Closing in on Dutch elm cure

December 01, 2005, vol. 34, no. 7
By Jennifer Gardy

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Imagine the worst house guest in the world. He shows up uninvited, completely destroys your home, and when it's all done he gets you to call him a cab so he can head over to his next appointment with destruction.

Sounds like a fun guy, right?

Fungi is more like it. Meet Ophiostoma novo-ulmi, the fungus causing Dutch elm disease. Ophiostoma clogs an infected tree's water-conducting vessels. Without an adequate water supply, a diseased elm can wilt and die in as little as three weeks.

Since hitch-hiking to New England aboard a diseased European log, Ophiostoma has spread westward, hopping between trees on the back of the native elm bark beetle. Now, thanks to biological sciences professor Gerhard Gries' discovery of how the fungus flags down its insect taxi, science is one step closer to stopping the pestiferous parasite in its tracks.

Gries is an expert in the field of semiochemicals - molecular messages that influence an organism's behaviour. Gries is especially interested in how semiochemicals affect insects. When the Saskatchewan Forest Service asked for his help in deciphering how Ophiostoma uses the elm bark beetle to travel between trees, his team went to work.

“The (healthy) elm tree releases odorants that are recognized by the elm bark beetle as they fly around,” Gries explains. “However, the attractiveness of the odorants is rather low. The elm doesn't release very much.”

When Gries looked at trees afflicted with Dutch elm disease, however, he found that the elm drastically increased production of these odorants in the presence of Ophiostoma. It appeared the fungus was amplifying the tree's natural semiochemical signals in order to attract more beetles.

Ophiostoma solved the transportation problem,” says Gries. “It turns the tree into a lure. Foraging beetles colonize the elm and when they leave, they take the fungus with them.”

After isolating the attractants in the lab, the researchers tested their hypothesis using a series of insect traps in a Saskatchewan elm forest. Traps baited with the semiochemicals snared about 10 times as many beetles as unbaited traps.

For Gries, the field study offered the ultimate proof that his hypothesis was correct. “You need to put your semiochemical out in the field and see the insects respond there - that's where it really counts.”

Gries' findings, published in a recent edition of Proceedings of the Royal Society: Biological Sciences, may help stop the spread of Dutch elm disease. If the mechanism by which Ophiostoma turns up semiochemical production can be unraveled, a management strategy involving suppression of this response could slow beetle traffic around infected trees, putting a stop to the fearsome fungus' steady spread.

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