Plant invasions are an important problem for natural and agronomic systems, being a threat for protected plant species and crops. An example is Cirsium arvense (L.) Scop, one of the most problematic weeds in North America and a common native weed in Europe. There have been many attempts to use biological control to limit spread and density of invasive species but the success of it, however, is not very high. Theoretical knowledge about effects of the herbivore insects on performance and population dynamics of species is relatively limited and more information here may improve efforts to implement successful biological control programs.

The aim of this work was to look at the effect of natural enemies on Cirsium arvense and how it varies in different abiotic conditions and localities. Our hypotheses were that occurrence of natural enemy damages and their effect on plant performance depends on moisture of the sites and that specific groups of natural enemies tend to co-occur more often together than expected by random.

We measured damages by different herbivore insect guilds and rust fungus Puccinia punctiformis in six different populations in the surroundings of Prague (the Czech Republic), divided by humidity gradient, during 2005 and 2006.

Results/Conclusions

Natural enemies were more common in sites near water streams, where the plants were taller, which shows Cirsium arvense tends to be more damaged in wet rather than in dry conditions. Insect guild damages also varies between seasons and sites.

The most common natural enemy damages found in Cirsium arvense were folivory, external stem damage and leaf necrosis and were not co-occurring together.

Folivory was co-occurring with leaf mines and with rust caused by Puccinia punctiformis. External stem damage was co-occurring with stem galls and top necrosis, eaten top and leaf cocoons. Leaf necrosis and eaten plant tops were not often co-occurring with other damage.

After studying insect effects in plants we concluded that when looking at insect guild damages of a plant there is a need to identity which insect species it cause for a better understanding and analysis of the plant damages and insect-plant interactions.

The study of particular insects and the connection between plant damage and insect in Cirsium arvense is needed for a better understanding of the system and the effect of the insect damages in the plant populations.

Key words: Cirsium arvense, natural enemies, plant-insect interaction, plant damage, population dynamics.