Invaders From Inner Space

13 June 2007 (All day)

Fukatsu et al./PLoS

Diet as destiny.
Researchers have found that populations of the common soybean pest, M. punctatissima, are quite harmless without their gut microbes.

When the plataspid stinkbug chows down on soybean crops, often laying waste to entire fields, it operates with the aid of a hidden accomplice. Researchers have found that a microbe living in the insect's gut governs its reproduction, allowing it to feast on soy by the millions. Remove the microbe, and the stinkbug's population plummets, effectively rendering it harmless. But inject those same bacteria into a related but benign stinkbug species, and that insect proliferates into a soy-munching army. The findings shed new light on insect evolution, but it's uncertain whether they will help crop researchers develop new strategies for protecting the world's food harvest.

Scientists have presumed that crop pests evolved genes that help them locate the crops they attack. Now researchers at the University of Tokyo have performed a simple experiment that has yielded startling results. They tinkered not with the genes of the plataspid stinkbug (Megacopta punctatissima) but with the symbiotic bacteria inhabiting its gut. The team cleaned out the microbes living inside a laboratory population of punctatissima and then inserted them into the gut of the stinkbug's cousin, M. cribraria, which does not normally prey on soy plants in huge numbers.

Voilà! As the researchers report online this week in the Proceedings of the Royal Society B, the experimental bugs switched roles: Suddenly M. punctatissima's egg hatching plummeted in the lab, whereas M. cribraria's soared. "The effect is not behavioral but physiological," says entomologist and team member Takema Fukatsu. Apparently, the bacteria can play a critical role in the life cycle of both stinkbug species, boosting the population of whatever species they inhabit. If M. cribraria became the predominant host, it would grab the mantle of soybean pest.

Fukatsu says the team is now sequencing the genome of the gut bacterium to see what makes it tick. He says the same techniques also could be applied to microbes that live in symbiosis with aphids, tsetse flies, termites, and other species where eliminating them could reduce the insects' effectiveness as pests.

The findings are certainly interesting in the context of evolutionary biology, says entomologist Tom Turpin of Purdue University in West Lafayette, Indiana. But whether they will become useful for developing pest-management strategies remains "a complex combination of biological, sociological, and ethical considerations," he says. For example, spraying antibiotics on a crop might eliminate the pest by killing their internal bacteria, says Turpin, but the residues might be perceived as harmful to the human population that relies on the crop.

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