Added soil nitrogen does not allow Rumex obtusifolius to escape the effects of insect-fungus interactions.
The rust fungus Uromyces rumicis and the chrysomelid beetle Gastrophysa viridula have been considered as biocontrol agents against Rumex obtusifolius. As this weed grows in widely varying soil nitrogen conditions, and the effect of insects and fungi on their hosts can vary greatly with changes in plant nitrogen fertilization, an investigation was carried out into the influence of three nitrogen fertilization regimes (0, 200 and 400 kg ha-1 year-1 added nitrogen) on the efficacy of these biocontrol agents, both individually and when combined, against R. obtusifolius seedlings, and on the nature of the insect-fungus interaction. Non-destructive measurements were made at monthly intervals from May, and plants were harvested at the end of October. The addition of 200 kg ha-1 year-1 nitrogen increased leaf area by 70% by September. At harvest, leaf weight was increased by 19% and root weight by 43% compared with unfertilized plants. There was no significant difference between the effect of 200 and 400 kg ha-1 year-1 added nitrogen. Herbivory alone caused a greater reduction in leaf area and decrease in root and shoot weight than rust infection alone. The effects of nitrogen fertilization, herbivory and infection were additive throughout the experiment. The combination of herbivory and infection had an increasing effect on leaf area as the year progressed, from a 46% reduction in July to 62% in October, compared with healthy plants. Unfertilized plants with combined herbivory and infection had a 65% reduction in leaf area and weight, and a 72% reduction in total harvested weight, compared to healthy plants with 400 kg ha-1 year-1 added nitrogen. It was concluded that increasing nitrogen fertilization did not allow R. obtusifolius to escape the effects of the insect and fungus and did not alter the additive nature of the insect-fungus interaction. However, the simple model developed in this paper indicates circumstances in which nitrogen fertilization may enable plants to escape the effects of these interactions, and this is discussed in relation to weed biocontrol strategies.