One of the significant factors causing seed mortality on the mother plant is pre-dispersal predation which has been intensively studied in many Asteraceae. Pre-dispersal predators could limit thistles propagation to new niches. Flower heads of thistles contain the most varied, specific and well-known insect fauna of any part of the plant. They present rich source of food, packed with achenes, and their inhabitants are protected from vertebrate predators by the tough spiny bracts.
Our study has focused on the occurrence and diversity of seed-feeding insects (pre-dispersal seed predators) in flower heads of over 30 Carduoideae species from Europe and North America, many of which are invasive in North America. The main goals of our study were: (1) to find out the occurrence of pre-dispersal seed predators, (2) to compare the strategy of pre-dispersal seed predators, which use thistle flower heads for their development and (3) to evaluate the utilizability of pre-dispersal predators as significant biological control agents of thistles.
Results/Conclusions
The presence and number of pre-dispersal seed predators were established. Importance and utilizability of each pre-dispersal predator was assessed according to the following parameters: prevalence (percentage of infested hosts in population), prevalence of attacked flower heads (ratio of prevalence of insect group to total prevalence), intensity (mean number of predators per infested host) and abundance (mean number of predators per potential host), and these data were compared with literature sources.
We determined seed-feeding species belonging to these insect families: Diptera: Tephritidae, Cecidomyiidae; Coleoptera: Curculionidae, Dermestidae; Lepidoptera: Tortricidae; Hymenoptera: Eulophidae, Eurytomidae, Pteromalidae. Each pre-dispersal predator species has different attack strategy and the way of seed consumption. The most effective seem to be weevils (Curculionidae) (their larvae do not attack only seeds, but also the receptacle) and/or a combination of two and more bioagents which together create multiple stresses on the plant (e.g. Tephritidae + Tortricidae (sometimes + Curculionidae) in C. heterophyllum).
We develop a database including all the above parameters for all plant and insect species. The database is complemented with data obtained from the literature. By summarizing the data, we will obtain information on host-specificity of each pre-dispersal seed predators. Future studies will compare thistles with their associated insects from geographically different localities. All this knowledge will provide useful background information for planning efficient biocontrol of the species in their invasive range. Our results will significantly contribute both to basic research but will also have practical implications for biological control.