Understanding dispersal rates of invading weed biocontrol agents.
Classical weed biological control programmes aim to rapidly establish biocontrol agent populations throughout the range of a weed. Release strategies, however, may often be suboptimal because the natural dispersal ability of a new agent is rarely known, potentially resulting in unnecessarily intensive release programmes for fast-dispersing agents or inadequate release programmes for slowly dispersing agents. We reviewed published dispersal data for 66 arthropod and 11 fungal pathogen weed biocontrol agents. We tested hypotheses regarding agent characteristics that were predicted to affect dispersal and whether agents that dispersed rapidly were more successful than those which dispersed more slowly. Dispersal rates varied by four orders of magnitude: the fastest agents dispersed several hundred kilometres per year and the slowest by only tens of metres per year. Approximately 30% of arthropod agents and four of 11 pathogen agents dispersed <1 km year-1, indicating that intensive redistribution is often required for rapid widespread establishment. Successful agents were equally likely to be fast or slow dispersers indicating that effort made redistributing slowly dispersing agents can often be beneficial. Both pathogen and arthropod dispersal rates were positively correlated with voltinism. Arthropod dispersal also significantly varied according to fecundity, dispersal type (crawling or passive wind dispersal vs. flight); taxon, life-style, habitat and the diversity of parasitoids attacking the agent in the native range. We developed dispersal models which explained up to 73% of the variance in arthropod agent dispersal rates and we validated these models by using them to predict the rate that invasive arthropod herbivores should invade. Synthesis and applications. We conclude that knowledge of a few parameters (agent voltinism, parasitoid diversity in the native range, habitat, fecundity, taxon and life-style category), prior to introduction of a biocontrol agent, could be used to predict how fast it is likely to invade a new environment. This should assist optimization of release strategies by determining the geographic scale at which to release agents, according to the agents' ability to rapidly close the gaps by natural dispersal.