Invasive species can have huge impacts on biological diversity when they invade. One of the reasons that has been suggested for this is the enemy release hypothesis. The release from natural enemies (parasites, predators, etc.) allows a population to expand without the pressure from natural enemies that native species are under. However, over time parasites of native species in the invaded range may adapt to utilise a novel host and influence the overall strength of the invasion. Invasive species also have the potential to bring with them biological weapons, in the form of pathogens and disease, which can contribute to the decline of native species. The harlequin ladybird has been described the most invasive ladybird on earth. Arriving in the UK in 2004, it has since become the most common ladybird in areas of South East England. This invader has caused worrying declines in native ladybird species in the UK and Europe. My PhD will investigate further the role of parasites in biological invasions, focusing on the Harlequin ladybird and its invasion in the UK. I aim to answer questions relating to parasite host switches, parasite adaptation to novel host species and biological weapons. Funded by NERC.