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

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.