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Time to scout for lygus bugs in canola and sunflower crops

Manitoba Insect and Disease summary for August 3

Summary

Insects: Insects of highest importance to scout for currently are Lygus bugs in canola and sunflowers, and banded sunflower moths in sunflowers. Only trace levels of soybean aphids have been found so far. Although a couple of traps monitoring adults of bertha armyworm had moderate counts in the Northwest, and a few traps had counts in the uncertain risk range, there have been no reports of high levels of larvae of bertha armyworms so far.

Plant Pathogens: Disease surveys for field crops are progressing rapidly and various diseases continue to be reported.

Disease update

Oilseeds: Sclerotinia is being reported in canola, sunflower and flax.

Canola: Root rots due to Fusarium spp and Rhizoctonia spp were reported at Manitoba Agriculture’s Crop Diagnostic lab. Blackleg is also being reported.

Corn: Common rust was found in low incidence in some corn fields. Goss’s wilt is also being found in some fields.

Soybeans: Sclerotinia, downy mildew and brown spot continue to be reported in soybeans.

Crop stages most sensitive to lygus bugs in canola

It is the early podding stage of canola that are most sensitive to potential damage from Lygus bugs. With adequate soil moisture, plants will compensate well for loss of flowers. It is feeding directly to seeds when the seeds are young that has the highest potential to reduce yields if levels of Lygus bugs are high enough.

At the current value of canola and control costs (assuming about $10.00 / bushel for canola) the economic threshold will be in the range of 10 to 15 Lygus bugs per 10 sweeps when the seeds in the lower pods are enlarging to full-size and translucent. When seeds in the lower pods are green the economic threshold would be about 15 to 20 Lygus bugs in 10 sweeps.

Once canola has reached the R5.3 stage, where seeds in the lower pods are mottled green-brown, the seeds are mature enough that Lygus bugs are no longer considered an economic risk. There are no economic thresholds for Lygus bugs in canola at the R5.3 stage and beyond.

A vigorously growing canola crop can be very difficult to walk through and sweep when it gets into the podding stage. If this is the case, samples can be collected from along the edge or at right angles from the edge of the field. Research has shown that samples taken along the edge of commercial fields and at various distances into the field all gave similar estimates of plant bug density. Sampling along the edge reduces effort during years when thick crop growth impedes access to the field. For edge sampling, the area selected for sampling should be at a crop stage similar to that in the main part of the field.

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