Insecticidal and growth inhibitory effects of some thymol derivatives on the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae) and their impact on detoxification enzymes.

BACKGROUND: Thymol is a known natural product with insecticidal activity against several insect species. A recent study on structural modifications of thymol to thymyl esters and their efficacy against Spodoptera litura suggested that such an approach could develop generalized novel insecticides/insect growth inhibitors and requires further studies to establish the efficacy against lepidopterans. RESULTS: Thymol and structurally modified eight esters were evaluated against beet armyworm, Spodoptera exigua using the topical application. Thymyl butanoate was the most toxic compound with a median lethal dose (LD50 ) of 2.33 and 1.62 µg/larva after 24 and 48 h posttreatment, respectively. All thymyl esters were potentially better than the parent compound thymol, except thymyl dibromoacetate, in their efficacy against Spodoptera exigua. Essentially, there were three levels of activity vis-à-vis the compounds used, that is, with the LD50 range of 1.5 to 5.0, 7.0 to 15.0, and > 20 µg/larva, respectively. Ovicidal activity and reduction in larval growth were also determined by treating third instars at sub-lethal doses, that is, LD50 doses of second instars. Thymyl butanoate treated larvae inhibited glutathione S-transferase, carboxylesterase, and acetylcholinesterase activities, whereas the other thymyl esters induced these enzymes. CONCLUSION: Thymyl butanoate exhibited higher toxicity against Spodoptera exigua and is the first to report about > 15.5× more toxicity than thymol and > 6.5× than thymyl cinnamate, which suggests that the efficacy was species-specific versus the chemical structural variation of the esters. © 2021 Society of Chemical Industry.

Insecticidal activity of isolated phenylpropanoids from Alpinia galanga rhizomes against Spodoptera litura.

Botanical insecticides as a means of controlling insects present an alternative approach that is safer than the use of synthetic insecticides. The present study identified the insecticidal activity of extracts of the rhizomes of Alpinia galanga (L.) Willd. and seven isolated phenylpropanoids against the second instar of Spodoptera litura Fab. by topical application. The ethyl acetate extract had the highest toxicity on this insect with LD50 values of 1.68 and 1.25 µg/larva after 24 and 48 h posttreatment, respectively. Among the seven phenylpropanoids separated from the ethyl acetate extract, 1'S-1'-acetoxychavicol acetate was identified as the most active compound with LD50 values of 1.63 and 1.40 µg/larva after 24 and 48 h posttreatment, respectively, followed by p-coumaryl diacetate. In addition, the two active compounds decreased glutathione S-transferase activity and increased acetylcholinesterase activity. p-Coumaryl diacetate also decreased carboxylesterase activity.

A Novel Insecticidal Molecule Extracted from Alpinia galanga with Potential to Control the Pest Insect Spodoptera frugiperda.

Spodoptera frugiperda, a highly polyphagous insect pest from America, has recently invaded and widely spread throughout Africa and Asia. Effective and environmentally safe tools are needed for successful pest management of this invasive species. Natural molecules extracted from plants offer this possibility. Our study aimed to determine the insecticidal efficacy of a new molecule extracted from Alpinia galanga rhizome, the 1'S-1'-acetoxychavicol acetate (ACA). The toxicity of ACA was assessed by topical application on early third-instar larvae of S. frugiperda. Results showed that ACA caused significant larval growth inhibition and larval developmental abnormalities. In order to further explore the effects of this molecule, experiments have been performed at the cellular level using Sf9 model cells. ACA exhibited higher toxicity on Sf9 cells as compared to azadirachtin and was 38-fold less toxic on HepG2 cells. Inhibition of cell proliferation was observed at sublethal concentrations of ACA and was associated with cellular morphological changes and nuclear condensation. In addition, ACA induced caspase-3 activity. RT-qPCR experiments reveal that ACA induces the expression of several caspase genes. This first study on the effects of ACA on S. frugiperda larvae and cells provides evidence that ACA may have potential as a botanical insecticide for the control of S. frugiperda.

Synthesis of thymyl esters and their insecticidal activity against Spodoptera litura (Lepidoptera: Noctuidae).

BACKGROUND: Thymol, a natural phenolic monoterpene originating from Thymus vulgaris, is recognized as a safe and potent botanical insecticide to many insects. The structural modification of thymol into thymyl esters is a potential approach for the development of novel insecticides, which showed more toxicity than thymol. However, there are no reports on the insecticidal activity of thymyl esters to control Spodoptera litura. RESULTS: Thymol was structurally modified into ten thymyl esters by esterification using a new reagent, PPh3 /Br3 CSO2 Ph. The insecticidal activity of these compounds was examined against the second instars of Spodoptera litura using a topical application. Among the ten thymyl esters evaluated, thymyl cinnamate was the most toxic with LD50  = 0.41 and 0.34 µg/larva after 24 and 48 h posttreatment, respectively. In addition, thymyl cinnamate-treated larvae showed increasing carboxylesterase and acetylcholinesterase activities in vivo experiment, whereas glutathione S-transferase activity showed no significant difference. CONCLUSION: Thymyl cinnamate were first reported to exhibit toxicity against S. litura 2.41-2.46 fold more efficient than thymol. However, the detailed biochemical interactions are necessary for further development of novel insecticides. © 2019 Society of Chemical Industry.

Economy of Catalyst Synthesis-Convenient Access to Libraries of Di- and Tetranaphtho Azepinium Compounds.

Efficient optimization procedures in chiral catalysis are usually linked to a straightforward strategy to access groups of structurally similar catalysts required for fine-tuning. The ease of building up such ligand libraries can be increased when the structure-modifying step (introduction of a substituent) is done at a later stage of the synthesis. This is demonstrated for the extended family of di- and tetranaphtho azepinium compounds, widely used as chiral phase transfer catalysts (PTC). Using 2,6-diiodo-4,5-dihydro-3H-dinaphtho[2,1-c:1',2'-e]azepine and 4,8-diiodo-6,7-dihydro-5H-dibenzo[c,e]azepine, respectively, as key intermediates, 18 spiro-azepinium compounds were synthesized in a total yield of 25-42% over 6-7 steps from 1,1'-binaphthyl-2,2'-dicarboxylic acid or diphenic acid, respectively. The replacement of iodo groups with aryl substituents was performed as the last or the penultimate step of the synthesis.