Not so Lady-like Ladybugs
Whether pure myth or based on an actual historical ploy, the story of the Trojan Horse has come to symbolize the process of surreptitiously getting inside another’s territory with sufficient forces to conquer the victim from within.
This process is initiated by fooling the victim into believing that what’s entering its territory (or body) is benign and harmless. Jared Diamond, in his book Guns, Germs, and Steel, makes the point that the hugely successful occupation of the Americas by Europeans was significantly aided by the (then invisible) germs they carried with them to the West. Unlike the Greek soldiers in the story of the Trojan Horse, the European carriers of these lethal germs had no awareness of the destruction they were spreading beyond the havoc their guns wrought. A similar story is now unfolding among those ubiquitous little bugs we call ladybugs (or ladybirds). An Asian variety of this beetle, Harmonia axyridis, introduced elsewhere around the world as a biologic control agent against insect pests, has displaced our native lady bug over wide areas by virtue of the fact that it carries a parasite that is lethal to most other species of ladybugs. The parasite, to which the Asian ladybug (also called the harlequin ladybird) is resistant, is a tiny spore-forming fungus that lives inside the ladybug’s cells.
There remains a mystery as to how this pathogen gets transmitted but there are two known possible routes. First, harlequin ladybugs gather into large, closely-packed, groups during the winter where the transfer of these pathogens among the individual beetles might be facilitated. This would enhance the Asian species’ resistance to the germ by keeping their immune system primed and increase the probability that all individuals have the capability of transmitting the germ to other species it came in contact with. There is also widespread predation by the larvae of one species on the larvae of other species and it was observed that when a larva of a non-Asian variety feeds on a harlequin ladybug larva it soon succumbs, during which time the spores of the fungal parasite can be found within the body of the dying native ladybug.
Why isn’t the Asian ladybug also killed by this fungal parasite? Currently, the best hypothesis that accounts for their resistance is the high concentration of a chemical called “harmonine” in the hemolymph (blood) of this bug. Harmonine is a broad spectrum anti-microbial agent and it may be this chemical that enables the Asian ladybug to survive most new pathogens it comes in contact with in its new habitats. Indeed the organisms that cause tuberculosis and malaria are both inhibited by this drug making it potentially useful in our efforts to design better therapeutic drugs to combat these widespread and difficult to treat human diseases.
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