THE EFFECT OF AN INSECT HERBIVORE AND A RUST FUNGUS INDIVIDUALLY, ANDCOMBINED IN SEQUENCE, ON THE GROWTH OF 2 RUMEX SPECIES

The chrysomelid beetle Gastrophysa viridula and the rust fungus Uromyc es rumicis both occur on leaves of Rumex crispus and R. obtusifolius. We investigated the effect of beetle grazing or rust infection individ ually and when combined in sequence on the growth of their hosts in th e field. Singly, beetle or rust reduced leaf area and plant biomass; t he effect was greater on R. crispus, and rust caused greater damage th an the beetle. Beetle grazing with subsequent rust infection caused da mage no greater than that caused by rust alone, although on R. obtusif olius damage was greater than that from beetle grazing alone. Rust inf ection of R. obtusifolius with subsequent beetle grazing produced dama ge similar to that from other treatments involving rust infection. In R. crispus this treatment produced the greatest reduction in biomass. The reductions in root and total plant weight from rust infection with subsequent beetle grazing were accurately predicted by a model includ ing the damage produced by beetle and rust alone and the length of tim e each was present on the plant. This model also predicted accurately the damage to R. obtusifolius from the beetle followed by rust treatme nt, but over-estimated by up to 40%, the damage to R. crispus. This ca n be explained mainly by an inhibition of rust infection by beetle gra zing.