Manitoba crop insect and disease update

Conditions as of July 21, 2015

Levels of armyworms are starting to decline in some fields as they turn to pupae. Currently, scout for grasshoppers around field edges. In corn, now is the time to be scouting for European corn borer. Also consider scouting for aphids in cereals and pulse crops.

Pulse Crops

Soybean aphids: Soybean aphids have just started to show up in a few fields in Central Manitoba. Levels are currently very low in fields where they have been found. The economic threshold for soybean aphids in soybeans is at least 250 aphids per plants on average and the population still increasing, and plants in the beginning bloom to beginning seed growth stages. So this is something to watch for when scouting soybean fields, but still a long way form economic levels.

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Soybean aphids do not overwinter in Manitoba and are blown in. We usually don’t have soybean aphids detected in Manitoba until some point in July. So one thing to note is that seed treatments containing insecticides likely won’t provide protection against soybean aphids in Manitoba, as they would not be at effective levels by July based on trials on length of protection done in the US. So regardless of seed treatment used, it is good to monitor soybeans for aphids this time of year. But soybean growers are cautioned against treating sub-economic levels of soybean aphids as natural enemies, once established, can sometimes cause aphid populations to plateau or decline.

Aphids on peas: It is common to find pea aphid on peas this time of year. Aphids on peas have been reported from some area of western Manitoba, however there are no reports of control being needed yet. The economic threshold for aphids on peas is if, at the beginning of flowering, there are 9 to 12 aphids per sweep or 2-3 aphids per 8-inch (20 cm) plant tip. If this is surpassed an insecticide application when 50 percent of plants have produced some young pods will be cost-effective.

Most of the damage that aphids do to peas is to the pods before they start to fill. If most of the pods have already started to fill, spraying would be too late and would not be economical.

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