The Seaside Nematode: Natural Born Killer
Here's a million dollar question for the TV show "Who Wants to Be a Millionaire?" What city in Oregon gave Heterorhabditus marelatus its name?
Give up? The answer is Seaside, where Jiu Liu, an entomology research associate, discovered a new species of nematode in a grassy area above the beach in 1995. The name comes from the Latin mare, for sea, and latus, for side.
The discovery isn't likely to replace the city's reputation as a beach resort and spring break destination for rowdy high school students. Indeed, it passed largely unnoticed except among nematologists-scientists who specialize in the study of nematodes.
Nematodes are the most numerous multicellular animals on earth. There are about 30,000 species of these simple worms that are so small you need a microscope to see them. Approximately 50 percent of the known nematode species are found in the ocean. Another 25 percent are free-living species found in soil or freshwater. Ten percent are parasites of plants.
The remaining 15 percent are parasites of animals. Dog heartworms are nematodes. About 50 species of nematodes attack humans, and an estimated 98 percent of the human population has had a nematode infection at one time or another. Pinworms are a common nematode infection in humans. Another nematode, the trichina, is the source of trichinosis, a sometimes fatal infection caused by eating raw or undercooked pork or wild game.
Although he is no longer at OSU, at the time of the Seaside nematode discovery Liu was a member of a team of researchers led by Ralph Berry, an OSU entomologist. "We were working with hundreds of nematodes from all over the world at the time. Dr. Liu's research showed that Heterorhabditus marelatus was a separate species," Berry said.
"What's the big deal about one more species of nematode among 30,000?" you might ask. For a scientist confronting a new species, there are many questions to answer. How does its life cycle compare to other species of nematode? How is its behavior similar to or different from other species? Is it harmful to plants or animals? Or could it have practical applications that would benefit humans in some way?
"We suspected it would be beneficial because the foredune area where Dr. Liu collected the specimens is a sandy, moist, cool habitat where beneficial nematodes are often found around the world.
That being said, however, there are a lot more bad nematodes that cause damage to crops than beneficial ones," Berry noted.
His first assumption was right. The Seaside nematode turned out to be what entomologists call an entomopathogenic nematode. In simple terms, an insect killer.
Because root weevils are found in soil conditions similar to those where Liu found the Seaside nematode, they were among the first insect pests Berry tested with the nematode. He found that the Seaside nematode attacked the larval stage of the root weevil and did so efficiently.
"I call it nature's natural weevil killer," Berry said, adding that the Seaside nematode has great potential to biologically control root weevils in strawberries, cranberries, mint and ornamental plants such as rhododendrons and azaleas.
Many Oregon home landscapers are familiar with the unsightly notches left by root weevils on rhododendron leaves. For commercial agriculture, the stakes are much higher. Each year weevils cause an estimated $5.4 million in losses to Oregon strawberry growers, $1 million to mint farmers. In Oregon's $554 million nursery industry, root weevils not only kill plants, they also feed on the leaves, deforming the plants and making them less appealing to buyers. Moreover, plants infested with weevils may cause interstate shipments of nursery stock to be quarantined.
For Oregon's nursery industry, concentrated in the heavily populated northern Willamette Valley where pesticides are subject to ever more restrictive Environmental Protection Agency regulation, the availability of an organic pest control method would be welcome news.
"Because the Seaside nematode is a completely natural insect killer, its use does not come under EPA pesticide regulations," Berry said.
For Berry, the first step was to find out how the Seaside nematode attacks and kills root weevils. The key, he found, was a bacterium that lives in a chamber inside the nematode. The nematode and the bacteria have a symbiotic relationship that benefits both of them--one cannot live without the other.
In essence, according to Berry, the nematode acts as a vector. Like the anopheles mosquito that transmits the malaria parasite from one human to another, the nematode carries a bacteria containing a toxin from one target host-in this case, the root weevil-to another. During the only free-living stage of its complex life cycle, the nematode enters the weevil and releases its bacteria. The bacteria toxin kills the weevil and the nematode continues to feed on the larval tissue and the developing bacteria.
Inside the weevil larvae, the nematode goes through at least two generations. The original invading nematode reaches adulthood and lays eggs that produce another generation of nematodes inside the dead weevil. Eventually a generation of nematodes escapes from the dead weevil and finds another root weevil to invade. It takes about two weeks for the Seaside nematode to complete its life cycle.
