Nematode squishes grape root borer


WOOSTER, Ohio – Two nematode species, one a native of Ohio, have been found to successfully control grape root borer, an insect pest responsible for major economic losses to the grape industry.

In fact, the nematodes are such effective biological controls that a product may be available for grower use as early as next year.

Nematodes are microscopic worms found abundantly in the soil that are pathogenic against many insect and mollusk agricultural pests.

“Farmers really have no control methods for the grape root borer larva and that is the most destructive phase of the insect,” said Parwinder Grewal, an entomologist with the Ohio Agricultural Research and Development Center. “We are very excited that there is a potential biological control.”

Lab, field success. Grewal’s research found that, in lab studies, the Ohio nematode (Heterohabditis bacteriophora) and a New Zealand strain (Heterohabditis zealandica) produced 92 percent and 86 percent control of grape root borer, respectively.

The researchers then duplicated field situations in the greenhouse and found that the Ohio species produced 16 percent control upon applications of 15,000 juveniles per grape plant and the New Zealand species produced as high as 93 percent control when 60,000 juveniles were applied per grape plant.

Grewal speculates that the higher the number of nematodes per application, the more effective the control.

“We took the New Zealand strain into the field and applied 2 billion nematodes per acre and tests showed over 70 percent control of the insect in a single application,” said Grewal. “Nematodes are easy to come by and recycle very easily. A single host can produce 300,000 to 400,000 new nematodes, which can then move on to the next victim. So control would continue for quite some time after the first application.”

Cruisin’ for food. The beauty behind the species’ effectiveness lies in their foraging behavior.

The nematodes are known as “cruisers,” named for their behavior of actively seeking out their hosts by following chemical trails the insects leave behind. Once the nematodes find their hosts, they enter the blood stream through a variety of natural body openings and release bacteria that multiply and kill the host within three to four days.

The nematodes then feed on the dead host, reproduce and migrate in search of additional hosts.

“Over the last 20 years or so, scientists have tried to control the grape root borer with nematodes, but have done so unsuccessfully. We then began discovering that different species have different kinds of foraging behavior, so one nematode species may not be as effective in controlling the pest as another,” said Grewal.

For example, “cruisers” exhibit different forage behaviors than “ambushers,” species that sit on the soil surface and wait for highly mobile hosts to pass by.

“An ‘ambusher’ released to kill an insect like the grape root borer which tunnels its way into vine roots and remains hidden is just a poor match,” said Grewal.

The researchers reevaluated 17 nematode species with “cruiser” behavior and found that the Ohio and New Zealand strains were the most effective against the grape root borer. An added bonus is that the New Zealand species has been found in Florida, eliminating any restrictions required to commercialize the species.

Devastating to crop. The grape root borer is a major pest of grapes in the eastern United States. Found in states south of Connecticut and east of Kansas, including Ohio, a single larva can reduce yield by 50 percent by feeding on the root system.

Two or three larvae within a root system can destroy an entire vine, affecting winter hardiness, fruit quality and yield.

Effective control of the grape root borer is difficult. The adults, which fly onto plants beginning in June, can be controlled with insecticide applications. However, once the larvae hatch and reach the root system, external control is ineffective.

This is where the success of entomopathogenic nematodes comes in handy, said Grewal.

“Nematodes are already used in some food production, such as mushrooms and cranberries. Florida applies nematodes to 50,000 acres of oranges each year to help control the citrus root weevil,” said Grewal.

“The same thing can be applied to grapes. Grapes provide the next potentially big opportunity for the use of nematodes. They can replace systemic insecticides and increase food safety.”

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