Phenology and management of the black cutworm in Ontario no-till corn

Black cutworm is an occasional serious pest of corn that annually recolonizes Ontario and causes severe stand losses and seedling injury in infested fields. First captures of adults occurred in early April with significant influxes occurring in mid April. Cutworm damage in Ontario corn occurred 137°C degree-days (base 10.4°C) after first capture of an adult. Larvae were present in cornfields before planting and mean instar of larvae increased with corn leaf stage. There was no evidence of new recruitment after planting.; Transgenic corn hybrids expressing Bacillus thuringiensis (Bt) toxins Cry9C or Cry1F and seeds treated with the neonicotinoid insecticide clothianidin were tested for efficacy in cutworm management. Expression of Cry9C was negatively affected in seedlings grown at temperatures below 15°C, mimicking Ontario spring conditions, raising concerns about the effectiveness of Bt corn hybrids for early season pest control in short season areas. No differences in toxin concentration were found between hybrids expressing the same toxin or when hybrids were treated with clothianidin.; Complex interactions occurred between instar, corn seedling development and temperature. Feeding on Cry9C Bt corn seedlings increased larval mortality and caused sublethal effects on larval weight gain of survivors. Clothianidin applied at the label rate of 0.25 mg kernel-1 to Cry1F Bt corn reduced larval weight gain, stand losses and seedling injury due to cutworm larvae additively. In contrast, weight gains of black cutworm larvae increased significantly when they were fed on non-Bt corn seedlings treated with clothianidin, suggesting phagostimulation, compensatory overconsumption or a hormetic response in the insect. Corn seedling weight also increased in Bt and non-Bt hybrids in response to the labelled rate of clothianidin, suggesting hormesis in planta.; Protection from cutworms should not be the deciding factor in choosing to grow a Bt corn hybrid in Ontario. Cry1F Bt corn alone or in combination with the labelled rate of clothianidin appears suitable as a means of suppressing black cutworm in no-till corn fields, but rescue treatments may still be necessary. Clothianidin alone should not be recommended for black cutworm control until further studies are completed.