Silicon suppresses a ubiquitous mite herbivore and promotes natural enemy attraction by altering plant volatile blends

Silicon (Si) accumulation in plants is widely recognised as an effective physical defence against chewing herbivores. However, its effects on some feeding guilds such as cell-content feeders are understudied despite being severe economic pests (e.g. Tetranychus urticae). Moreover, most studies focus on direct impacts of Si, but there is growing evidence that Si also impacts indirect defence. We examined the effects of Si on French bean, Phaseolus vulgaris, defences against the two-spotted spider mite, T. urticae. We grew plants hydroponically with (+ Si) or without (-Si) silicon, assessed T. urticae performance and tested the preference of the predatory mite, Phytoseiulus persimilis, for volatiles from T. urticae-infested (+ M) or uninfested (-M) plants. The provision of Si to plants suppressed T. urticae egg-laying, population growth and leaflet damage, and partially ameliorated T. urticae-induced reductions in stomatal conductance and net photosynthesis. Furthermore, T. urticae infestation increased foliar Si accumulation. Predatory mites were more attracted (64%) to volatiles from + Si plants experiencing herbivory than to -Si plants. The relative emissions (%) of volatile compounds, viz. E-2-hexanyl benzoate, hexanal, E-trans-β-ocimene, D-limonene, β-caryophyllene and methyl salicylate were elevated from + Si + M plants, while the relative emissions of 3-hexanol, trans-calamenene, o-xylene and o-cymene were lowered compared to -Si + M plants. Our results show, for the first time, that Si defences are inducible and effective even in low Si-accumulating plants against T. urticae and suggest that Si could play a role in pest biocontrol.