Invasive pests threaten sweet potato crop in southeastern North Carolina

George Wooten III of Columbus County, N.C., says he and his father have moved their sweet potato production to land farther north in Columbus County, N.C., to avoid the damage caused by white grubs in years past.
Photo by Rocky Womack.

The sweet potato industry in North Carolina is reeling from the devastation that white grubs can possibly cause in the future. Their numbers appear to be growing and may spread farther into regions of the state with high sweet potato production.

In the fall of 2006, George Kennedy, a William Neal Reynolds professor and head of the Department of Entomology at North Carolina State University (NCSU), and Mark Abney, an NCSU assistant professor of entomology, began hearing about sweet potato damage in the southeastern part of the state. They received a call from a grower in Columbus County saying he had significant infestation to his crop. Abney and Kennedy traveled down to the grower’s farm and met the extension agent. What they saw was devastating.

“There were some potatoes there, but the damage was just unbelievable, I mean completely covered with feeding scars. He had 500 acres of potatoes that were like that. I can’t remember what he estimated the loss at, but it was in the hundreds of thousands of dollars just in that year,” Abney says.

The adult white grub beetle emerges from the ground at dusk and looks to mate before shortly returning to the ground at night.
Photo by Rocky Womack.

George Wooten III says his sweet potatoes were damaged in 2006, 2007 and 2008. Wooten, who farms with his father, George Wooten Jr., estimates that the grubs caused an average of 50 to 60 percent damage, with some fields suffering 100 percent damage. The Wootens raise about 1,200 acres of sweet potatoes.

The new species of white grubs was a consistent problem in 2006, 2007 and 2008. In 2007, Kennedy says it flexed its numbers and caused a huge amount of damage.

If the numbers and damage continue at that pace, “growers aren’t sure how long they’re going to be able to grow potatoes there if something’s not done about it, if they can’t come up with a way to manage it,” Abney says.

Species

Abney says the species of white grub, Plectris aliena, was first identified by an expert in North Carolina in 2006, but its roots might have been here before then. “When the growers think about it, they say, ‘Well, we probably had this for at least two or three years for sure before 2006,’ but the levels of damage were low enough that they didn’t really think of it as being a problem that was different than just the regular low levels of grub damage they had always seen. But 2006 was really when it broke loose,” he says.

Wooten believes he probably had a little damage as far back as 2004 and continued to see more in 2005 and beyond. “In 2006, we didn’t know what was going on,” he says. “We thought that we didn’t use something in our insecticide program.”

Plectris aliena is fairly new to the state, but not necessarily to the United States. Abney says it is native to South America; however, it was introduced into South Carolina around the turn of the 20th century, so it was introduced here somewhere in the early 1900s.

The Plectris aliena species of white grub feeds on sweet potatoes.
Photo courtesy of Mark Abney.

So why has it taken this long for these pests to spread farther into North Carolina and become more active? Abney says he and Kennedy don’t know why, but one thing is for certain. “It’s been a hundred years getting here, and then it’s gone from relative obscurity to it could cause the end of sweet potato production in Columbus County over the course of just a couple of years. It’s pretty dramatic,” Abney says.

Columbus County, which is near the South Carolina line, is actually isolated from the majority of the sweet potato production area. They are also grown in Bladen County, but the majority of the crop is raised in the North Carolina counties of Nash, Wilson, Johnston, Wayne, Sampson and Duplin. Abney says the state has about 40,000 acres of sweet potatoes in production, and only about 5,000 acres are grown in Columbus County.

Research

Because of the increase in numbers and damage, Abney started some extensive research in a special project. In 2007, he and Kennedy started an efficacy trial. They planned to test anything that was registered in sweet potatoes, which had grub activity based on other information that they had on record. They looked at several different insecticides and rate combinations in about 10 treatments.

Entomologists Mark Abney and George Kennedy set up traps to catch and track adult white grub beetle numbers.
White grubs damage the roots and produce scars from feeding on sweet potatoes.
Photo courtesy of Mark Abney.
The sweet potato vines, raised by George Wooten III and his father George Wooten Jr., shine while receiving a much-needed shower in May 2010. The Wootens moved their sweet potatoes to land farther north so they could hopefully avoid the damage caused by white grubs.
Photo by Rocky Womack.

“Our best treatment had over 75 percent damage,” Abney says, “and that was applying preplant and coming back in and applying insecticides into the soil after planting. They were totally doing nothing. That’s the bottom line. Even if it was 50 percent, that’s not acceptable. There’s no way they can sustain production down there [Columbus County] at 50 percent.”

The adult beetle looks like any insect flying around a porch light at night, but Abney says it is not attracted to light. He says the biology of the insect is amazing. The adults don’t eat anything. They feed at the grub stage, on top of the surface and right up to sweet potato harvest.

Kennedy says because the adults don’t feed, they are difficult to control since they “don’t present much of a target.”

In late May, the adults emerge from the soil to mate for about 15 minutes. This is done at dusk. They fly around, mate, then go back underground and don’t emerge again until the next night. These adults, which are bigger than a ladybug, do this for slightly more than a month. They peak about June 1, then the numbers start to tail off through the end of June.

Based on their research, Abney and Kennedy believe the insect has at least a one-year life cycle, and perhaps a two-year cycle. Abney says they lay eggs starting in May, probably going into July, and the eggs hatch in late June and into July.

Abney and Kennedy also set out traps in Columbus County to track the numbers of adult beetles. In the first week of June, the flight intercept traps were catching about

300 male and female adults. By the end of June, the numbers decreased.

When the beetles fly around from ground level to knee-high and sometimes waist-high, thousands can be seen in a field. Abney says the sheer numbers are almost unbelievable.

“Then as quickly as they come out, they’re gone again, and there’s not a trace of them anywhere. They just go right back into the ground, and you won’t see another one until the following evening,” he says.

Slight damage decrease

In Abney’s research plots in 2009, the white grub damage was significantly less than in previous years. He’s not sure why, so it will require more research.

Wooten says he also saw less damage in 2009, of about 20 to 25 percent. Unfortunately, he planted fewer sweet potatoes because of damage in previous years.

In 2010, the Wootens went back to the same acreage of sweet potatoes as they had in 2007 and 2008, but planted them on land that is farther north, away from the heavier white grub populations.

Based in Danville, Va., Rocky Womack has been writing for more than 25 years and is a contributing writer for numerous national and international publications.