Contrasting effects of foliar silicon application on a citrus scale insect through tissue‐specific silica allocation

BACKGROUND Silicon (Si) and silica (SiO 2 ) are widely used agrochemicals, yet their effects on scale insect populations, leaf growth, and tissue‐specific allocation remain poorly understood. We examined the effects of foliar silicon application on population dynamics of different developmental stages of the arrowhead scale Unaspis yanonensis infesting Satsuma mandarin, leaf area growth, and tissue‐specific SiO 2 allocation under greenhouse conditions. Either silicon or water (control) was sprayed once on both leaf surfaces in two experiments: (i) application after nymphal settlement, with insect populations monitored weekly for 11 weeks; and (ii) application before settlement to paired leaves to evaluate first‐instar nymphal choice between water‐ and silicon‐treated leaves. RESULTS Silicon treatment significantly reduced population growth rates of first‐ and second‐instar nymphs and decreased first‐instar production by adults, whereas adult population change was unaffected. Silicon treatment also strengthened negative density dependence among nymphs. Despite these suppressive effects, first‐instar nymphs preferentially settled on silicon‐treated leaves. Neither silicon application nor scale infestation affected leaf area expansion. Silicon increased leaf toughness and promoted SiO 2 accumulation primarily in the mesophyll, with the lowest accumulation in the abaxial epidermis. Principal component analysis revealed positive associations among mesophyll silica content, leaf toughness, and nymphal density. CONCLUSION Foliar silicon application exerted contrasting effects depending on treatment timing. Post‐settlement treatment suppressed scale population growth, whereas pre‐settlement treatment increased settlement of first‐instar nymphs. Silicon application enhanced leaf toughness through mesophyll SiO 2 accumulation. These findings demonstrate foliar‐tissue‐mediated plant–insect interactions and highlight the importance of application timing in silicon‐based pest management approaches.