Influence of Environment and Genotype on the Expression of Host Plant Resistance in Sweet Potato to the Sweet Potato Weevil, Cylas Formicarius (Fab.).

Field and laboratory experiments were conducted to study the influence of environment and genotype on the expression of sweetpotato resistance to the sweetpotato weevil (SPW), Cylas formicarius (Fab.). Roots of four genotypes ( W-244 , W-250 , Beauregard , Centennial ) were evaluated for SPW feeding and oviposition at three different storage times. Roots from Louisiana, South Carolina and Mississippi were also evaluated. Genotype had a significant effect on feeding and oviposition. Storage time and production sites appeared to affect resistance expression; the outcomes depend on the genotypes. The effect of nitrogen on sweetpotato resistance to SPW was studied. Four genotypes were grown in the field under three nitrogen regimes (0, 45 and 135 kg N/ha). Harvested roots were evaluated for SPW feeding, oviposition, larval survival, and pupal weight. Significant nitrogen effects were found on oviposition. Genotype significantly affected feeding, oviposition, and larval survival where W-244, Excel and Sumor had lower rates than Beauregard and W-250. No significant nitrogen and genotype interaction was found. The effects of drought and manual defoliation on SPW feeding, oviposition, larval survival and pupal weight were studied on Beauregard and Excel. Drought, stressed plants received significantly more SPW feeding punctures and eggs than non-stressed plants, but had lower larval survival. Manual defoliation (67% of leaf area at different growth stages) had a significant effect on oviposition, but not on feeding, larval survival or pupal weight. Experiments were conducted to investigate the effect of root and foliage feeding by other insects on sweetpotato resistance to SPW using four genotypes. In the field, banded cucumber beetle (BCB), Diabrotica balteata LeConte, and armyworm, Spodoptera latifascia (Walker), were released into cages (1.8 x 1.8 x 1.8m) placed over sweetpotato plants, and were allowed to feed and deposit eggs throughout the growing season. Harvested roots were evaluated for SPW feeding, oviposition, larval survival, and pupal weight. Root feeding by BCB and defoliation by armyworm tended to increase SPW feeding and oviposition, but had no effect on larval survival and pupal weight. Genotype had a significant effect on feeding, oviposition, and larval survival, suggesting both antixenosis and antibiosis as resistance mechanisms