The first Harlequin ladybirds (Harmonia axyridis) arrived to the UK in 2004 and 2005. The dataset presents the SILAM atmospheric chemical transport model (CTM) simulatios of the arrival of H. axyridis to the UK in 2004 and 2005 and inverse CTM simulations from the first H. axyridis recods in 2004. These SILAM simulations are theoretical. They are intended to help assess whether it was possible for ladybirds to fly over the English Channel. LadybirdsSILAMforward.tar includes SILAM CTM forward simulation for 2004 and 2005. PM10 and PM2.5 particles represent ladybirds. The source is daily 5 -18 UTC from some cities in France, Belgium and Netherlands. The fileformat is GrADS. HarlequinsFootprintsFirstObsUK.tar includes the inverse simulations of H. axyridis from the first records in the UK in 2004. PM10 and PM2.5 particles represent ladybirds. Fileformat is GrADS. Ctl-files are examples for the grads-file, and they also describe output resolution and output timesteps for the SILAM model simulations.