Elevated atmospheric CO2 triggers compensatory feeding by root herbivores on a C3 but not a C4 grass

Predicted increases in atmospheric carbon dioxide (CO2) concentrations often reduce nutritional quality for herbivores by increasing the C:N ratio of plant tissue. This frequently triggers compensatory feeding by aboveground herbivores, whereby they consume more shoot material in an attempt to meet their nutritional needs. Little, however, is known about how root herbivores respond to such changes. Grasslands are particularly vulnerable to root herbivores, which can collectively exceed the mass of mammals grazing aboveground. Here we provide novel evidence for compensatory feeding by a grass root herbivore, Sericesthis nigrolineata, under elevated atmospheric CO2 (600 mmol mol21) on a C3 (Microlaena stipoides) but not a C4 (Cymbopogon refractus) grass species. At ambient CO2 (400 mmol mol21) M. stipoides roots were 44% higher in nitrogen (N) and 7% lower in carbon (C) concentrations than C. refractus, with insects performing better on M. stipoides. Elevated CO2 decreased N and increased C:N in M. stipoides roots, but had no impact on C. refractus roots. Root-feeders displayed compensatory feeding on M. stipoides at elevated CO2, consuming 118% more tissue than at ambient atmospheric CO2. Despite this, root feeder biomass remained depressed by 24%. These results suggest that compensatory feeding under elevated atmospheric CO2 may make some grass species particularly vulnerable to attack, potentially leading to future shifts in the community composition of grasslands.