Refine and validate economic threshold for Lygus bugs in canola production in Alberta

Project Code: PRR12-030

Project Lead

Hector Carcamo  Agriculture and Agri-Food Canada

Objective

To make available to growers an improved economic threshold, as part of a spray decision support system for Lygus control in canola crops in Alberta

Summary of Results

Background

Lygus bugs (Lygus spp.) are a serious threat to many oilseed and forage crops in the Canadian prairies, including canola. Once considered an intermittent pest of canola in Alberta, the frequency of Lygus bug outbreaks has increased over the last decade. Insecticide sprays are commonly used to control Lygus bugs in canola crops.

In Northern Alberta, growers are particularly concerned about Lygus damage at the early bud stage and thus, may add insecticide to their last herbicide spray. In southern Alberta, some growers are spraying at the late pod stage. Economic thresholds for the early pod stage were developed in Manitoba in the mid-1990s and were based on conventional canola varieties. However, since then a number of new hybrids, including herbicide-tolerant cultivars with superior agronomic traits have entered the market and been adopted extensively. Managing Lygus across regions is complicated by the distinct climatic conditions in the northern Peace Region and southern Alberta, as well as by the differences in Lygus species present and their life cycles relative to crop growth. This project aimed to refine and validate the current Lygus economic thresholds in the context of currently used canola cultivars to reflect specific environmental, biological, and climatic conditions across Alberta. The goal was to enhance Lygus management strategies and reduce unnecessary pesticide use.

Approaches

On-farm trials conducted in southern Alberta during 2012 and 2013 assessed the need to spray for Lygus control at early, mid and late pod stages. A total of 10 canola fields was included over the two years of the study. To compare yield benefits, growers applied a pyrethroid insecticide during different pod stages and left large control plots unsprayed.

Cage studies were used in both southern (Lethbridge) and northern (Beaverlodge) Alberta to validate economic thresholds for Lygus in canola. Individual cages were infested with varying numbers (4, 10, 20, 50, or 80) of Lygus alone, or as a mix of 10 Lygus and 10 cabbage seedpod weevil. Cage studies in Beaverlodge used two newer cultivars of canola (RR7345, L150) and the older cultivar Westar to determine how these varieties respond to feeding from two species of Lygus (Lygus lineolaris and Lygus keltoni). Cages were infested with Lygus at bolting stage and removed for analysis at harvest.

Results

The two growing seasons were very different in terms of moisture and temperature during the canola flowering periods. Dry and hot conditions in 2012 were conducive to large Lygus populations and poor seedset. In 2013, conditions were humid and cool, favouring seed production and reduced Lygus populations.

In on-farm trials, three fields sprayed at early pod when Lygus abundance was higher than 5 per sweep showed a yield increase of about 222 kilograms per hectare (kg/ha). However, there also appeared to be a yield penalty associated with trampling the plants while spraying insecticide. At the other sites, Lygus were less abundant and yield responses to insecticide spraying were highly variable and did not differ significantly, especially when the crops were irrigated. In two dryland fields sprayed at early pod (soon after the end of flowering) there was a trend towards higher yield in sprayed strips than in those unsprayed when Lygus density was around 5 per sweep. Trampling by the sprayer appeared to reduce yields by 55-110 (kg/ha), so the net benefit of spraying is expected to be lower.

In 2012 cage studies in southern Alberta, Lygus numbers were very high inside the cages, reaching over 1000 per cage in the 50 and 80 infestation level treatments. Yields in infested cages were lower than both the control cages with no Lygus and the uncaged plots, and showed very high variability between treatments. The control cages that received no insect inoculation had the highest yield of 1,162 kg/ha. These yields are about 13% lower than the commercial farm average of 1,500 kg/ha for 2012, likely the result of late seeding. Only the two treatments with 50 and 80 Lygus per cage had significantly lower yield than the control cages. The yield in cages with the highest Lygus treatment of 80 Lygus/cage was 730 kg/ha, and 44% lower than the control cages. In 2013, a wet and cool year, there were no treatment effects. These results suggest that growers should not reduce economic thresholds below 1 per sweep as suggested in the current table of economic threshold when canola prices are very high (over $600/tonne).

The cage studies in northern Alberta also support the hypothesis that moderate Lygus bug densities (20 Lygus per meter squared or less than 1 per sweep) do not affect canola yield regardless of the cultivar. At harvest time, Lygus numbers averaged 50-100 per cage for the three canola cultivars, regardless of infestation densities at bud time. Abundances of 100 per cage would translate to about 5 Lygus per sweep. A regression analysis showed no relationship between Lygus density, canola cultivar and yield. The newer cultivars (RR7345 and L150) had higher yield than the older conventional cultivar (Westar), but there was no indication that they were affected differentially by Lygus feeding.

Some preliminary recommendations can be made from these studies about the need to spray for Lygus at the pod stage. The results suggest that irrigated fields are unlikely to suffer from Lygus damage, despite high abundance of Lygus bugs, and growers can consider not spraying irrigated fields once the pods are ripening. Generally, growers are advised not to spray when the crop is at an advanced pod stage (8-10 days from swathing) unless Lygus bugs are clearly at high densities (greater than or equal to 5 per sweep). However, further research is needed to more accurately determine the need to spray for Lygus at this stage.

Following completion of this project, the cage studies will continue for two more years (through March 2016) with support from the Alberta Crop Industry Development Fund, the Alberta Canola Producers Commission and the Alberta Pulse Growers Commission.

For more details about this project please contact Hector Carcamo, Ph.D. 

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