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. That much is well established.
A 2013 paper in Science proposed a second mechanism. Harlequin haemolymph carries microsporidian spores, related to Nosema thompsoni, that appear harmless to the harlequin itself; injected into the seven-spot ladybird, Coccinella septempunctata, they killed every individual within two weeks. The paper drew published comments questioning whether injection reflects any natural route, and a 2015 follow-up in which seven-spots actually ate infected harlequins recorded only about eleven per cent mortality over four months.
The declines themselves are not in doubt. In Britain the two-spot ladybird fell substantially in the decade after the harlequin's 2004 arrival, and native species have declined in North America too. What drives them is unsettled: direct predation, competition for aphids and parasite transfer are all candidates, and the parasite's share now looks smaller than the first headlines suggested.