THE EFFECT OF INSECT-FUNGUS INTERACTIONS ON THE AUTUMN GROWTH AND OVER-WINTERING OF RUMEX CRISPUS AND R-OBTUSIFOLIUS SEEDLINGS

1 The rust fungus Uromyces rumicis infects leaves of Rumex crispus and R. obtusifolius mainly between August and October, during which time the chrysomelid beetle Gastrophysa viridula also occurs on these leave s. This experiment investigated the effect of beetle herbivory or rust infection, individually and when combined, on the growth of seedlings from August and also on regrowth the following spring. 2 Between Augu st and October, the beetle alone removed up to 79% of leaf area. Rust infection was slow to develop, but caused a decrease of approximate to 50% in leaf area by October. The combination of herbivory and infecti on had an additive effect by October, causing a reduction in leaf area of 92% (R. crispus) and 88% (R. obtusifolius). 3 There was no plant m ortality. At the harvest in February, previous herbivory reduced area and weight of regrowth by 78% in both species, but infection had a sig nificant effect only on R. crispus. The combined effects of herbivory and infection were additive, leading to a 88% (R. crispus) and 84% (R. obtusifolius) reduction in shoot dry weight, and a 82% (R. crispus) a nd 72% (R. obtusifolius) reduction in root dry weight. 4 Herbivory and infection both led to a significant, additive, increase in the concen tration of alcohol-soluble carbohydrates in the roots. Herbivory also had an effect on fructans, starch, and total nonstructural carbohydrat e in the roots, reducing concentrations by up to 26%. 5 Overall, herbi vory alone or combined with infection had a severe effect on regrowth capacity and root quantity and quality of both species, although infec tion alone had little effect on R. obtusifolius. The possible effects of insect-fungus interactions on the subsequent reproductive success a nd survival of these plants in natural habitats are discussed.