PROJECT RESULTS

Classical Biological Control of Root Maggots, Delia spp. on Canola

Applicant: 

Dr. N. J. Holliday
Department of Entomology
University of Manitoba
Winnipeg, Manitoba  R3T 2N2  Canada

Table of Contents:

• Background and Objectives
• Procedure and Project Activities
• Results and Discussion
• Conclusion

ARDI Project:

#99-273

Project Status:

Completed November, 2003

Background and Objectives:

Adult cabbage root maggots are similar to a house fly, but greyish and smaller. Female flies lay eggs near the base of stems of canola. The eggs hatch into legless, headless maggots that browse on the roots, producing surface grooves and later tunnels in the root. Rotting of root tissues frequently results. After repeated moulting and feeding, the mature maggot swells into a brown barrel-like resting stage. It over-winters in this stage and the following spring adult flies emerge and search for mates.

 The effect of root maggots feeding on canola roots has not been fully quantified, but it is clear that large numbers of maggots reduce yield, and can cause plant death. Throughout the last four decades there has been a steady increase in root maggots in prairie canola, and since the 1980s, there has been significant crop damage in Alberta.  In Saskatchewan and Manitoba economic damage has not been reported, but the trend of increase has reached a point in Manitoba where the prospect of economic loss is not distant.

No insecticides are available to control root maggots in canola because the vulnerable maggot stage occurs in mid-July. Insecticidal seed treatments that would be active in July are too persistent to be used. Research is being done to find non-insecticidal control methods. This project focuses on the potential for using classical biological control, and its objective was:

“To identify candidate agents that could be used in classical biological control of cabbage root maggots in canola in prairie Canada and to begin detailed studies on them.”

Attention was focused on parasitoids as potential natural enemies of root maggots. Parasitoids are insects in which the immature stages develop inside, or attached to the outside of a host insect.  They ultimately kill their host. Because of the close association of the parasitoid with the host, parasitoids are the most host specific natural enemies, and are least likely to pose a threat to non-target organisms.

Procedure and Project Activities:

In 2000, the assemblage of parasitoids attacking root maggots in canola in the Prairies was investigated by regular collections of eggs, larvae and pupae of root maggots from two canola fields in each of the three Prairie Provinces. The immature root maggots were individually reared to adulthood to determine whether a root maggot fly emerged or whether a parasitoid emerged. In 2001 and 2002, the researcher used the same methods to investigate the assemblage of parasitoids attacking root maggots in Europe. Collections were made in winter and summer canola and brassica vegetables in locations ranging from the Baltic shore of Germany to central Switzerland.

Once the researcher identified the most promising candidate for introduction, he investigated reports that it was already present in North America. He also did supplementary studies of the over wintering biology of root maggots collected in canola in the Canadian Prairies, so that the characteristic time of emergence that would need to be matched by any parasitoids chosen for introduction would be known.

Results and Discussion:

In canola in the Canadian prairies, the principle parasitoids of root maggots are Trybliographa rapae, Aleochara bilineata and Aleochara verna. In canola in Europe, the principle parasitoids of root maggots are Trybliographa rapae, Aleochara bilineata and Aleochara bipustulata.  Comparison of the parasitoids in Europe and Canada suggest that Aleochara bipustulata could be a candidate for introduction to Canada for classical biological control.

Studies of the records and specimens of collections of alleged North American records of Aleochara bipustulata show that these are erroneous. There is no evidence that the species currently occurs in North America. Further evidence that Aleochara bipustulata shows promise as a biological control agent includes its coexistence with important Canadian parasitoid species in several locations in Europe, and its apparent adaptation to northern climates.

In laboratory studies it was determined that the time of spring emergence of cabbage root maggots in canola in prairie Canada is delayed relative to that observed in previously-studied populations of the same insect. This indicates the need for selecting parasitoids for introduction from populations that can accommodate this late emergence.

Conclusion:

Aleochara bipustulata should receive intensive study as it is a promising candidate for introduction for biological control of cabbage root maggots in canola in the Prairie Provinces of Canada.

Acknowledgements:

This project was made possible by funding from the Governments of Manitoba and Canada through the Canada-Manitoba Agri-Food Research and Development Initiative (ARDI), by funding from the Canola Council of Canada through the Canola Agronomic Research Program (CARP), by funding from the Government of Canada through Agriculture and Agri-Food Canada and the Canadian Commonwealth Scholarship and Fellowship Office, and through funding from the Entomological Society of Canada.

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