Grub threatens sweet potato crop

Managing labor, keeping production costs in line and finding reliable markets are all major challenges for North Carolina sweet potato growers. But these headaches are being over-shadowed by a small white grub that is threatening production in some areas of the state.

Wayne Bailey Produce Company in Chadbourn, N.C., has been particularly hard hit by white grubs since 2006.

George Wooten III, farming operations manager for his family’s farming operation, says these insects generally do only superficial damage to sweet potatoes, but this damage often leaves the spuds unmarketable.

The damage first occurred in 2006 at Wayne Bailey Produce Company, which is owned and operated by the Wooten family, and has gotten progressively worse. In some fields last year over 80 percent of the sweet potatoes were damaged by these voracious insects. Of the damaged potatoes, Wooten says more than 30 percent were not marketable. The 2009 crop doesn’t look much better, he says.

North Carolina State University Extension Entomologist Mark Abney says the scientific name for this pest is Plectris aliena. It is so rare and so little is known about it that it has no common name — right now it is simply referred to as the Plectris grub.

“We know these insects were found in the Charleston, S.C., area as far back as the 1930s. There was some work done with them on turf, but they don’t appear to have caused significant problems with crops. A lack of knowledge about Plectris aliena is a big challenge in finding ways to manage it,” Abney says.

“When George Wooten found these insects in his sweet potatoes in 2006 that was the first time we knew we had a problem, or even that the pest was present in North Carolina. Since then we have had one grower reporting a problem on peanuts and near catastrophic damage on sweet potatoes in some fields,” he explains.

These insects are amazing in the way they damage potatoes, Abney says. They feed on the surface, making large, ugly gouges and craters on the sweet potato. In too many cases affected potatoes aren’t even marketable as processing potatoes.

In the processing plant there is no way to stop and cut out the affected part of the potato. Though the potato doesn’t die from the damage, in most cases the gouges and craters on the surface make them unmarketable.

“Wayne Bailey Produce seems to be in the epicenter of the outbreak, with damage radiating out in all directions. We don’t know if the insects are in other areas of the state, or really much about its movements.”

A big concern, Abney says, is that the insect spreads into the heart of North Carolina’s sweet potato production area — generally north and east from Chadbourn. “If it moves into this area before we can develop management strategies for it, Plectris aliena could be catastrophic to North Carolina sweet potato production.”

So far, the North Carolina State University scientist says no chemical control has been found to slow down the voracious feeding habits of these grubs. “In 2007 we tried combinations of Lorsban and Mocap which have good activity against other white grub species. In the best treatments, we still had over 70 percent damage,” Abney says. “We did see some encouraging results in 2008 field tests with the insecticides clothianidin and imidacloprid, but we are still a long way from an effective management strategy.

“To say these little critters are hard to kill is an understatement. We don’t know much about the biology of the insect, so we don’t know if it is tolerant to insecticides or whether we’re not getting the chemical in contact with the grub at a susceptible stage in their growth cycle,” he adds.

An interesting characteristic of this insect is that it only feeds in the larval stage. Once they become an adult, they don’t eat anything. The adult beetles only come out of the ground for a few minutes a day — just at dusk — to mate. Combined, these characteristics make control very difficult.

Controlling the adult beetle with contact insecticides does not look promising because the adults don’t eat anything and are only above the soil surface for a short time each day. They swarm at dusk and are quickly gone, so timing is a problem in trying to use a contact material to control them.

So far the best management tool has been to simply abandon fields with high infestations and move sweet potato production to new areas.

Wooten says he is trying to move his sweet potato production to new land, with no history of white grub populations. Abney concurs this is about the only strategy available to growers, but he notes that the insects may already be in these new areas.

Richard Ward, who grows organic sweet potatoes in Whiteville, N.C., says white grubs are the major problem he has in his crops. None of the natural insecticides that we are allowed to use on organic crops does any good, he adds.

Wooten notes that recent high grain prices have escalated the cost of acquiring new land to rent. Selling prices for farmland in the Chadbourn area have doubled in the past 4-5 years, and in some cases, farmland is simply not available at any cost.

If white grubs are found in a field of any crop, Abney urges the grower to collect samples and send these insects to him for positive identification. “There are a number of other white grubs that attack sweet potatoes, but they aren’t nearly so big a problem. Grower cooperation is going to be very important as we work to develop management tools. The grub samples that growers send will allow us to determine the current distribution of Plectris aliena in North Carolina and will help us track movements of the pest and get a better handle on how to manage it,” Abney says.

Ideally, growers can put grubs in a solution of 70 percent ethyl alcohol and send them to Abney, but simply placing live grubs in a small container with soil and sending would be okay. Dead grubs will decay rapidly and are often impossible to identify. These samples can be sent to: Mark Abney, Department of Entomology, Campus Box 7630, North Carolina State University, Raleigh, N.C. 27695.

“If you know what you are looking for, these damaging white grubs are fairly easy to identify. Plectris aliena larvae have a distinctive pattern of ridges on their head that is not present on any other white grub found in North Carolina. With just a little bit of training a grower can become proficient at identifying these pests,” Abney says.

“We know the beetles, or adult stage of the insect use phermones to find each other for mating. Finding the chemical makeup of this pheromone may be the first step in finding ways to manage the beetles.

“We also know that it over-winters as a third instar, or fairly large grub in North Carolina. And, in tests this year we found it feeds on corn, but there is just not much information available on which crops are susceptible to feeding damage.

“It seems that peanuts would be a natural target, but so far only one grower has reported damage from Plectris grubs in peanuts. We are hoping there is something in the insecticide program for peanuts that in some way reduces feeding damage of these pests,” Abney says.

In today’s global economy, it is not unheard of for a pest to suddenly show up in a particular area, cause catastrophic economic damage to a particular crop and have no history of causing problems in other areas.

This is actually more common today than people realize. Insects are transported from their native range and are deposited in a new environment complete with food resources but no natural predators/pathogens. Often, the insects are not pests until they are released from the effects of natural enemies.

For farmers like George Wooten and researchers like Mark Abney, starting from scratch in finding out how to manage a pest with little or no history is proving to be quite a challenge.

e-mail: [email protected]

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