Effect of nitrogen fertilization on flea beetle (Phyllotreta cruciferae) and cabbage seedpod weevil (Ceutorhynchus obstrictus) injury and crop yield in dry land canola

Abstract

Nitrogen (N) availability is an important factor affecting the canola (Brassica napus L.) yield. While N fertilization is commonly used to enhance the yield and quality of canola, it is also known to influence the incidence of insect pests in the crop. The flea beetle Phyllotreta cruciferae (Goeze) and the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsh), are two economically important pests of canola in the northern Great Plains of the United States. This study investigated the effects of N fertilization (0, 56, 112 and 168 kg/ha) with or without insecticide seed treatment application on P. cruciferae and C. obstrictus injury levels, canola seed yield, and quality in replicated field trials at the two locations (Conrad and Sweet Grass) of north central Montana. The study showed that N fertilization had no influence on P. cruciferae and C. obstrictus injury levels, regardless of the study location or sampling date. However, insecticide seed treatment application significantly influenced P. cruciferae injury ratings at the both locations. In Conrad, insecticide-treated plots had significantly lower injury levels at the four leaf, pre-flower, and pod formation stages but without an effect at the cotyledon stage of canola plants when compared with untreated control plots. Similarly, in Sweet Grass, injury levels were significantly lower in insecticide-treated plots from cotyledon to pre-flowering stages but not at the pod stage of canola plants. In contrast, insecticide seed treatment had no impact on pod infestation rates by C. obstrictus at either location. Insecticide seed treatment, averaged over all the N rates, improved canola seed yield and quality parameters compared to the untreated plots.