Evaluation of two biological control strategies against the European corn borer (Ostrinia nubilalis Hübner) in fresh market sweet corn

In Québec, Canada, fresh market sweet corn growers spray chemical insecticide as many as six times per season to fight the European corn borer (ECB), Ostrinia nubilalis Hübner. The insect attacks all parts of the plant, damages to the ears being the most problematic for fresh market sweet corn. The present study comes within the scope of a larger project aiming at the replacement of chemical insecticides used against sweet corn pests by environmentally safer and reduced-risk products. The two principal biological control measures against ECB are the application of Bacillus thuringiensis-based insecticides and the introduction of parasitoids of the hymenopteran family Trichogrammatidae. The objective of the study, conducted in 2009, was to compare the efficiency of two biological control strategies against the ECB in fresh market sweet corn of southern Québec. The first strategy (Tricho) consisted in the release of 150 000 Trichogramma ostriniae per hectare, five times during the season. The second strategy (Tricho+Bt) consisted in a unique release of T. ostriniae (same dose), followed by weekly applications of B. thuringiensis, according to ECB infestation levels. Control efficiency of both strategies was evaluated and compared to controls by counting larvae and damages on the day of harvest. The Tricho strategy was more efficient than the Tricho+Bt strategy by reducing ear damages by 30% and number of larvae by 60% compared to control plots, while the Tricho+Bt strategy only reduced 17% ear damages and 8% larvae. However, none of the strategies could reduce ear damages under acceptable levels.