A spatial ecology study of the effects of field conditions on the incidence of Plectris aliena Chapman (Coleoptera:Scarabaeidae) grub damage to sweetpotato roots

Plectris aliena is an invasive white grub that was introduced into the United States from South America in the early 1900’s and was discovered in Columbus County, North Carolina in October of 2006 feeding on sweetpotato.  Since its discovery, growers have estimated that P. aliena has caused more than $16 million in lost revenue to the sweetpotato industry.  Little was known about P. aliena prior to 2006, with only a few references on its distribution and taxonomy, including limited knowledge of its biology. As an introduced soil pest in North Carolina, environmental effects that contribute to P. aliena's survival and spread is unknown. The objective of this study was to determine the effect of various field conditions such as soil drainage class, soil texture, field size, elevation, and adjacent habitats on the incidence of P. aliena grub damage to sweetpotato roots. Agricultural fields (n=9 in 2010 and n=19 in 2011) in Columbus County, NC were sampled for sweetpotato roots damaged by P. aliena. Root sampling was conducted every 23 m in sweetpotato rows and 20 m across rows in a grid pattern.  At every 30 m in the row, sweetpotato roots (n=10 roots in 2010 and n=15 roots in 2011) were randomly selected within the row and scored for grub damage. The number of roots damaged out of the total number of roots sampled, i.e. the proportion of roots damaged, was recorded. A hand-held GPS unit recorded the coordinates of each sampling point and the distance from each sampling point to the edge of border habitats was determined in ArcGIS 10. The soil series, soil texture, and drainage class were obtained from Soil Survey Geographic (SSURGO) database maintained by the USDA-Natural Resource Conservation Service (NRCS) made available from NRCS via a shapefile format for use in ArcGIS 10.  Elevation of sampling points was obtained from Google Earth Pro (Google Inc., 2012). The effect of field conditions on the incidence of insect-damaged roots in sweetpotato fields was analyzed using proc glimmix (SAS, 2008) with the additive random effect  of year*location.  Results showed that drainage and soil texture had an effect on the incidence of grub-damaged roots in sampled fields (P < 0.0001 for drainage and P = 0.0001 for soil texture) with more damage in sandy loam and poorly drained soils. A crop rotation of soybean planted the year before sweetpotatoes were sampled showed significantly more damage to roots (15%) than all other crop rotations (0 – 5%) (P = 0.0017) The adjacent habitats of soybean and turf areas also showed an effect with less damage when roots were closer to a soybean (P = 0.0200) or grass adjacent habitat (P = 0.0074). Growers who use crop rotation as a management technique and avoid planting sweetpotatoes the year after soybeans may reduce the incidence of P. aliena larval feeding on sweetpotato roots.  Environmental conditions such as fields with poor drainage and certain border habitats may be avoided, or selected, by growers in order to reduce risk of damage to roots by P. aliena.