This study investigated the relationships between numbers of Delia species (Delia spp. Rob.-Desv., Diptera: Anthomyiidae) flies or eggs and root injury levels of Brassica napus L. and Brassica rapa L. (Brassicaceae) canola (oilseed rape), and between root injury levels and canola seed yields. Wild-collected Delia species adult flies or laboratory-obtained Delia radicum (L.) eggs were introduced into laboratory or field cages containing B. napus or B. rapa plants. The degree of root injury was strongly associated with infestation level using both flies and eggs. Linear response curves between numbers of Delia flies and root injury levels and quadrilinear response curves between numbers of D. radicum eggs and root injury levels showed that root damage increased with increasing levels of infestation; at high egg infestation levels, root damage levelled off in both Brassica species. Seed yield was not clearly related to fly or egg infestation rate, nor to ensuing root damage. No consistent yield relationship in B. napus ‘AC Excel’ was found with levels of root damage seen in the investigation. Conversely, B. rapa cultivar ‘AC Boreal’ seed yields tended to increase above uninfested controls when feeding damage was light but decreased with increased root injury. Despite procedures used to prevent infestation in Delia-free control cages, some root maggot damage was found in control cages in all 4 yr of the field study. Changes in climate, land use patterns, and cropping systems could influence the economic impact of this pest complex on canola in the future.
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