Sweet potato growers in North Carolina will soon begin the process of planning for their 2012 crop and a big part of that process will be developing a management plan insects — most prominent of these the ever-present wireworm.

North Carolina is the country’s largest sweet potato producing state, totaling nearly half the entire production in the U.S. Despite annual production of 600 million pounds or so, worth more than $175 million, until recently there was no consistent plan for managing wire worms — the leading insect threat to sweet potato production in the state.

Now, thanks to a multi-state research program and the recent addition of Mark Abney to the research and Extension program at North Carolina State University, growers have a uniform program to follow.

Abney worked for a while on a USDA regional sweet potato project to determine which insect pests were causing problems in sweet potatoes. He brought with him to his current position a keen interest in sweet potatoes and extensive knowledge of sweet potato pest management.

Wireworms, which live in the soil and feed on sweet potato roots, are the No.1 insect enemy of sweet potato growers. Left uncontrolled, these insects can cause devastating yield reductions.

Prior to 2006, wireworm management strategies were much the same from North Carolina to Texas. Growers relied almost exclusively on a pre-plant insecticide, most recently Lorsban, followed by up to 12 foliar applications during the growing season. Most North Carolina growers were spraying 4-6 times with a wide array of foliar-applied insecticides, Abney says.

Sometimes, the foliar applications worked, sometimes they didn’t work. The major shortcoming of this long-term management strategy is the treatments didn’t kill wireworms below ground. Adults don’t feed above ground on sweet potato, so the practice was often ineffective.

Nevertheless, growers were compelled to spray because of the high value of the crop and the high probability of wireworm damage.     

Larval stage of click beetle

Wireworms are the subterranean larval stage of click beetles (Coleoptera: Elateridae). Though wireworms feed on corn, sorghum, small grains, tobacco, and various vegetables, they are particularly damaging to potatoes.

These pests can and do feed on seed pieces (on Irish potatoes) early in the season, causing weak, sporadic stands. However, the big damage to both Irish potatoes and sweet potatoes is damage done to maturing tubers and roots that make harvested potatoes less valuable and in many cases unmarketable.

Abney is among a team North Carolina State researchers, who developed a new insect management strategy that saved sweet potato growers an estimated $250 million last year.

The new management strategy also reduces the amount and impact of pesticides released into the environment.

Abney, speaking at the recent North Carolina Vegetable Growers Association annual meeting, says “Prior to 2006, in all the sweet potato growing states in the Southeast, we really didn’t know what the major pests were. We knew we had wireworms, we knew we had several other insect pests, but there was no composite picture as to how many of what insects were in sweet potatoes, nor how much damage these insects were causing.”

Abney was part of the multi-state project that ran from 2004-2007 and was designed to document which insects were major pests of sweet potatoes and begin the process of developing comprehensive IPM programs to best manage these pests.

After three years of taking sweet potatoes from 95 grower fields and analyzing damage in a lab, it was not a surprise that wireworms were our biggest insect problem. More than 20 different growers agreed to leave a portion of these fields untreated; they found 17 percent of the sweet potatoes had wireworm damage. Sweet potato flea beetles were second, causing damage on 11 percent of roots and a combination of white grub, white fringe beetles and other insects made up a smaller percentage of damage.

“This damage was corrected for marketability, so 17 percent of the sweet potatoes we tested over three years were unmarketable due only to wireworm damage. Had we tested just for damage, regardless of whether the sweet potatoes were marketable or not, probably more than 40 percent were damaged,” Abney says.

“So, growers who don’t treat with an insecticide for wireworm control on sweet potatoes should expect 40 percent or more of their crop to be damaged by wireworms and for 17-20 percent of the crop to be unmarketable,” he adds.

“Though we have at least 8 different species of wireworms in sweet potatoes in North Carolina, the tobacco wireworm makes up 85 percent of the wireworm complex. So, if we can manage this one species, we have made good progress in managing wireworms,” Abney says.

However, he adds, corn wireworm, which makes up a relatively small percentage of the total wireworm complex causes tremendous damage — much more so than tobacco wireworms.

Damage can be severe

“In sweet potato fields with high corn wireworm populations damage can be so severe that most of the crop will be unmarketable,” Abney says.

The biology of the two species of wireworms is very different.

The tobacco wireworm has a one-year life cycle. Adults are highly mobile, so populations that are high in a field in one year, don’t necessarily mean populations will be high the next year. Also, these wireworms are not affected by crop rotation — the adult females will lay eggs in most any crop.

Corn wireworm females, by contrast much prefer to lay their eggs in corn. So, these highly damaging, but infrequently occurring wireworms are much more likely to be in sweet potato fields that were in corn the previous year. 

“The catch is that this wireworm species has up to a five-year life cycle, so having corn in the rotation as long as five years back could increase the risk to corn wireworm damage,” Abney says.

Knowing field history is important in determining which species of wireworms are present in a sweet potato field. One doesn’t occur very often, but when it does it causes severe damage. The other occurs more often, but often doesn’t do enough damage to render sweet potatoes unmarketable.

“Regardless of the species of wireworms in a sweet potato field, there is a management strategy than can work and be economically efficient in managing these pests,” Abney says.

“First, know where you are going to plant your sweet potatoes and avoid fields with a history of corn in the rotation in the field. Also, avoid fields with a history of white fringe beetle infestations, because we don’t have a good management strategy to control it,” the North Carolina entomologist adds.

“Second, use a preplant insecticide, regardless of where you plant sweet potatoes. Don’t expect any of the commonly used preplant insecticides to control corn wireworms, but do expect to knock down populations of tobacco wireworms.

“Third, apply preplant materials as near to transplanting as possible.

“Fourth, apply a soil barrier treatment. This is a directed insecticide spray that allows for coverage of as much of the soil as possible, from the top of the hill to the bottom. Then come in with a rolling cultivator and incorporate the material. This treatment targets tobacco wireworm larvae that hatch early in the season.

“Timing is critical for the soil barrier treatments. In some fields split application may help, but it’s all about timing one or two applications when adults are emerging and laying eggs. In North Carolina, this is historically between June 20 and July 7. If insecticides are applied late, the newly hatched wireworms will be down in the soil where insecticides cannot reach them.

“Finally, be careful to scout fields at least 10 days before sweet potatoes are picked and spray if needed to kill foliar feeding caterpillars.”

This is critical, Abney says, because if these insects are in the field at harvest and make it into the bins in a sweet potato house they will continue to feed and damage the roots. “Once they get in with harvested sweet potatoes, there is no viable way to kill them,” he adds.

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