PROJECT RESULTS

Management of Root Maggots, Delia spp. on Oilseed Rape in Manitoba

Applicant: 

Dr. N. J. Holliday

Department of Entomology

University of Manitoba

Winnipeg, Manitoba  R3T 2N2  Canada

Table of Contents:

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

ARDI Project:

#98-166

Project Status:

Completed January, 2004

Background and Objective:

Adult cabbage root maggots are similar to a house fly, but grayish 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 occasional 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. We have been doing research to find non-insecticidal control methods. These methods include manipulations to production practices that can be implemented now, and explorations of biological control that may provide a permanent solution in the future. This project focuses on the manipulation of production practices, and its objective was:

“To reduce injury caused by cabbage maggot population in Manitoba through tillage and other cultural practices.”

Procedure and Project Activities:

Large-scale field plot trials were conducted for three years to test the effect of seeding rate and tillage practice on the number of cabbage root maggots and the amount of damage caused. Treatments consisted of a zero tillage regime and a conventional tillage regime (fall and spring tillage) and two seeding rates, 4 and 8 kg/ha. In each year, numbers of root maggots and predators and root damage were assessed at weekly intervals. Total quantity of yield and components of yield were also assessed.

In field plots in 1999, and in commercial fields from 2000 onwards, sampling was done to examine the distribution of maggot larvae and root damage near the edge of the canola crop. Root samples were taken at fixed intervals from 0 to 20 m into the crop and also near the middle of the crop. Roots were taken to the laboratory for damage rating and for dissection to count maggot larvae.

Data was analyzed from a large-scale survey of commercial canola fields being conducted by Dr. J. Gavloski (Manitoba Agriculture and Food), and Dr. D. McLaren (Brandon Research Station, Agriculture and Agri-Food Canada) to examine the effect of production practices on maggot damage in commercial canola production. In each surveyed field, a total of 100 plants were taken as 20 plants from each of five widely separated locations.

Results and Discussion:

Major findings were:

• Fields with short rotation intervals invariably had higher levels of root damage.

• Root damage was less after zero tillage than after conventional tillage.

• Root damage was lower when higher seeding densities were used or plant density was higher.

• Wind-breaks reduced the number of larvae and root damage within 20 m of field margins.

• Particularly in the absence of wind-breaks, larval numbers and root damage were elevated within 20 m of crop margins.

• There was evidence of yield loss when root damage ratings exceeded about 3 (25–50% of root area damaged). These rating levels were exceeded in individual plants in plots and near field margins throughout the study, and in 2000, five commercial fields had average damage rating over 3.0.

• Quality characteristics of yield, including oil content, gluosinolate concentration and protein content appear to be related to root ratings when they are above about 3.0.

Major recommendations to reduce maggot damage are:

• Short rotation intervals between canola crops should be avoided.

• Zero tillage practices should be used to maintain crop residues on the soil surface before seeding canola, but after a maggot-infested canola crop conventional tillage should be practiced.

• Higher seeding rates should be used to promote higher plant densities.

• Wind-breaks should be established around canola fields.

Data on the distribution of maggots near crop margins suggest that the following practices be adopted:

• Sampling to assess the general levels of root maggots in canola fields should be done no less than 20 m from the field margin.

• If insecticidal methods of root maggot control become available, consideration should be given to applying insecticides only at the field margins.

• Small plots, such as those used by plant breeders, are particularly vulnerable to high levels of root maggot attack, and can be protected by embedding them within a larger stand of canola or another root maggot host, or by surrounding them with fly-proof barriers > 1 m in height.

Conclusion:

Root maggots are an increasing pest to canola in the Canadian Prairies, are already of economic significance in Alberta, and are expected to attain this status in Manitoba within a few years. The absence of an effective insecticidal control means that modification of production systems is the only currently available tool for management. The research resulted in recommendations for agronomic measures that, under Manitoba conditions, will reduce the root damage associated with root maggots, and so reduce the impact on canola yield. Until other solutions, such as classical biological control or plant resistance, are developed, agronomic methods are the best tactics available to producers for avoidance of economic loss in the face of increasing root maggot populations.

Acknowledgements:

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