Investigation of insecticidal activity of two Rhododendron species on stored-product insects

Anti-insect activity of benzylacetone and its structural analogues against Tribolium castaneum (Coleoptera: Tenebrionidae)

Several studies have confirmed that the volatile oil of Rhododendron anthopogonoides Maxim has good anti-insect activity and is expected to become an environmentally friendly botanical insecticide, with benzylacetone as its main active component. Benzylacetone has a simple chemical structure, containing only three elements of carbon, hydrogen and oxygen, low toxicity, and easy degradation. Therefore, in this study, benzylacetone and its seven structural analogues were selected for testing their anti-insect activity to Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), in order to explore the key structural sites where benzylacetone plays a role in combating T. castaneum. Benzylacetone demonstrated better repellent activity compared to the positive control pyrethroid, and when the single bonds at positions 3 and 4 were transformed into double bonds, the percent repellency increased. Additionally, the fumigation activity of benzylacetone exhibited a concentration-dependent trend and was significantly higher than that of the positive control pyrethroid. After losing two methylene groups at positions 3 and 4, the LC50 decreased to 13.10 mg/L air, which was approximately 9.4 times lower than that of benzylacetone. In terms of contact activity against T. castaneum, benzylacetone was about 2 times more effective than pyrethroids. Moreover, when the single bond at positions 3 and 4 became a double bond, the contact mortality was 2.5 times higher than that of benzylacetone, and the LD50 reached 10.47 μg/adult. Based on the results of repellent, fumigation, and contact toxicity experiments, it was found that the side chains of benzylacetone, carbonyl groups on the side chains, and benzene ring unsubstituted were the key structures for its prevention and control against T. castaneum. This study is an exploration of the structure-activity relationship of benzylacetone in the anti-insect of T. castaneum, providing ideas for its structural modification.

Insecticidal activity of two Pelargonium essential oils and head transcriptome analysis of stored-product pest Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in response to citronellyl formate fumigation

Tribolium castaneum (Herbst) is one of the most common stored-product pests, causing enormous economic losses and developing widespread resistance to chemical insecticides. Natural products derived from essential oils (EOs) are well-known for insecticidal activity against agricultural pests, especially in the management of stored-product pests. In this study, the chemical constituents and repellent, contact and fumigation toxicity activities of two Pelargonium EOs against T. castaneum were evaluated. Moreover, the fumigation mechanism of citronellyl formate was assessed by head transcriptome sequencing and RNA interference (RNAi). A total of 28 and 39 compounds were identified by gas chromatography-mass spectrometry, accounting for 98.58 and 97.33 % of P. roseum and P. asperum EOs, and the major components were citronellol, (1S)-(1)-β-pinene and citronellyl formate. (1S)-(1)-β-Pinene exhibited strong contact toxicity activity (LD50 = 19.72 μg/adult), while citronellyl formate exhibited strong repellent and fumigation toxicity activities, with the LC50 value of 11.93 mg/L air. Under the stress of citronellyl formate, 1222 differentially expressed genes were identified, including 554 up-regulated and 668 down-regulated genes, which was further verified by qRT-PCR. Among odorant-binding proteins (OBPs), only TcGOBP70 was up-regulated, suggesting that GOBP70 is more likely to be involved in the defense of T. castaneum by recognizing, binding and transporting citronellyl formate. Additionally, RNAi against TcGOBP70 dramatically increased the contact and fumigation toxicity activities of citronellyl formate, with mortality rates of 73 and 75 %, respectively. Therefore, our findings not only provided theoretical bases for the comprehensive utilization of the Pelargonium species but also established potential targets for controlling T. castaneum adults.

Can volatile sulphides act as fumigant or contact toxicant in cowpea weevil, Callosobruchus maculatus F.?: a comparative analysis

For understanding the toxic action of volatile sulphides, we have evaluated the insecticidal efficacy of sulphide mixture (diallyldisulfide, diallylsulfide and diallyltrisulfide) against Callosobruchus maculatus F. adults by fumigant toxicity (FT) and contact toxicity (CT) bioassays. Further, diffusional characteristics of sulphides inside the treatment vials were analysed by GC-FPD and microstructural impacts on treated beetles were observed using SEM. In the comparative analysis, maximum level of insecticidal toxicity (0.88 µl/L of LC50), residue persistence (32.28 ppm/adult) and microstructural aberration in beetle elytra was observed in CT compared to FT bioassay (1.60 µl/L of LC50 and 22.67 ppm/adult of residue). Whereas, the GC-FPD based diffusion analysis indicated even distribution of volatiles inside the vials in both FT and CT bioassays. The study results suggests that volatiles act as fumigant/vapours in both bioassays with respect to even diffusion and toxicity occurs based on insect-volatile contacts inside the treatment chambers.

Phytochemical and pharmacological profiling of Trewia nudiflora Linn. leaf extract deciphers therapeutic potentials against thrombosis, arthritis, helminths, and insects

The objective of the current study was to examine the phytochemical and in vitro thrombolytic, anti-arthritic, anthelmintic, and insecticidal effects of Trewia nudiflora (TN) methanolic leaf extract with its methanol (MTN), chloroform (CTN), and ethyl acetate (ETN) fractions. Pheretima posthuma and Tribolium castaneum were employed for evaluating the antihelmintic and insecticidal properties, respectively. All the tested extracts showed the presence of copious potential constituents in phytochemical analysis. Among all extracts, MTN extract exhibited the utmost clot lysis (35.95 ± 4.81%) property compared to standard streptokinase (SK) (53.77 ± 7.52%). All samples displayed striking protein denaturation activity in a dose-dependent manner (100–500 µg/mL), where the highest inhibition was observed for MTN (67.26 ± 6.39% at 500 µg/mL). Each extract demonstrated considerable anthelmintic activity at 25–75 mg/mL dose ranges. ETN showed the strongest anthelmintic activity at the highest dose. Among all samples, the CTN extract displayed the utmost mortality rate (77.22%) in the insecticidal test. The results of the study suggest that T. nudiflora leaf extracts may have potential against thrombosis, arthritis, helminths, and insects, which warrants the necessity of extensive isolation and identification of bioactive compounds to develop newer effective drugs upon preclinical and clinical investigations.