Nematodes Vs. Slugs – To the Victor Goes the Soil

The name Phasmarhabditis hermaphrodita means literally “non-binary ghost rod,” but this nematode is alive and well and busy turning slimy slugs into just slime. The question is, can these nematodes be turned into money? At Oregon State University, Professor Dee Denver and Associate Professor Rory McDonnell are finding out.  

Slugs cause billions of dollars’ worth of crop damage worldwide, eating crops and contaminating them with their slime trails and their feces, which lowers the quality of foods grown and can carry diseases. Because of all of this, a method of slug control that’s more reliable, cheaper, and causes less environmental damage matters a lot.  

In Oregon, slugs eat some of the most valuable crops, including grass seed and nursery plants. That’s why Denver, head of OSU’s Department of Integrated Biology and nematode specialist, and McDonnell, who studies crops and soils at the university’s College of Agricultural Sciences, are co-operating on a study of ways to eliminate the slug problem 

“The damage they cause is a massive issue,” McDonnell told Oregon State University 

Newsroom. “We surveyed 200 growers throughout the Willamette Valley to gather their opinion of controlling slugs with pesticide. We found only 30% said they were happy with the performance of chemicals.” 

hermaphroditahas been sold in Europe as a non-toxic pesticide for decades under the brand name “Nemaslug,” but a life form which is safe and effective in one ecosystem can be useless or harmful in another. Such as the damage caused by careless use of biocontrol measures like the cane toads which were supposed to protect Australia’s sugarcane but instead devastated Australian wildlife. Closer to home, the gray field slug Deroceras reticulatum, which Denver and McDonnell hope the nematodes will discourage, is itself an invasive species, accidentally brought to North America as eggs in the soil surrounding potted plants.  

The U.S. Environmental Protection Agency wants to know more before approving this nematode for sale here.  

As Dr. McDonnell put it, “The thought process is that if it works in Europe and we find it here and it works here, it might be easier to get it registered by the EPA. If we can provide evidence it’s native, that makes a strong case for developing it as a bio-control. But we want to make sure there are no effects on native slugs or snails. We don’t want bio-control gone awry. That’s very, very important.” 

There are nematodes teeming in almost every patch of soil in the country, but it took McDonnell three years of searching to find P. hermaphrodita and prove that it was already present in North America, outside a region in California where they had already been found. In 2014, he found it – in Corvallis, on the OSU campus. Once he’d found it, though, he had to prove that it really was the nematode he was looking for and that the tiny wrigglers he’d found were not some phantasmal Phasmarhabditis.  

That was where Denver came in. An expert at distinguishing between nematode species, she studied McDonnell’s specimens from the outside, and then from the inside, sequencing their DNA and comparing it with DNA sequences in a national repository of plant and animal DNA. This information not only served to confirm that P. hermaphrodita existed outside its previously known range, it might eventually lead to the discovery of other nematodes with similar slug-control powers which could be used in places where P. hermaphrodita doesn’t exist. 

The power of the nematodes to destroy slugs is immense – and a bit disgusting. 

“When a slug is infested with nematodes, it liquifies,” Denver said to OSU Newsroom. “You end up with a swarming pile of worms. It’s pretty gruesome. Nematodes are abundant and diverse – there are millions of them in every aspect of the earth’s biosphere. They are really understudied and, with Rory’s lab as one of a very few in North America devoted to slugs and snails, we’re in a good position to do this research.” 

A full report on Denver and McDonnell’s research can be found in the journals PLoS and Biological Control. 

By: John M. Burt