What is in this article?:
• 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, sweet potato growers have a uniform wireworm control program to follow.
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.