Management breakthrough saves time and money

PHOTOS COURTESY OF NORTH CAROLINA STATE UNIVERSITY.
George Kennedy evaluates sweet potato roots for insect damage.

Though soil insects are still a concern in the southeastern part of the United States, sweet potato growers and university scientists have fought hard to better manage these pests.

The fight started in 2004 when plant pathologists, entomologists, horticulturalists, researchers and cooperative extensions from North Carolina State University (NCSU), Auburn University, Louisiana State University and Mississippi State University began collaborating on the Southern Sweetpotato Integrated Pest Management (IPM) Project.

Mark Abney, left, and George Kennedy scout for insects in a sweet potato test plot.

Those four states are producing 80 percent of the sweet potatoes in the United States, with North Carolina being the largest, producing about 40 percent of the crop’s annual yield of 1.6 billion pounds. About 40,000 North Carolina acres are devoted to the crop, which is worth more than $85 million annually. In the state, most of the crop is grown in the counties of Nash, Wilson, Johnson, Sampson, Duplin, Columbus, Lenoir, Greene, Edgecombe and Wayne.

The goal of the four-year project was to address the ineffectiveness of growers to manage soil insects, ones that feed on a plant’s roots, and to better manage post-harvest and post-packing diseases that cause rot in sweet potatoes.

The project was funded by Gerber Products Co. and the U.S. Department of Agriculture (USDA) under the Cooperative State Research, Education and Extension Service Risk Avoidance and Mitigation Program.

Basic assumption

Starting out, the scientists assumed that the basic problems were the same in the sweet potato-producing regions in the southeast. “Everyone was trying to manage the problems in the same way, and it wasn’t working,” says George Kennedy, a William Neal Reynolds professor and head of the department of entomology at NCSU.

Realizing the pests were getting the upper hand, Kennedy and the other scientists stepped back and re-evaluated how to better understand the pest control systems in place. They wanted to review the species of soil insects involved; what production practices growers were implementing; and how they could adjust them to improve control.

“We were trying to identify factors that contributed to risk, that you were going to sustain damage and then modify those,” Kennedy says. “We were trying to evaluate the various approaches, understand the biology and ecology of the different pest species, and come up with a management system that would reflect those various components.”

Field experimentation

The project involved research conducted on farms and at university experiment stations, with the majority of it done on commercial operations. Growers gave the scientists unlimited access to their fields and agreed to set aside eight to 24 rows about 300 to 400 feet long. One plot was for an untreated check, so they could compare this with a treated plot. On the treated plot, growers applied insecticides based on their normal production practices. On a third plot, the scientists implemented experimental practices.

In North Carolina, the project was done on 30 fields for four years. In each of the other states, it was conducted on 5 to 15 fields for four years. About 15 total growers participated in the study from Mississippi, Alabama and Louisiana.

“One of the first things we found was that problems were different in the different states,” Kennedy says.

In North Carolina, the primary soil insect problem on sweet potatoes was wireworms. In Louisiana, the major concern was white grubs and rootworms. In Mississippi and Alabama, the problem was wireworms, white grubs and rootworms. Growers were sharing information and applying pest management practices based on what others said.

“The approach they had been using everywhere was largely to put a soil insecticide down prior to planting,” Kennedy says, “and then spray the foliage to try to kill adult soil insects before they could lay eggs. They laid their eggs in the soil, and larvae would feed on the developing roots.

“The idea was to kill the adult before it could lay eggs,” he continues, “but as we began to look at that we found it had absolutely no effect.”

Corn wireworm damage is evident on this sweet potato and can ruin the sale on the market.
Soil-barrier insecticide treatments have little effect on the corn wireworm on sweet potatoes, says George Kennedy, head of the department of entomology at North Carolina State University.

The scientists began studying the biology of the insect and determined that eight species of wireworms were present in North Carolina sweet potatoes, with two species of major concern: the tobacco wireworm and the corn wireworm. Growers had been applying three to 12 foliar sprays of organophosphates in North Carolina on a weekly basis. The cost was high with minimal results, suggests Mark Abney, NCSU assistant professor of entomology.

The tobacco wireworm has a one-year life cycle. The adults lay their eggs from late June through July, and the eggs hatch in July.

However, most growers were applying foliar sprays late in the season, because they thought the most damage occurred then. “They felt frustrated with the results they were getting,” says Will Connell, a crop consultant with Agricultural Consultant, Inc. of Stokes, N.C.

Kennedy and Abney realized the growers were spraying too late, so they developed a procedure that applies a bifenthrin soil inecticide product such as Capture, which directs the product into the soil surface while the growers are cultivating with rolling cultivators. “You had the insecticide incorporated into the top 1.5 to 2 inches of soil when the adults laid their eggs, and when the larvae hatched in that insecticide barrier it killed them,” Kennedy says. “That one application soil-directed and immediately incorporated was far more effective than anything we had been doing so far,” Kennedy stresses. “Combined with the preplant application soil insecticide, you’ve got pretty acceptable control.”

