suggests EL-NEs as promising nano-biofumigant against the S. oryzae adults for eco-friendly Integrated Pest Management (IPM).

Persistence and ingestion characteristics of phytochemical volatiles as bio-fumigants in Sitophilus oryzae adults.

Fumigant toxicity of phytochemical volatiles has been widely reported against stored product insect pests. Such volatiles are considered as natural fumigants and bio-fumigants in post-harvest food protection research. In the present study, persistence and ingestion of diallyl disulfide, citral, eucalyptol, eugenol and menthol were investigated in Sitophilus oryzae adults in comparison with fumigant toxicity and microstructural impact in elytra. The fumigant toxicity bioassay was performed with increasing concentrations of phytochemical volatiles at 25, 125, 250 and 500 µL/L air against S. oryzae adults in 50 mL glass vials. The phytochemical residues were examined from the treated adults by Gas Chromatography coupled with Flame Ionization Detector (GC-FID) and their pathological impacts on the elytral surface was observed under Scanning Electron Microscopy (SEM). After 72 h of fumigation, diallyl disulfide and eucalyptol were identified as potential fumigants with 5.24 and 8.30 µL/L air LC50 values, respectively. GC-FID analyses showed that diallyl disulfide and eucalyptol molecules persistence (1.29 and 2.60 ppb persistence with 0.94 and 0.90 r2 values respectively at 72 h exposure) on the body surface of weevil was positively correlated with the fumigation exposure and toxicity. Whereas, phytochemical molecules ingestion into the body of weevils was not directly linked with the insect mortalities. The SEM observations indicated that diallyl disulfide and eucalyptol molecules caused severe microstructural impacts on the elytra of weevils compared to other molecules. As a result, the present study suggested that phytochemical fumigants are persisted on the body surface and caused insecticidal toxicities in S. oryzae adults. In addition, it was predicted that persisted molecules might be entered into the body of weevils via cuticular penetration.