WHEN GRAZED BY TIPULA-PALUDOSA, LOLIUM-PERENNE IS A STRONGER COMPETITOR OF RUMEX-OBTUSIFOLIUS

1 Experiments were conducted to investigate the effect of root-grazing by Tipula paludosa on competitive interactions between Rumex obtusifo lius and Lolium perenne in the glasshouse. 2 Preference tests found th at T paludosa larvae preferred L. perenne to R. obtusifolius plant mat erial, consuming twice as much of the former when presented with eithe r the shoot material or the whole plant. 3 Competition experiments inv olved replacement series at a range of planting densities and the meth odology of Jolliffe et al. (1984). 4 Leatherjacket grazing significant ly reduced L. perenne but not R. obtusifolius dry weights in monocultu re in the glasshouse. 5 Reduced L. perenne total plant dry weights in grazed monocultures consisted primarily of reduced amounts of root mat erial, reflected in a lower L. perenne root: shoot ratio in the presen ce of tipulids. 6 Intraspecific competition in R. obtusifolius monocul tures was not significantly affected by tipulid grazing, but in L. per enne monocultures, it was significantly greater in grazed than ungraze d treatments. 7 Tipulid grazing significantly reduced L. perenne plant dry weight in mixtures, while having no significant effect on R. obtu sifolius dry weights. 8 Tipulid grazing of L. perenne reduced the impa ct of interspecific competition from R. obtusifolius on L. perenne, im plying that grazed L. perenne was a stronger competitor than ungrazed L. perenne. 9 It appears that root grazing of the grass led to a relat ive increase in the shoot production of L. perenne which together best owed an increased competitive ability on grazed L. perenne when in com petition with R. obtusifolius.