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Although nanoparticles have gained attention as efficient alternatives to conventional agricultural chemicals, there is limited knowledge regarding their effects on herbivorous insect behavior and plant physicochemistry. Here, we investigated the effects of foliar applications of nano-silica (SiO₂NPs) and nano-selenium (SeNPs), and bulk-size silica (SiO₂) on the choice behavior of the arrowhead scale insect on mandarin orange plants. One leaf of a bifoliate pair was treated with one of the three chemicals, while the other was treated with water (control). The respective SiO₂, SeO₂, calcium (Ca), and carbon (C) content levels in the leaf epidermis and mesophyll were quantified using SEM–EDX (or SEM–EDS); leaf toughness and the arrowhead scale density and body size were measured. First-instar nymphs preferred silica-treated leaves and avoided SeNP-treated leaves. SiO₂ content did not differ between control and SiO₂NP-treated leaves, but was higher in bulk-size SiO₂-treated leaves. The SiO₂ level in the control leaves was higher in the SiO₂NP treatment compared with that in the control leaves in the bulk-size SiO₂ treatment. Silica-treated leaves increased in toughness, but SeNP-treated leaves did not; leaf toughness increased with mesophyllic SiO₂ content. The insect density per leaf increased with leaf toughness, SiO₂ content and, in the SiO₂NP treatment, with epidermal C content. There was no correlation between SeO₂ content and insect density. This study highlights the potential uses of SeNPs as an insect deterrent and of silica for enhancing leaf toughness and attracting scale insects. Full article