The uptake of silicon (Si) by plants is known to reinforce plant tissues against invertebrate herbivores, but whether there is also direct antibiosis from ingesting Si remains in question. To investigate for antibiotic effects, Si-bearing minerals, wollastonite (CaSiO3) or olivine (Mg/FeSiO3), were added to artificial diets as finely ground powders (40–50 μm) before or after acidulation to determine whether Si-reduced larval growth. Newly hatched Helicoverpa armigera (Hübner, 1808) and H. punctigera (Wallengren, 1860) (Lepidoptera: Noctuidae) were placed onto the diets and weighed once the larvae on the control diet had completed feeding (day12 for H. armigera; day 14 for H. punctigera: 25.5°C). Acidulated olivine at rates of 0.4–1.7% Si w/dw (weight Si/dry weight of diet) reduced larval weight of H. armigera by 95–99% compared with control diets. Non-acidulated olivine also appeared to cause a decline in larval weight. Wollastonite at rates of up to 3.3% Si had no significant effect on larval weight whether acidulated or not. A similar effect was observed for H. punctigera. Very few insects survived to emergence on diets containing the higher rates of acidulated olivine. Olivine contains quantities of heavy metals, particularly nickel, cobalt and chromium, which can be toxic to insects. Given the lack of toxicity when Si was included as wollastonite compared with similar quantities as olivine, the heavy metals are implicated as the antibiotic agents. Acidulation increased the toxicity of olivine probably by rendering the metals more biologically active. The results of this bioassay do not support the hypothesis that Si is directly antibiotic to Helicoverpa spp. via ingestion.