The effects of soil fertility and other agronomic factors on infestations of root maggots in canola

The quantity and availability of soil nutrients can affect the amount of damage inflicted by insect herbivores on their host plants and the ability of infested plants to compensate for herbivory, but this relationship has not been investigated previously in canola agroecosystems. Two field experiments were conducted in central Alberta, Canada, from 1999 to 2001, for a total of five experiment-years, to determine the relationship between soil fertility (0.5, 1.0, 1.5, and 2.0 times the recommended rates of N, P, K, and S) and root maggot (Delia spp.) (Diptera: Anthomyiidae) infestations in canola (Brassica napus L.). The experiments also investigated effects of cultivar, timing of weed removal, and seeding rate on root maggot infestations. In both experiments, fertilizer rate significantly affected larval damage to taproots, with greatest damage occurring at the two highest levels of soil fertility. Egg deposition, however, was unaffected by the fertility treatments, suggesting that increased canola nutrient levels enhanced survival and development of root maggot larvae. Results of this study support the plant vigor hypothesis, which holds that insect herbivores perform better on plants growing vigorously than on plants under physiological stress. Although the yield advantage from improved plant nutrition at high rates of fertilization was diminished by losses due to greater attack by root maggots, vigorous plants were better able to compensate for damage than plants grown with less fertilizer. Optimal crop management in areas where root maggots are annually responsible for significant economic damage should continue to incorporate recommended fertilizer applications, in view of the enhanced compensatory ability of canola grown in nutrient-rich soil.