Linking feeding microstructure and systemic delivery for preventive root-zone control of Megalurothrips usitatus in cowpea

Megalurothrips usitatus is difficult to manage in cowpea because adults and larvae feed and oviposit in protected young tissues that are poorly covered by sprays applied after infestation. This study examined whether root-zone delivery of cyantraniliprole could place the active ingredient in the tissues used by thrips and provide longer protection than foliar spraying. Of six candidate insecticides, cyantraniliprole gave the strongest activity by 72 h. In seedling assays, 1 mg·plant -1 reduced leaf damage from 79.95 to 55.33 mm 2 and produced measurable residues in leaves. Histological sections showed that feeding occurred mainly within the epidermis and upper mesophyll, with stylet penetration concentrated at 10–20 μm and extending to 40–50 μm. Mass spectrometry imaging showed that cyantraniliprole taken up by roots accumulated in these leaf layers. In two field trials, root-zone application suppressed M. usitatus in a dose-dependent manner and was more persistent than foliar spraying; at 960 g a.i.·ha -1 , relative control efficacy reached 76.67–87.78% at 5 days after treatment and remained higher than foliar spraying at later observations. Residue profiling showed continued delivery to newly formed tender tissues after root-zone treatment, whereas foliar residues declined rapidly and were not detected in newly emerged leaves. Short-term 16S rRNA sequencing detected no significant restructuring of rhizosphere bacterial communities 20 days after treatment, although longer-term effects and responses of other microbial groups were not assessed. Supplementary assessments further showed that cyantraniliprole residues in cowpea pods remained below the established MRL and that root-zone treatment did not significantly increase mortality of bees or lady beetles under the tested conditions.