I find invasive species fascinating. Either on their own steam or with a little help from us they have expanded their geographical distribution and, with time, they start forming part of the network of ecological links in each environment. Furthermore, - although they might have a negative impact on the economy and/or native species - they provide unique natural experiments that allow us to witness evolution in action and to investigate which evolutionary forces are be involved. One such invasive species is the Harlequin ladybird. This beautiful ladybird has gone from a native Central and Eastern Asian species to be considered a worldwide plague, via its use by us as an agent for the biological control of aphids. Despite the Harlequin having been used as such control in the U.S for decades, its range did not immediately expand and there seemed to be little room for concern. However, in 1988 the Harlequin started to become invasive and went to invade large areas in the U.S, Europe, South America and South Africa. In the U.K. the first sightings occurred in 2004 and it appears that by 2010 the ladybird had expanded into most available habitat.
Facon B, Hufbauer RA, Tayeh A, Loiseau A, Lombaert E, Vitalis R, Guillemaud T, Lundgren JG, & Estoup A (2011). Inbreeding Depression Is Purged in the Invasive Insect Harmonia axyridis. Current biology : CB, 21 (5), 424-7 PMID: 21333536
Maderspacher, F. (2011). The benefits of bottlenecks Current Biology : CB, 21 (5)