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Scouting for Soybean Pests

By: Joel Peru, PAg, CCA, Irrigation Agrologist

Soybeans are an attractive option for Saskatchewan producers to include in their crop rotation. This is largely because of new earlier-maturing varieties, because of their tolerance to excess moisture and because they are a non-host for Aphanomyces root rot.  Statistics Canada reported a jump in soybean acres to 850,000 this year from 240,000 last year. They also report 3.1 million acres across the  provinces, compared to 1.9 million last year. With this large increase in acres, higher disease and pest pressure is likely.

Thistle caterpillar
Photo of a thistle caterpillar in
North Dakota.

The painted lady butterfly caterpillar, or thistle caterpillar, is a pest that has been found in Saskatchewan soybean fields this year (photo 1). This pest is most abundant at the edge of fields and feeds on soybean leaves. Peak damage is seen between the V2 and V3 stages of the plant, although only in extreme cases is chemical control economical. Soybeans are a hearty plant and can take significant damage to leaves with minimal yield loss. Spraying for thistle caterpillar is only warranted if 25 per cent of the leaves have been defoliated (photo 2) or if half the plants in the field contain the insect. Other known pests that can impact soybeans in Saskatchewan include grasshoppers, corn earworm, fall armyworm, seed corn maggots, wireworms, cutworms and aphids. Soybean aphids are an important pest of soybean in other soybean regions, but do not over winter here. They can be brought with southern winds into Manitoba often later in the season and thus are something to scout for on the eastern side of the province.

Levels of defoliation in a soybean field  caused by thistle caterpillar
Levels of defoliation in a soybean field
caused by thistle caterpillar.

Soybeans may be a non-host for Aphanomyces root rot, but they are still susceptible to root rots caused by other pathogens, including Rhizoctonia solani, Pythium spp. and Fusarium species. As a result, it is recommended that you apply a seed treatment to protect developing seedlings from early seedling root rots. Since soybeans are a new crop in Saskatchewan, pressure is relatively low for leaf diseases such as septoria, bacterial blight and downy mildew. It is currently not economical to control leaf diseases on soybeans with a fungicide in northern regions such as Saskatchewan. Soybeans, along with most other broad-leaved crops , are susceptible to white mould caused by Sclerotinia sclerotiorum. To reduce inoculum pressure within the field, include non-host (cereal crops) in the crop rotation. When disease pressure is high and the environmental conditions are favourable for disease development, a registered foliar fungicide can be applied to prevent yield loss.

There are no other major soybean disease issues at the moment in Saskatchewan. However, Phytophthora root rot has been confirmed in both Manitoba and Alberta soybean fields and thus could be present in Saskatchewan. If you suspect phytophthora root rot in a soybean field, please contact to see about having the sample submitted as part of a phytophthora survey. 

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