Wireworms a nemesis for potato growers

Most Southeastern potato farmers, both sweet and Irish, contend a wireworm is a wireworm and the only good one is a dead one.

Back in the halcyon days of multiple insecticidal hammers, soil fumigants and numerous other management systems, that may have been okay, though still probably not economically the best route to take.

With a considerable amount of price instability and ever-increasing input costs, today’s potato farmer in the Southeast simply must manage wireworms in order to survive.

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 early in the season, causing weak, sporadic stands. However, the big damage to both Irish potatoes and sweetpotatoes is damage done to maturing tubers and roots that make harvested potatoes less valuable and in many cases unmarketable.

Unlike most weed, disease and insect pests of agricultural crops, wireworms are nearly impossible to eradicate — once you got ‘em, you got ‘em — as long as there is a crop or native host plant for food.

Wireworms often spend multiple years in the soil and a complete life cycle from egg to adult may take 1 to 5 years depending on environmental conditions and wireworm species. The insects remain in the soil for the entire larval period progressing through multiple developmental stages.

Fields with a history of high wireworm densities will tend to maintain those populations over many years. Since grasses are excellent hosts for wireworms, fields recently cleared, previously in sod or pasture, or planted with grass cover crops are more prone to high wireworm densities. Such fields should be avoided for potato production if there is a low tolerance for wireworm injury on tubers. Wireworms are often more abundant in low-lying areas and in fields high in organic matter.

The tools for managing these subterranean pests are ever dwindling. In the 1960s, 1970s, and 1980s, cyclodiene insecticides provided effective control of wireworms. However, because these chemicals were so long-lasting in the soil (a characteristic that made them so efficacious against wireworms), they were eventually banned from use in the U.S. for environmental reasons.

For the past 25 years or so, potato growers have struggled to find efficacious and cost effective control. The primary remaining weapons are carbamates and organophosphate insecticides. Virginia Tech Researcher and Entomologist Tom Kuhar says results from a number of organophosphate insecticides have been inconsistent, sometimes even contradictory over the past 20-plus years.

One of the most consistent of these materials, diazinon, lost its Federal label for potatoes in 2003. The remaining organophosphate insecticides that have shown good, though still inconsistent, efficacy on wireworm. Ethoprop (Mocap) and phorate (Thimet) are under close scrutiny and may not be available much longer.

Two relatively new materials: pyrethroid bifenthrin (Capture) and phenylpyrazole fibronil (Regent) have shown similar control efficacy as the organophosphate materials, but with much less environmental and human risk, Kuhar says.

North Carolina State University Entomologist Mark Abney says wireworms have become an increasingly difficult management problem for his state’s growers. The reasons, he says, are open for debate, but the bottom line is available materials aren’t working so well.

In on-farm testing in North Carolina in 2009, Abney found that a drag-off application of Capture can reduce damage in Irish potatoes. It is most likely to be effective, he says, if a grower did not use any pre-plant insecticide and/or if there is some environmental factor that leads the grower to think the pre-plant or at-plant insecticide is not working.

“I’ve been working with sweet potatoes for a number of years in North Carolina, and we have confirmed eight different common species of wireworms. These species have different biology and react differently to different insecticides. One species might live three or four years and another only one year.

“If there is a species out there and we don’t know its biology, it’s hard to tailor a management system for it,” Abney says.

Problems with unknown, or little known species of insects isn’t a stretch even with today’s sophisticated cyber-driven communication options. North Carolina growers, for example, had no history with a white grub, Plectris aliena, until recently. Now, it is threatening to put some areas of sweetpotato production in the state out of business.

In extensive testing in the eastern part of North Carolina, where a majority of potato production in the state occurs, most of the wireworms were corn wireworms. “In that part of the state we have a predominant rotation of potatoes, corn, and soybeans. Based on the crop mix, corn wireworm is what I expected to find,” Abney says.

The majority of wireworms in the survey were corn wireworms, but what he didn’t expect to find was wireworms in the genus glyphonix, about which little is known, the North Carolina State scientist says. It made up a fairly large percentage of the total species of insects in the tests conducted in Irish potato production areas of the state in 2009.

“We can determine these larvae are in the genus glyphonix, but we don’t know what species. Again, that creates a problem in figuring out how to manage it, without knowing a great deal about its biology,” Abney adds.

It is important to know which species of wireworms are the most damaging from an economic standpoint. It would also be very beneficial to be able to predict which fields are most susceptible to a particular species. The North Carolina scientist says that’s the direction in which his research program is headed.

North Carolina typically produces 17,000 to 18,000 acres of Irish potatoes annually. A relatively new area of production comes from organic farmers who really struggle to manage wireworms.

Whether it be in sweet potatoes, conventional Irish potatoes or organic potato production, knowing your wireworm — and more importantly how to manage these pests — is critical to success in 2010.

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