Using parasitic nematodes to control insects in agricultural crops and home landscapes is not new. But each nematode species has specific attributes that contribute to its success on different insects. Some nematode species stay in the upper level of the soil and ambush target insects as they pass by. Other species of nematode display what Berry calls "cruiser behavior" and actively search for target insects deeper in the soil.
"Because root weevil larvae are found below the surface, nematodes that display the cruiser behavior are more effective against them. The Seaside nematode is a cruiser," Berry said.
Soil temperature plays a key role in the effectiveness of nematodes for control of insects. The Seaside nematode turned out to be well adapted to cooler temperatures. Berry's research found it controlled strawberry root weevil at temperatures as low as 50 F to 60 F. "This provides an opportunity to use H. marelatus earlier in the spring when soils are colder and before plant damage has occurred. It gives Northwest nursery growers a temperature window to control root weevils that they didn't have before," Berry said.
Ideally, insect control is carried out before insect populations reach damaging levels to avoid needless expense and waste of pesticides. But there is no practical way to sample for root weevils without damaging or destroying the plants. So the Seaside nematode may be a way to protect against root weevils rather than eliminating them once they are present, just like flu shots protect people from Asian flu rather than postponing medical treatment until the sniffles and sore throat arrive.
A disadvantage: You can't recycle the nematodes. Because they don't survive in the soil after they have killed all the weevils, they have to be applied every year. "It costs about $300 an acre to treat plants with H. marelatus," said Tom Peerbolt of Peerbolt Crop Management, an agricultural consulting business. "So even though the nematode has the advantage of working at temperatures below 50 F, the cost must be brought down."
Another obstacle to widespread use of the Seaside nematode by commercial growers is the lack of a reliable supply of nematodes. Effective control of root weevils requires 1 to 2 billion infective juveniles per acre, according to Berry. In terms of nursery plant containers, that translates to 160 to 320 nematodes per square inch.
The responsibility for perfecting a way to mass produce the huge quantities of nematodes necessary for commercial use is in the hands of Pete Gothro, a research assistant on Berry's team. Working in a tiny room in Cordley Hall on the OSU campus, Gothro uses a fermenter to culture the bacteria that lives in the Seaside nematode. "The bacteria is the key to everything. You keep the nematode happy by providing it plenty of bacteria to feed on," said Gothro.
Making bacteria, he says, is much like making a good microbrew beer. "You put in the ingredients, then adjust conditions to promote growth of the bacteria." When the bacteria is ready, nematodes are placed in it. If all goes well, they reproduce in large numbers.
But Gothro discovered that the complex mating behavior of the Seaside nematode makes it difficult to reproduce in straight bacteria. He describes H. marelatus as being "persnickety" about the bacteria. "I'm still trying to create conditions conducive to mating," he said.
The ultimate aim is to develop a reliable method for producing mass quantities of the Seaside nematode for use in the Pacific Northwest. Unfortunately, perfecting such a method is not the final roadblock to bringing the Seaside nematode to market, according to Tom Hinks, an executive with MicroBio, a company that supplies beneficial nematodes to the agricultural industry.
"It would be difficult for a company to survive on a regional niche market for H. marelatus because it is limited to, roughly, a six-week sales period each year," Hinks said.
Two other members of Berry's research team are investigating other aspects of the nematode puzzle. Tae-Hwan Kim, a South Korean graduate student, is researching the bacteria found in other species of beneficial nematodes. Most of these species are found in subtropical regions. He wants to switch bacteria among the species to study them for Oregon's climate and growing conditions.
Christine Armer, a graduate student from California, has found that the Seaside nematode is a highly effective biological control against the Colorado potato beetle, which causes serious damage to potato crops in the Pacific Northwest. But the nematode doesn't reproduce inside the body of the dead beetle as it does in root weevils, so it can't seek out other beetles after its initial killing foray.
Armer suspects that the Colorado potato beetle takes in a toxin from the potato that prevents the Seaside nematode from reproducing. If her research confirms her suspicion, she will seek ways to reduce the level of toxin in the potato.
While Berry's research team has revealed the life cycle and behavior of the Seaside nematode as well as its potential use as a pesticide, a number of practical considerations remain to be solved before it joins the grower's arsenal against insect pests. For instance, an economical method of mass production needs to be perfected. And more work needs to be done on application methods as well as other potential target insects.
While scientists continue studying nematodes to understand more about these most numerous multicellular animals on earth, you can keep up with the latest developments online. Here are some Web sites you can check out that offer an exhaustive amount of nematode information: