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Fusarium levels low, soybean aphid numbers increasing

Manitoba crop insect and disease update for August 4, 2015

Lygus bug on a canola pod

Plant Pathogens: Levels of fusarium head blight are being reported as low in many areas.

Increased levels of mycosphaerella are being reported in later seeded field peas in the southwest as a result of the cooler and wetter conditions. Some soybean fields are showing increasing damage due to excess moisture and subsequent root rots. Weather conditions have been conducive for white mould in edible beans, and bacterial blight has been reported in some dry beans.

Damage from sclerotinia is noted in some canola fields. Blackleg is also being noted throughtout the province, both stem lesions and basal canker. Sunflower rust is being noted in the Elm Creek area.

Insects: Lygus bugs monitoring continues in sunflowers and later-seeded canola.  Scouting for soybean aphids continues; they are present in many fields but no economic populations have yet been reported.


Soybean aphids have been showing up in many fields, but so far populations have stayed below the economic thresholds. A few things to note regarding economic thresholds for soybean aphid:

  • The economic threshold for aphids in soybeans is 250 aphids per plant, and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages.
  • The reason that “and the population is increasing” is part of the threshold is because the actual economic injury level, where control costs will equal yield loss, is actually about 670 aphids per plant. The economic threshold, where control is suggested, has been set much lower than 670 to allow time for the spray to be applied before increasing populations could potentially reached 670 per plant. The population doubling time for soybean aphids can be as low as about 7 days if they are not being regulated well by natural enemies or weather.
  • Aphid populations do not always continue to increase. So we can not assume that because a certain number is present this week that even more will be present the next week. Populations can plateau or start decreasing due to natural enemies or weather conditions or events.


Crop Stage and Lygus Bugs in canola: It is the early podding stage of canola that is 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.

The economic thresholds for Lygus are actually the same as the economic injury levels, in contrast to the economic threshold for soybean aphid. So for Lygus bugs the economic thresholds can be adjusted for the value of the crop and the cost of the control. At the current value of canola and control costs the economic threshold will be in the range of 10 to 15 Lygus bugs per 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.

To read the full Manitoba Disease and Insect update for August 4, visit the Manitoba Agriculture, Food and Rural Development website.

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