Interactions between the rust fungus Puccinia punctiformis and ectophagousand endophagous insects on creeping thistle

1. Plant-pathogen-insect interactions may have various outcomes that are of great interest when considering the combined use of insects and pathogens in the biological control of weeds. Here effects of rust infection on herbi vore attack and of herbivory on rust infection were analysed to ensure that these possible control agents do not impede each other. 2. The system studied was the weed creeping thistle Cirsium arvense, the sp ecialized rust fungus Puccinia punctiformis, and ectophagous and endophagou s insect herbivores. On 20 sites around Gottingen, Germany, the abundance o f ectophagous and endophagous insects on systemically rust-infected and uni nfected plants was quantified by suction samples and the dissection of shoo ts. 3. Abundance of ectophagous insects was not reduced by rust infection, and aphids as well as several beetle species were more abundant on systemically infected plants. Preference of endophagous insects differed between specie s: larvae of Apion weevils were more abundant in infected shoots, whereas M elanagromyza aeneoventris, Urophora cardui, all of them oligophagous, and s ome leaf miners preferred uninfected plants. In dual-choice feeding tests, larvae of the oligophagous chrysomelid beetle Cassida rubiginosa were more likely to feed on leaf segments of uninfected thistles. 4. Enhancement of rust infection rate by simulated herbivory was analysed. Thistle leaves were perforated and infection rate was recorded after 2 week s under natural Puccinia punctiformis spore concentrations. Numbers of ured osori were significantly higher in perforated leaves. Artificial inoculatio n resulted in an even greater increase in uredosori, both in perforated and unperforated thistles. 5. As the majority of ectophagous and endophagous insects did not discrimin ate between systemically infected and uninfected thistles, herbivore damage together with infection with Puccinia punctiformis can be expected to incr ease stress on creeping thistle when used in biological control of the weed . Insects may not only affect plant growth by herbivory but may also increa se the susceptibility of healthy thistles to local rust infection by woundi ng of leaves.