The white grub, Plectris aliena, was first documented near Charleston in the 1930s and has caused few problems in South Carolina, but in a small area in North Carolina, it has caused severe damage to sweet potatoes and keeping it confined to that area has been an ongoing challenge for the past few years.

Sweet potatoes have become a superstar among health-conscious Americans and for good reason. Consumption of sweet potatoes has been linked to reduced heart disease, and to contributing to improved immunity to a number of other disease causing pathogens.

And, its nutritional value ranks among the top crops grown anywhere in the world.

Unfortunately, sweet potatoes are not marketed based on their nutritional or medicinal value, instead they are still marketed primarily on how they look.

White grubs, or plectris grubs, make sweet potatoes look bad, very bad.

Though the burrowing damage, caused by feeding of these insects on sweet potatoes, doesn’t reduce nutritional or health value of the crop, it can virtually destroy its marketability.

When plectris grub first showed up in Columbus County, N.C., it made an immediate impact, and not a good one, on growers there. The more immediate concern was to figure out how to prevent the insects from moving into even more highly intensive sweet potato growing areas of the state.

“When George Wooten found these insects in his sweet potatoes in 2006, that was the first time we knew we had a problem, or even that the pest was present in North Carolina, says North Carolina State University Entomologist Mark Abney.

“Since then we have had near catastrophic damage on sweet potatoes in some fields,” he says.

Since it was first documented on sweet potatoes in North Carolina this pest has caused an estimated $16.5 million in losses, or about one-tenth the annual value of sweet potatoes in the state.

Despite intense research efforts, there remains no definitive chemical control for these pests.

In recent research, plectris grubs have been found in corn, soybean and peanut fields, creating significant concern but so far no real damage.

Only cotton a non-host

Among the major crops grown in the region, only cotton appears to be a non-host for these insects. The biggest economic impact has been felt by sweet potato growers.

These insects are amazing in the way they damage potatoes, Abney says. They feed on the surface, making large, ugly gouges and craters on the sweet potato. In too many cases affected potatoes aren’t even marketable as processing potatoes.

In the processing plant there is no way to stop and cut out the affected part of the potato. Though the potato doesn’t die from the damage, in most cases the gouges and craters on the surface make them unmarketable.

North Carolina is the No. 1 sweet potato growing state in the country and currently produces more than 40 percent of the U.S. crop.

Though it has been found in only a small part of the North Carolina sweet potato growing area, it has created concern across the entire U.S. sweet potato industry.

Abney has been at the forefront of containing white grub damage and has worked in sweet potatoes for most of his professional life.

After looking at numerous insecticide options, he says it looks like a two-pronged attack with insecticides and natural predators of the insect may offer hope in containing and managing plectris grubs.

Abney uses the term ‘entomopathogens’ to describe one biological approach he and his research team has undertaken to stem the movement of these white grubs.

One of his graduate students, Amber Arrington, has been using entomopathogens in combination with the most effective insecticide family used to combat white grubs.

“Using traditional organosphosphate insecticides has proven to be mostly futile in managing white grubs,” she says.

“By switching to neonicotinoids, a relatively new family of insecticides, control was better, but we were still seeing 50 percent damage in some fields,” she adds.

Control of plectris grubs with neonicotinoids continues to show promise, but damage is too high to prevent economic losses in sweet potatoes.

Previous research demonstrated some synergistic activity between neonicotinoids and pathogenic fungi and nematodes when used in combination to kill white grubs.

Paralyze target insect

The mode of action of neonicotinoid insecticides is to essentially paralyze the target insect. Plectris grubs, as do most white grubs, go through a grooming process which eliminates many naturally occurring fungal and nematode pathogens.

More of these pathogens are eliminated as grubs move through the soil.

By slowing the movement of plectris grubs, neonicotinoid insecticides give biological pathogens a chance to attach to these insects where they penetrate the body eventually causing death.   

In her research Arrington used both naturally occurring pathogenic soil fungi and nematodes in treatments with and without neonicotinoids, each alone, and the combination of the fungi and nematodes.

The tests were done in Columbus County, in the heart of plectris grub damage to sweet potatoes. Insect levels were extremely high, and in non-treated plots, damage to sweet potatoes was as high as 80 percent.    

For her tests, Arrington found a nematode species that actively moves in the soil and actively seeks a host. This particular nematode was applied in a soil drench in July, along with a neonicotinoid insecticide.

The number of roots damaged by plectris grubs did not differ significantly from one treatment to another.

However, there was a significant impact on the severity of damage when a combination of a neonicotinoid insecticide, in this case Admire, and pathogenic nematodes were applied.

“The results showed that early-season application of nematodes and insecticide worked better than when the products were applied later. This may be because younger grubs are more susceptible to the treatments than older grubs,” she says.

“We know the adults of this species don’t feed at all, and we documented that the biological parasites would only infect third instar grubs.

“So, we know growers won’t need to apply these nematodes late in the season and it is likely one application per season is adequate to reduce damage caused to sweet potatoes by plectris grubs,” she says.

“This is just one test in one year, so it’s hard to make sweeping conclusions, but it is clear that some combination of a neonicotinoid insecticide and pathogenic nematodes can significantly reduce damage caused to sweet potatoes by plectris grubs, she adds.

“Obviously, we need more than one year of data from one site. Things like how environmental conditions affect the movements of these biological pathogens through the soil and the persistence of these pathogens within the soil will be critical to refining their use in combined management practice with neonicotinoid insecticides.”

rroberson@farmpress.com