Assessment of larval feeding damage of the cabbage maggot (Diptera: Anthomyiidae) in relation to oviposition preference on canola

A choice bioassay was used in the laboratory to investigate oviposition preference of the cabbage 957g962, Delia radicum (L.), for canola, Brassica campestris L. cv. Tobin, in different developmental stages. Given a choice of two different ages of canola plants, females invariably preferred to oviposit in sand around the oldest plants. Greatest preference was shown for 5-wk-old plants in the stem elongation stage. The effect of oviposition on plants in the five-leaf (3-wk-old), stem elongation (5-wk-old), and initial flowering (7-wk-old) stages was also examined by artificial infestation with 10 eggs per plant and assessment of root damage and numbers of puparia recovered. Root damage or survival from eggs to pupation did not differ significantly with stage of plant development. In a separate experiment, root damage was related (r2 = 0.76) to introduction of 10, 25, or 50 eggs per plant in the stem elongation stage. However, percentage survival from eggs to pupation and pupal weight did not differ significantly among treatments. Microscopic examination of root damage revealed that larvae consumed a narrow band of tissue bordered on the outside of the taproot by a thin layer of periderm and phloem tissue and on the inside by the parenchyma cells of secondary xylem. Independent of larval density, wounds never penetrated to the center of the root. Consequently, no plants in any of the development stages examined were killed by larval feeding. Plants appeared to compensate for damage by a significant increase in root weight with increasing level of infestation.