Stem-Borer Larval Infestation of Ryegrass Swards Under Rotationally Grazed and Cut Conditions

(1) Dipterous stem-boring larval populations were assessed in rotationally grazed or cut ryegrass swards (Lolium perenne L.) throughout 1980 and 1981. (2) Total larval populations were markedly higher under grazed than under cut conditions. Oscinella frit larvae, which were separated into two forms, were much more common in the grazed swards. O. vastator, (present in large numbers only in 1981) was more evenly distributed in both regimes. (3) Vegetation characteristics studied were tillering, soluble nitrogen, water-soluble carbohydrate and silica contents. Tillering was a major factor implicated in larval infestation: the higher tiller numbers in grazed swards is associated with an increase in larval numbers. However, grazed tillers also appeared more susceptible to larval attack than cut tillers. (4) The increased larval populations in the grazed swards did not occur as a result of the increased soluble nitrogen levels from dung and urine voided by the grazing animals, nor as a result of a difference in the level of silica which was higher under grazing. Water-soluble carbohydrate levels were higher under the cut conditions and this may have led to a reduction in larval numbers.