The innocuous-looking harlequin ladybird (Harmonia axyridis, shown left) wields a biological weapon of mass destruction. Europe and North America imported the insects in the early 20th century to control pesky aphids. But the harlequin, native to Asia, began to flourish, crowding out the native seven-spotted ladybug (Coccinella septempunctata, shown inset). Scientists previously thought that the harlequin prospered because of an unusually strong antimicrobial immune system, which would protect it from disease in a foreign environment. But the beetle's more potent secret is a fungal parasite, in the insect-afflicting Nosema genus, which lives in the beetle's blood. The parasite doesn't affect the harlequin but fatally overwhelms seven-spotted lady beetles within 2 weeks  of infection, researchers report online today in Science. Ladybugs commonly eat the eggs of competing species, so when seven-spotted beetles feast on the harlequin's parasite-laden eggs, the parasite strikes back. Researchers say that foreign invaders fare better when they bring along diseases that they're already tolerant of, while other, closely related species (such as the seven-spotted ladybug) might not enjoy such conquistador-like success.
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