Is two company or a crowd: How does conspecific density affect the small-scale dispersal of a weed biocontrol agent?

To predict the growth and spread of an insect population introduced for the biological control of weeds, one must first understand the factors affecting the movement of individuals in the population. The purpose of this study was to determine how the dispersal rate of Aphthona lacertosa (Rosenhauer) (Chrysomelidae) was affected by conspecific density and by the characteristics of leafy spurge (Euphorbia esula L.: Euphorbiaceae) in patches where these beetles feed. In 2002 in Manitoba and in 2003 in Alberta, Canada, between 200 and 2500 insects were released in small patches (<10 m²) of spurge. The number and location of beetles within patches was monitored over subsequent days. In 1 m² plots within patches, spurge ramet density, the proportions of vegetative and reproductive ramets, and ramet height were measured. In both years, beetle movement within patches and emigration from patches, was not affected by conspecific density. In Manitoba in 2002, beetles aggregated non-randomly on either vegetative or reproductive ramets within plots, but plot characteristics were not related to the formation of aggregations. In Alberta in 2003, plots in which beetles aggregated had significantly higher spurge density but did not differ in either the proportion of vegetative ramets or in the amount of non-spurge vegetation. These results suggest that density-dependent dispersal does not limit the population's ability to reach densities up to 2500 beetles/m².