The harlequin is an intraguild predator. Where aphids run short, its larvae attack the eggs and larvae of other ladybirds, and it is generally the winner in those encounters because it is larger, faster-developing and better chemically defended.
Research published in 2013 identified a second mechanism. Harlequin haemolymph carries microsporidian spores that appear harmless to the harlequin itself but are lethal to at least some native coccinellids, including the two-spot ladybird, when they eat harlequin eggs or larvae. The parasite therefore functions as a biological weapon carried by an animal immune to it.
The consequences have been measured. In Britain, the two-spot ladybird declined substantially in the decade following the harlequin's 2004 arrival, and similar declines have been recorded for some native species in North America. What remains contested is how much of the decline is predation, how much is parasite transmission, and how much is competition for aphids in a shared food supply.