Silica in grasses as a defence against insect herbivores: contrasting effects on folivores and a phloem feeder
Fergus P. Massey, Department of Biology and Environmental Science, University of Sussex, Falmer, Brighton, East Sussex, BN1 9QG, UK. Tel: 01273 873502; Fax: 01273 6784333. E-mail: F.P.Massey@sussex.ac.uk
- 1Silica, deposited as opaline phytoliths in the leaves of grasses, constitutes 2–5% of dry leaf mass, yet its function remains unclear. It has been proposed that silica may act as an antiherbivore defence by increasing the abrasiveness and reducing the digestibility of grass leaves, although there is little direct experimental evidence to support this.
- 2We investigated the effects of manipulated silica levels on the abrasiveness of the leaves of five grass species. We also examined the effects of silica levels on the feeding preferences, growth performance and digestion efficiency of two folivorous insects and one phloem-feeding insect.
- 3Silica addition resulted in increases to leaf abrasiveness in four of the five grass species studied. Silica addition also deterred feeding by both folivores and reduced their growth rates and digestion efficiency.
- 4These effects resulted in lower pupal mass of the lepidopteron larvae Spodoptera exempta and compensatory feeding by the orthopteran, Schistocerca gregaria. In contrast, silica had no effects on the feeding preference or the population growth of the phloem feeder, Sitobion avenae.
- 5Our results demonstrate that silica is an effective defence against folivorous insects, both as a feeding deterrent, possibly mediated by increased abrasiveness, and as a digestibility reducer. The effects of silica on pupal mass and development time may impact on herbivore fitness and exposure to natural enemies.
- 6These results are the first demonstration of a direct effect of silica on the abrasiveness of grasses and the adverse impact of silica on herbivore preference and performance.