Canola Watch #17

Four the week

Lygus schmygus. Sweep net counts approaching thresholds have been reported in many areas across the Prairies, but high counts may not require a spray if canola is growing vigorously and has good moisture conditions.

Low bertha. Trap counts for bertha armyworm adults are fairly low across the Prairies, except for a few hotspots in central Alberta. This suggests a minimal threat of heavy worm damage overall, but don’t let your guard down completely.

Diamond backtrack. Beneficials seem to be doing a good job on diamondback moth larvae. Unless you see significant pod feeding and counts exceed 20-30 per square foot at a few random sites in the field, spraying will not likely provide an economic return.

Why the miSing podS? Six leading factors can cause blanks up the stem. Sulphur deficiency is one of them, and could be a leading contender this year.

Mature bertha armyworm. 


Map of the week

This map from StatsCan’s Crop Condition Assessment Program uses NDVI satellite imagery to show crop growth. This map compares growth to July 21 this year with the same period last year. Yellow indicates that growth is about the same as last year. Light brown is behind. As you can see, crop growth for most of the western Prairies is around the same pace as last year while most of the eastern Prairies is behind.

July 30 Quiz

How well do you know your green worms? See if you can identify alfalfa looper, bertha armyworm, cabbageworm and diamondback moth larva. Photo credit: Wes Anderson

Lygus thresholds


Include lygus nymphs, which have telltale black dots, when doing sweep net counts for lygus. Source: Roy Ellis

Threshold tables for lygus suggest, for example, that if canola is $12 per bushel and spray costs $8 per acre, the threshold at the early pod stage is 5 lygus adults or late instar nymphs per 10 sweeps.

Current thinking is that 5 lygus per 10 sweeps (0.5 per sweep) is too few to warrant a spray, and that the economic threshold is likely quite a bit higher. This is certainly the case for canola growing strong with decent moisture.

Wise and Lamb acknowledged this in their research paper used to set the lygus threshold. In “The Canadian Entomologist 130” pages 825-836 in 1998, they wrote: “When precipitation is greater than 100 mm from the onset of bud formation to the end of flowering, the crop may partially compensate for plant bug damage.” Recent work confirms that a fairly large population of lygus has almost no effect on yield in canola with good yield potential and ample moisture, but that same population can cause significant yield loss in a stand with moisture stress and low yield potential.

Rough estimates suggest that 5 lygus per sweep (50 per 10 sweeps) at pod stages could cause a 2 bu./ac. reduction in yield, generally speaking. At $12 canola, a spray at this threshold could save $24 worth of canola, but your time and expenses must be paid for from these two bushels.

An $8 spray would provide an economic return in this case, but does an $8 budget account for all costs? For instance, does it account for 0.5 bu./ac. in lost yield from trampling by the ground sprayer? Killing beneficial insects with the spray is another cost.

Pods are the focus. The most vulnerable crop stage for lygus feeding is after flowering and when seeds are enlarged on lower pods. (This would be around stage 5.2 for a dense stand and 5.1 for a thinner stand.) After sweep netting, look for sticky sap spots on pods, pedicels and stems before making the spray decision. Oozing on pods suggests active feeding. If sweep net counts exceed thresholds, but there is no evidence of pod damage, growers could decide to hold off on spraying.

Lygus thresholds for the bud and flower stages are not needed because economic loss is rare at these stages. Research shows that low level lygus feeding at flowering can actually stimulate yield. The only situation where spraying may be warranted at flowering stages is if numbers are so high that large areas of the crop are not coming into flower because the flowers are being eaten.

Situations to consider:

Rainfall: Rain improves crop growth, lowering the risk from lygus. A heavy rain at podding will also knock lygus to the ground, where ground beetles and other beneficial insects can eat them.

Other pests present: If lygus are at or above thresholds and other pests, such as diamondback moth larvae, are also feeding on pods and are near thresholds, this combination improves the potential economic return from a spray.

Lygus and cabbage seedpod weevil.

The decision to spray for cabbage seedpod weevil is made at early flower. This is too early to see an economic benefit from a lygus spray, so don’t let lygus in the sweep net at CSPW spray timing influence the CSPW spray decision. Make a spray decision based on CSPW numbers only. Spin off benefits in terms of lygus control may occur but cannot be predicted or expected. The adult lygus you find at flower are not the ones that cause economic damage to the crop at the pod stage. The damaging lygus — the older nymphs and resulting new adults — belong to the second late summer generation whereas the previous adults at flower belong to the first late spring-early summer generation.

Source: Alberta Canola Producers Commission