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The stalk borer Eldana saccharina Walker (Lepidoptera: Pyralidae) is a major limiting factor in South African sugarcane production, while yield is also reduced by sugarcane thrips Fulmekiola serrata Kobus (Thysanoptera: Thripidae). Borer management options include appropriate nitrogen (N) and enhanced silicon (Si) nutrition; the effect of N on sugarcane thrips is unknown. We tested the effects of these nutrients, in combination with resistant (N33) and susceptible (N27) sugarcane cultivars, on E. saccharina and F. serrata infestation. Two pot trials with three levels of N (60, 120, and 180 kg ha-1) and two levels each of calcium silicate and dolomitic lime (5 and 10 t ha-1) were naturally infested with thrips, then artificially water stressed and infested with borer. Higher N levels increased borer survival and stalk damage, while Si reduced these compared with controls. Silicon significantly reduced stalk damage in N27 but not in N33; hence, Si provided relatively greater protection for susceptible cultivars than for resistant ones. High N treatments were associated with greater thrips numbers, while Si treatments did not significantly influence thrips infestation. The reduction in borer survival and stalk damage by Si application at all N rates indicates that under field conditions, the opportunity exists for optimizing sugarcane yields through maintaining adequate N nutrition, while reducing populations of E. saccharina using integrated pest management (IPM) tactics that include improved Si nutrition of the crop and reduced plant water stress. Improved management of N nutrition may also provide an option for thrips IPM. The contrasting effects of Si on stalk borer and thrips indicate that Si-mediated resistance to insect herbivores in sugarcane has mechanical and biochemical components that are well developed in the stalk tissues targeted by E. saccharina but poorly developed in the young leaf spindles where F. serrata occurs.

Silicon's role in ameliorating a range of biotic and abiotic plant stresses is beyond doubt, yet means to maximise its uptake via the roots from applied silicon sources and thereby enhance crop yields have not been fully explored. Our study found that reduction of rhizosphere pH through provision of nitrogen fertilizer to sugarcane as ammonium rather than nitrate increased silicon uptake from a low-silicon soil amended with calcium silicate slag. We propose that ammoniacal fertilizers have potential for enhancing the solubilisation of silicate slags by acidifying the rhizosphere and increasing silicic acid solubility and availability for plant uptake.

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