Abstract
Defoliation by insects is a major source of damage in field crops such as canola and an economic evaluation of resulting yield losses is essential for guiding management decisions. In this study, different intensities of defoliation were simulated at different growth stages of rapeseed plants in three sets of experiments. In the first experiment, 25, 50, or 75% of the leaf area was removed once at either the 2-, 4- or 8-leaf stage. In the second experiment, similar defoliation treatments were imposed at one of four flowering stages, pre-bloom, early bloom, mid-bloom, or late bloom. In the third experiment, 80% of leaf area was removed progressively in three treatments: 10% defoliation weekly throughout the 8-week period of flowering, 20% defoliation in each of the first 4 weeks, and 20% defoliation in each of the last 4 weeks. In all experiments, various plant metrics, components of yield, and seed yields per unit area were compared to undefoliated (control) plants. Plant phenology (onset of flowering, time to flowering, and ripening time) was delayed, and growth and yield components were reduced by defoliation occuring in vegetative, prebloom and early bloom stages, but plants compensated for up to 25% defoliation that occurred mid to late flowering. The economic injury level (EIL) was determined to be 3.7, 5.5, and 5.7% defoliation at the 2-, 4- and 8-leaf stages respectively, whereas it was < 2% just prior to flowering and 5.6% during early bloom. Moderate levels of defoliation late in the flowering period resulted in some overcompensation (yields greater than controls). The economic threshold for defoliation during flowering was calculated to be 3.2 and 3.7% for gradual and sudden defoliation events, respectively. These findings can be linked to leaf consumption by defoliators to aid decision making by farmers.