A rapid, precise and low‐cost method to quantify silicon for the determination of food intake and utilisation for insect herbivores

Abstract

AbstractDetermining intake and subsequent digestion and absorption of nutrients is fundamental for understanding host plant quality for insect herbivores. This typically involves considerable time spent weighing leaf material, with various associated sources of error and inaccuracy. Biomarker techniques based on silicon (Si) for estimating food intake and utilisation are well described, but rarely used, because there is no standard technique to either extract or quantify Si, there is a lack of congruence regarding the efficacy of existing techniques, and these techniques rely on large sample sizes, slow and laborious procedures, highly hazardous chemicals and/or expensive specialized equipment. To determine Si in milligram plant and faecal samples, we adapted the molybdenum blue assay to a microplate technique. This assay determines silicon (Si in 2% NaOH) over the range 0.3–1.5 μg Si/0.18 mL (1.7–8.3 μg/mL) and was able to detect changes of 1.0 × 10−9 g Si. Four of the most commonly used digestion techniques were compared, and we found that a technique based on the autoclave‐induced digestion method allowed the rapid and precise extraction and determination of silicon found in concentrations greater than 0.001% from ca. 2.5 mg of dry ground leaf material, a level of precision that far exceeds the 0.1–10% that silicon typically contributes to dry leaf matter. Using silicon as a biomarker reduced the weighing effort involved in traditional gravimetry by up to 95% and improved the precision (±SE) of estimates of food utilisation. Both the extraction and quantification steps require relatively inexpensive equipment that is available in most laboratories, uses micromasses of plant material and chemicals and is amenable to automation and high‐throughput processing.