Silicon, selenium, iron, and zinc nanoparticles suppress arrowhead scale populations and modify citrus leaf and petiole traits

Abstract

Nanoparticle (NP)-based agrochemicals are increasingly explored as sustainable alternatives to conventional pesticides, yet their effects on piercing–sucking pests and the underlying plant-mediated mechanisms remain poorly understood. We examined how foliar application of four elemental NPs—silicon dioxide (SiO₂)NPs, selenium (Se)NPs, iron oxide (Fe₂O₃)NPs, and zinc oxide (ZnO)NPs—influences citrus leaf and petiole traits and the population dynamics, settlement behavior, and body size of the arrowhead scale Unaspis yanonensis on satsuma mandarin Citrus unshiu. After nymphal settlement, leaves were treated once with NPs or distilled water, and scale populations and leaf areas were monitored for six weeks under greenhouse conditions. All NP treatments reduced weekly population-density change in nymphal and adult stages, with ZnONPs and SiO₂NPs showing the strongest suppression of adults. SeNPs, Fe₂O₃NPs, and ZnONPs promoted leaf growth only under scale infestation. In bifoliate choice assays, first-instar nymphs showed the previously reported preference for SiO₂NP-treated leaves and avoidance of SeNP-treated leaves. NP treatments also modified leaf and petiole toughness and tissue-specific elemental composition. SiO₂NPs increased leaf and petiole toughness in association with elevated adaxial epidermal Si and Ca. ZnONP treatment reduced adult scale size. Across treatments, tougher tissues were generally associated with reduced adult body size. These results suggest that elemental NPs can suppress arrowhead scale populations through direct and/or plant-mediated effects involving changes in citrus tissue structure and elemental allocation. ZnONPs appear particularly promising as pest-suppressive materials that may also enhance leaf growth under infestation.