Abney adds, “With an untreated field compared to a field that just has a preplant soil insecticide, you can expect damage to be reduced by about half. If you add a soil-barrier treatment to that, you’ll cut that in half again.”

Kennedy points out growers will not reduce damage to zero, but they will lower it to a tolerable level. At the same time, it will cost fewer dollars and have less impact on the environment, because they are spraying insecticides less often.

While the soil insecticides aren’t cheap, Connell says his growers were able to shift the way they applied the materials, using fewer pounds of insecticides per acre, and found the applications to be more effective.

Growers also are running the equipment over the land less often, since they are applying the soil-barrier insecticide at cultivation rather than in a separate application or more, Abney says.

Sweet potato grower Jim Jones of Bailey, N.C., says he reduced his chemical application costs by at least $150 per acre as a result of the Southern Sweetpotato IPM Project. Jones, who raises 750 to 800 acres of sweet potatoes, used to spray eight to 10 times with a foliar spray.

Now, he applies the soil insecticide, Capture, twice and says those two applications work better than the several he has applied before.

Corn wireworm

Abney and Kennedy learned that the corn wireworm was influenced by crop rotation, especially if sweet potatoes were planted directly behind corn or soybeans.

Kennedy says insecticides are not effective against the corn wireworm, so avoiding the planting of sweet potatoes two years after planting corn and soybeans will greatly reduce the risk of damage, because the insect larvae can survive in the soil for as much as three years.

Abney and Kennedy found that soil-barrier treatments applied during cultivation to control tobacco wireworms did not have any effect on corn wireworms.

Post-harvest diseases

The Southern Sweetpotato IPM Project also involved the study of post-harvest handling diseases. This research was done primarily in North Carolina and Louisiana under the direction of plant pathology professors Gerald Holmes and Chris Clark, respectively.

“One of the key findings that they came up with was that harvest handling and post-harvest handling dramatically influenced susceptibility to diseases,” Kennedy says.

To come to this conclusion, the scientists studied the sweet potatoes going through packing lines at packinghouses. They took a wireless mechanical device—a SmartSpud, which is shaped like a sweet potato—and ran it through the line. The device measured the impacts received and sent a radio signal to a computer read by the scientists. When they saw high impacts, they searched for ways to reduce them. They also compared the rough handling to laboratory studies so they could define the impacts compared with dropping a sweet potato. That is important because “post-harvest rots oftentimes don’t show up until after the sweet potatoes are packed, so you’ve got all your production costs, all your storage costs, and then you have a rot problem after it’s packed,” Kennedy says.

Abney adds that shipping sweet potatoes with rot or rots across the country or overseas will greatly hurt that sale and future sales to customers. Some common rots in sweet potatoes include bacterial soft rot, fusarium root rot, fusarium surface rot, rhizopus soft rot and scurf.

Treatment

The standard treatment for rot disease was to apply a dicloran fungicide such as Botran. That worked, but Botran is being targeted by the Environmental Protection Agency as one of the chemicals for elimination.

The challenge for scientists was to identify alternatives for rot control. They tested several and came up with two possible choices. Holmes indicates that they were able to identify the fungicide Scholar as a potential alternative. Scholar gives similar control to Botran and with reduced-risk chemistry. Another product identified was the biological control agent BioSave. It offers fair to poor biocontrol for chemical-free markets, Holmes says, but the company is working to improve efficacy.

Bottom line

In North Carolina, the Southern Sweet-potato IPM Project changed the soil insect-management approach of growers, eliminated unnecessary foliar sprays, improved the level of pest control and reduced growers’ pesticide costs. The project delivered new pest management recommendations that reduced the need for high-risk insecticides targeted under the U.S. Food Quality Protection Act. In addition, the project identified ways to reduce damage on packing lines.

“Virtually all the industry has adopted the insect management practices,” Kennedy says. “One can say the cost of risk management has been reduced. It basically eliminated foliar sprays for soil insects and replaced them with a more modern product, which reduced potential for exposure.”

When the project started, Abney says growers were applying an average of three foliar insecticide applications. By 2006, that number was down to an average of one foliar spray per year.

Before the project started, Kennedy and Abney surveyed growers on their foliar insecticide use. In the fourth year, they surveyed them again, and the growers indicated that they reduced the use of three common foliar insecticides—a phosmet product such as Imidan (by 42 percent), an endosulfan product such as Thiodan (50 percent) and a carbaryl product such as Sevin (73 percent).

“I did some calculations just to see what that might mean in terms of dollar values and figured about a quarter of a million dollars a year just in the cost of those materials, not trying to figure in other application costs,” Abney estimates.

Rocky Womack has written about agriculture and business for more than 25 years and currently serves as a contributing writer and correspondent for agriculture and business magazines, domestically and internationally. In the past, he has worked as a magazine editor and daily newspaper writer. Womack has won numerous awards for his interviewing, writing and in-depth reporting.