Physical defences wear you down: progressive and irreversible impacts of silica on insect herbivores

Abstract

Summary 1.Silica in the leaves of grasses can act as a defence against both vertebrate and invertebrate herbivores. The mechanisms by which silica affects herbivore performance are not well characterized. Here we expose an insect herbivore Spodoptera exempta to high-silica diets and test two mechanisms by which silica has been proposed to act as a defence. First, that silica reduces the digestibility of leaves and second, that silica causes wear to insect mandibles, both of which could potentially impact on herbivore performance. 2. Silica reduced the efficiency with which S. exempta converted ingested food to body mass and the amount of nitrogen absorbed from their food, leading to reduced insect growth rates. The measure of how efficiently herbivores utilize digested food (ECD) was unaffected by silica. 3. These effects occurred even with short-term exposure to silica-rich diets, but they also increased markedly with the duration of exposure and affected late instars more than early instar larvae. This appears to be due to the progressive impacts of silica with longer exposure times and suggests that herbivores cannot adapt to silica defences, nor do they develop a tolerance for silica with age. 4. Exposure to silica-rich diets caused increased mandible wear in S. exempta . This effect was extremely rapid, occurring within a single instar, further reducing feeding efficiency and growth rates. These effects on insect growth and feeding efficiency are nonreversible, persisting after the herbivore has switched diets. Up to a third of this residual impact can be explained by the degree of mandible wear caused by previous silica-rich diets. 5. The impacts of silica on S. exempta larvae were progressive with exposure time and could not be compensated for, even by switching to a different diet. Thus, herbivores cannot easily adapt to physical defences such as silica, suggesting this defence will have major implications for herbivore fitness.

Mandibles of Leaf-Cutting Ants: Morphology Related to Food Preference

“…Moreover, silica plays an important role in the interaction between herbivores and plants. Massey et al (2006aMassey et al ( , 2006bMassey et al ( , 2007Massey et al ( , 2008Massey et al ( , 2009 showed that silica accumulation in grasses increases leaf abrasiveness, causing the wear and deterioration of mandibles and impairing insect feeding.…”

Study of Potassium Silicate Spraying in Coffee Plants to Control <i>Oligonychus ilicis</i> (McGregor) (Acari: Tetranychidae)

“…It has also been reported that the resistance conferred by the leaf silicon application causes chemical and physical changes in three varieties of Poaceae (Deschampsia caespitosa L, Festuca ovina and Lolium perene L.) such as reduced digestibility, increased tissue stiffness and increased abrasiveness [30]. However, with application of calcium silicate in the soil, it was already observed in coffee that the mealybug, P. citri, reached the phloem feeding place without problems, indicating that in this study, no mechanical barrier was performed to avoid the penetration of the buccal stylets [31].…”

Silicon in banana plants: uptake, distribution and interaction with the disease fusarium wilt

“…Although the role deposited silicon plays in enhancing defences against plant pathogens is well established, there is also evidence that silicon content is directly correlated with resistance to herbivory (Massey and Hartley, 2009;Hunt et al, 2008). This resistance to herbivory is thought to be due to a reduction in palatability for the herbivore.…”

“…This resistance to herbivory is thought to be due to a reduction in palatability for the herbivore. Massey and Hartley (2009) found that organisms feeding on silicon-rich material plant material did not adapt with age and theorised that organisms have a limited capacity for developing a tolerance to silicon-rich plant tissue. This suggests that silicon deposition provides plants with an evolutionary advantage against herbivores, and it would be interesting to compare silicon deposition with the presence of anti-herbivore toxins to see if there is a negative correlation.…”

“…Both vertebrate and invertebrate herbivores suffer adverse effects when feeding on silica-rich plants (Massey et al, 2007). Grasses that accumulate silicon are less digestible to insects and amorphous silicon deposits in plant tissue increase mandible wear, thus decreasing insect fitness (Massey and Hartley, 2009;Hunt et al, 2008). Grazing rabbits have been shown to avoid feeding on silicon-treated winter wheat (Cotterill et al, 2007).…”