Tara P. Smith and Abner M. Hammond
Sweet potatoes are an important agricultural commodity in Louisiana. More than 14,000 acres of sweet potatoes were planted in Louisiana in 2007 with a farm gate value of $65 million. The total value of the crop after packing and processing exceeded $112 million. Sweet potato producers face many obstacles each year in crop production. Insect pests are often cited by growers as their primary concern.
The sweetpotato weevil is the most damaging insect pest of sweet potatoes in both tropical and subtropical areas of the world. Sweetpotato weevils can attack sweet potatoes in the field and in storage, and larvae feeding on sweet potato roots can cause major economic damage and yield loss. Reported yield losses due to sweetpotato weevil damage range from 5 percent to 80 percent. Feeding larvae tunneling through the roots induce chemical changes, and these infested roots reportedly taste bitter, rendering them unsuitable for human consumption.
An effective integrated pest management program is multidisciplinary and includes numerous management options. Several tactics employed to manage the sweetpotato weevil in Louisiana include cultural practices, monitoring with pheromone traps and routine application of insecticides. Insecticides traditionally have been the primary defense against root damage by these insects, and they are a valuable tool in pest management systems. However, they must be used judiciously to reduce the development of insect resistance.
Currently, sweetpotato weevils are established primarily in southern regions of Louisiana, and a quarantine exists to minimize the spread of weevils into northern areas of the state. All commercial sweet potato fields and seedbeds in the state are monitored with pheromone traps for the presence of sweetpotato weevils.
Sweet potatoes are produced in two primary production areas in Louisiana. Approximately 80 percent of Louisiana sweet potato production is concentrated in the northeast parishes of West Carroll, Franklin, Morehouse and Richland. The remainder of the acreage is in south Louisiana, primarily in Avoyelles, Acadia, St. Landry, Evangeline and Rapides parishes.
The northeastern part of the state is defined as a “green tag” area. Sweet potatoes grown in green tag areas are monitored for the presence of sweetpotato weevils, but distribution is not restricted by quarantine regulations. South Louisiana is referred to as a “pink tag” area, and movement of sweet potatoes and any plant material originating from this area is restricted to areas where sweetpotato weevils are established or where quarantine restrictions are not imposed on pink tag material.
In addition, a mandatory insecticide use program has been outlined by the Louisiana Department of Agriculture and Forestry and is required for all sweet potato production fields and seedbeds in pink tag areas. The statewide monitoring program and the mandatory spray program have been successful in two ways. First, sweetpotato weevil numbers in South Louisiana have been below average in recent years, and second, sweetpotato weevils have been contained within the pink tag production areas.
The mandatory spray program was established in 2001. Since its inception, sweet potato production fields and seedbeds have been treated with labeled, recommended insecticides on a 10-to- 14-day spray schedule. A study initiated in 2004 compared the response of two populations of sweetpotato weevils to selected insecticides used in the required spray program.
The Louisiana population of sweetpotato weevils used in these bioassays was obtained from a laboratory colony established in 2002 from field-collected sweet potato roots taken from Avoyelles Parish. Sweet potatoes grown at this location received numerous applications of one or more of the insecticides evaluated in this study, indicating that weevils collected from this location had experienced multiple insecticide exposures. A Texas colony was collected from pheromone traps placed near wild sweetpotato weevil host plants in Frio County, Texas, and this population had limited or no previous exposure to insecticides.
A glass vial technique was used to determine the susceptibility of the two populations of sweetpotato weevils to the insecticides. Technical-grade insecticides were dissolved in acetone to yield the desired concentrations. Insecti cides evaluated included methyl parathion, bifenthrin, cyfluthrin, carbaryl and phosmet. All chemicals evaluated are recommended for control of the sweetpotato weevil and are labeled for use on sweet potatoes in Louisiana. The interior surface of each vial was coated with the appropriate insecticide solution. Control vials were treated with acetone only. Vials were then rotated until all the acetone had evaporated and a residue of insecticide was deposited on the interior surface of the vials.
One adult sweetpotato weevil was placed in each of the vials. A minimum of 10 insects were used for each concentration. Three trials (with a minimum of 20 insects per concentration) were conducted with Louisiana weevils for each insecticide tested and two trials (with a minimum of 10 insects per concentration) were conducted with Texas weevils for each insecticide. No food was provided to insects during testing, and all assays were conducted at about 75 degrees. Mortality was determined 24 hours after exposure for all insecticides tested. Weevils were considered dead when they were unable to maintain an upright posture and perform coordinated movement after being dislodged from the vials.
More sweetpotato weevils died with each increase in concentration of all insecticides tested, and the results indicated Louisiana weevils were less susceptible than Texas weevils. Methyl parathion was the most toxic of the chemicals evaluated, followed by bifenthrin and cyfluthrin.
The Louisiana population’s lower susceptibility than the Texas population’s is cause for concern because weevils in South Louisiana are repeatedly exposed to the insecticides evaluated in this study. Surveying insect populations for changes in susceptibility is an integral part of insecticide resistance management, and determining the range of initial resistance frequencies among insect populations can allow for early detection of changes in susceptibility to insecticides.
Continued monitoring of the Louisiana sweetpotato weevil population’s susceptibility to currently labeled insecticides is necessary to ensure that controls don’t fail in the field. The mandatory weevil quarantine program initiated by the Louisiana Department of Agriculture and Forestry has been a valuable asset to overall sweet potato production in the state. We will continue to monitor sweetpotato weevil populations for resistance development and modify the management programs when appropriate. Tara Smith, Assistant Professor and Research Coordinator, Sweet Potato Research Station, Chase, La.; and Abner M. Hammond, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.
Read "Insect Pest Management in Louisiana Sweet Potatoes."
(This article was published in the winter 2008 issue of Louisiana Agriculture.)