Antifeeding effects of azadirachtin on the fifth instar Spodoptera litura larvae and the analysis of azadirachtin on target sensilla around mouthparts

Abstract

To clarify the types, number, and distribution of sensilla on the head of the fifth instar Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) larvae and identify the main sensilla of azadirachtin acting on larvae, scanning electron microscopy was used to study the morphology of the head and sensilla on the mouthparts. The four sensilla—sensillum basiconicum, sensillum chaeticum, sensillum styloconicum, and sensillum trichodeum—on the head of the fifth instar larvae were treated with 0, 0.1, 0.5, 1, 2, and 4 mg/kg azadirachtin by a microdrop method. The larvae showed an obvious antifeeding effect with azadirachtin. And higher the concentration of azadirachtin, the more obvious the phenomenon of antifeeding activity. The sensillum styloconicum and the sensillum trichodeum were the main sensilla for azadirachtin. When 1 mg/kg azadirachtin was used to treat sensillum styloconicum and sensillum basiconicum, the fifth instar larvae of S. litura showed obvious antifeedant activity and the cumulative feed intake for 24 hr was no more than 30% of the leaf area. Quantitative reverse‐transcription polymerase chain reaction verified the expression patterns of some Grs, indicating that Grst43a was upregulated by 1.3‐ and 3.9‐fold, Gor24 was upregulated by 2.5‐ and 3.3‐fold, Gr5a was downregulated by 0.6‐fold and upregulated by 2.0‐fold, and Gr28a was downregulated by 0.8‐fold and upregulated by 3.6‐fold upon treatment with 0.5 mg/kg and 1 mg/kg azadirachtin in 24 hr. Gr genes participated in the identification of bitterness and we speculated that Gr genes may indirectly lead to the occurrence of antifeeding behavior.

Azadirachtin-Based Insecticide: Overview, Risk Assessments, and Future Directions

“…Azadirachtin has a variety of physiological effects on many insect pests, such as antifeedancy (Qin et al, 2020), growth and development inhibition (Zhao et al, 2019), impairment of oocyte structure, inhibition of fecundity, and egg viability (Bezzar-Bendjazia et al, 2016;Amaral et al, 2018;Oulhaci et al, 2018;Ferdenache et al, 2019). Despite extensive studies of the mechanisms that highlight the physiological effects of azadirachtin, the behavioral effects remain more controversial (Charleston et al, 2006;Hasan and Ansari, 2011;Tomé et al, 2013).…”

Azadirachtin downregulates the expression of the CREB gene and protein in the brain and directly or indirectly affects the cognitive behavior of the Spodoptera litura fourth‐instar larvae

“…The cumulative feeding rate increased with time within 24 h. The cumulative feeding rate decreased with increasing azadirachtin concentration and was lower than that of CK at the same time point. It reached maximum values of CK (85.3 ± 3.8%) > 0.1 mg kg −1 (83.7 ± 6.0%) > 0.5 mg kg −1 (74.3 ± 4.0%) > 1 mg kg −1 (68.0 ± 4.6%) > 2 mg kg −1 (57.7 ± 5.5%) > 4 mg kg −1 (53.7 ± 1.5%) at 24 h. It is guaranteed that azadirachtin is ingested by insects 46 …”

“…44,45 This is mainly because azadirachtin is a good anti-feedant. 46 We studied the effect of azadirachtin on the feeding behavior of S. litura larvae. We found that the feeding rate of larvae treated with azadirachtin in 2 mg kg −1 was 60% compared with control, which proved the larval intake of azadirachtin.…”

“…It reached maximum values of CK (85.3 ± 3.8%) > 0.1 mg kg −1 (83.7 ± 6.0%) > 0.5 mg kg −1 (74.3 ± 4.0%) > 1 mg kg −1 (68.0 ± 4.6%) > 2 mg kg −1 (57.7 ± 5.5%) > 4 mg kg −1 (53.7 ± 1.5%) at 24 h. It is guaranteed that azadirachtin is ingested by insects. 46 Transcriptome sequencing has shown that the effect of azadirachtin can be divided into three aspects: biological process, cellular component, and molecular function. In all, 490 genes were differentially expressed upon azadirachtin treatment, of which 221 were upregulated and 269 were downregulated, resulting in anti-feeding, 47 inhibition of growth and development, 48 avoidance, 49 and cytotoxicity.…”

“…Because of its high efficiency and low toxicity 7,8 it can be used as a substitute or in rotation with current synthetic insecticides in integrated pest management programs 44,45 . This is mainly because azadirachtin is a good anti‐feedant 46 . We studied the effect of azadirachtin on the feeding behavior of S. litura larvae.…”