Tara P. Smith, Julien M. Beuzelin, Theresa F. Arnold and Dana M. May

Sweet potatoes are an important agricultural commodity in Louisiana, with a total production value exceeding $85 million in 2013. Soil insect pests, which reduce root yield and quality, are often cited as one of the most limiting factors affecting production. The sugarcane beetle (Figure 1) has emerged as the primary target of soil insect management programs for sweet potato producers in Louisiana during the past 10 years. Unlike most soil insects feeding on sweet potatoes, only adult sugarcane beetles cause crop damage. These adult beetles feed on sweet potatoes late during the growing season by creating gouges on the surface of the roots (Figure 2). Because sweet potatoes are an aesthetic crop, minimal damage negatively affects marketability of the roots.

Limited options are available to manage sugarcane beetles in sweet potatoes. The insecticides Belay 2.13SC (clothianidin) and Brigade 2EC (bifenthrin) are presently labeled for soil applications preplant or approximately three weeks after planting. Belay can control the sugarcane beetle and is currently recommended. Root injury has been reduced in field plots treated with these and other labeled insecticides; however, results have varied among locations and production years. Thus, there is a need to better document activity of clothianidin and other insecticides of interest to refine sugarcane beetle management recommendations.

A study conducted in 2013 at the LSU AgCenter Sweet Potato Research Station in Chase examined the efficacy of selected insecticide treatments against controlled sugarcane beetle infestations in sweet potatoes. All insecticides evaluated in the study are labeled for use on sweet potatoes in Louisiana and were applied to experimental plots prior to planting or 20 days after planting. Sweet potato transplants were planted at a rate of one plant per foot of row, consistent with commercial practices. Insecticide treatments were applied to experimental plots with a carbon-dioxide-charged spraying system calibrated to deliver 20 gallons per acre.

Polyvinylchloride pipes 12 inches in diameter and 15 inches tall were placed 3 inches into the soil around individual sweet potato plants in each plot (Figures 3 and 4). Adult sugarcane beetles were collected using black lights during September and were maintained on sweet potato roots until needed for infestations. Four sugarcane beetle adults were individually placed on each plant surrounded by a pipe in each plot. Following this mechanical infestation, pipes were covered with a mesh screen cap secured by a rubber band.

Feeding injury to the sweet potato roots, number of beetles recovered and beetle mortality were evaluated 10 days after insect release began by harvesting whole plant samples. Each sweet potato plant and surrounding soil contained within the pipe were placed into a plastic tub (Figure 5). All of the soil was sifted (Figure 6) to recover beetles. Each sweet potato root (Figure 7) was also examined for the presence of feeding injury. T

he majority of sugarcane beetles – 79-90 percent – were recovered (Figure 8). Injury ranged from 6 percent to 17 percent of sweet potato roots with sugarcane beetle feeding (Figure 9). However, differences in injury were not detected for any insecticide treatment evaluated in this study. Beetle mortality ranged from 2 percent to 34 percent, with the Lorsban 4E treatment resulting in significantly higher beetle mortality (Figure 10). Belay applications were not associated with increased mortality.

Occurrence of and damage incurred from sugarcane beetles in sweet potatoes are highly erratic. The insecticides evaluated in the study have been shown to significantly reduce sugarcane beetle damage in naturally infested small-plot field studies. In field trials conducted from 2011 to 2014, sugarcane beetle damage ranged from zero to 40 percent in 2011, 12-57 percent in 2012, zero to 5 percent in 2013 and zero to 12 percent in 2014.

The current study design allowed researchers to evaluate sugarcane beetle damage in sweet potatoes in a contained system. Insufficient numbers of sugarcane beetles were collected during 2014, which prevented successful replication of the study. LSU AgCenter scientists plan to use the current design to evaluate insecticide management options for the sugarcane beetle in sweet potatoes at the Sweet Potato Research Station and the Dean Lee Research and Extension Center in subsequent years. Studies evaluating cropping systems adjacent to sweet potatoes and alternative monitoring techniques for the sugarcane beetle are also in progress. Results from these studies will be integrated into a multidisciplinary pest management program for the sugarcane beetle in sweet potatoes.

Acknowledgement: This project was supported by the Specialty Crop Block Grant Program, U.S. Department of Agriculture, through grant agreement 12-25-B-1464, NIFA Hatch Funds and the Louisiana Sweet Potato Commission.

Tara P. Smith is associate professor and research coordinator at the Sweet Potato Research Station, Chase, La.; she is also the director for the Northeast Region. Julien M. Beuzelin is an assistant professor at the Dean Lee Research and Extension Center, Alexandria, La. Theresa F. Arnold is research associate and graduate student at the Sweet Potato Station, and Dana M. May is research associate at Dean Lee.

(This article was published in the winter 2015 issue of Louisiana Agriculture.)