Effects of root-feeding by scarabaeid larvae on growth of perennial ryegrass plants

(1) Growth of perennial ryegrass turfs was measured under glasshouse conditions, in pots containing different densities of root-feeding third-instar larvae of Sericesthis nigrolineata. (2) Larval feeding caused a reduction in root yield, but there was a reduction in green foliage yield only when plants were also regularly defoliated. (3) There was a reduction in new root and foliage growth but no increase in dead foliage as a result of larval root-feeding. The reduction in root yield was greater than the estimated larval consumption at all insect densities and was proportional to the control yield. (4) Larval feeding caused water stress in the foliage, as measured by a reduction in leaf relative water contents, which probably reduced the growth of plants. (5) The relationships between root yield and insect numbers were J-shaped with the greatest reduction in root yield occurring between the plants with no larvae and those with the lowest insect density. Higher insect densities had comparatively less influence on plant yield. The influences of both plant growth and insect feeding on these relationships are discussed.