| Summary: | 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 (SiO<sub>2</sub>NPs) and nano-selenium (SeNPs), and bulk-size silica (SiO<sub>2</sub>) 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 SiO<sub>2</sub>, SeO<sub>2</sub>, 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. SiO<sub>2</sub> content did not differ between control and SiO<sub>2</sub>NP-treated leaves, but was higher in bulk-size SiO<sub>2</sub>-treated leaves. The SiO<sub>2</sub> level in the control leaves was higher in the SiO<sub>2</sub>NP treatment compared with that in the control leaves in the bulk-size SiO<sub>2</sub> treatment. Silica-treated leaves increased in toughness, but SeNP-treated leaves did not; leaf toughness increased with mesophyllic SiO<sub>2</sub> content. The insect density per leaf increased with leaf toughness, SiO<sub>2</sub> content and, in the SiO<sub>2</sub>NP treatment, with epidermal C content. There was no correlation between SeO<sub>2</sub> 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.
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