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Pest Scouting 101: Mid-Summer

By Kaeley Kindrachuk, TechAg, Crops Extension Specialist, Outlook

July brings us to the peak of scouting season. Insect and disease pest issues will change annually, but it's best to assess each field separately as crop pests will have varying degrees of populations, severity and scouting windows. Here are some of the things you will want to look for this year.

Cabbage seedpod weevils are one of the first insects to scout in canola crops. These insects are the easiest to scout with a sweep net as the weevils will "play dead" when they are disturbed. Start sweeping when the crop is starting to bud. The economic threshold (ET) are two adult weevils/sweep on average when canola is over $8 per bushel.

Diamondback moth larvae will feed on any green plant material, often causing holes in leaves first, but will move to chewing on buds, flowers and pods. Larvae will wriggle when disturbed and drop off the plant attached with a silken thread. The ET for diamondbacks are a nominal threshold of 100-150 larvae/m2 (10-15/ft2) in immature to flowering plants, or 200-300 larvae/m2 (20-30/ft2) in plants with flowers/pods. A quick way to find and count them is to pull up plants and bang them on the hood of a vehicle.

Economic thresholds for Bertha armyworm on Argentine canola:

Cost of Spraying ($/acre) Expected seed value - $/bushel
6 7 8 9 10 11 12 13 14 15 16 17 18
# larvae/ metre2
6 17 15 13 11 11 10 9 8 7 7 6 6 6
7 20 17 15 13 12 11 10 9 9 8 8 7 7
8 23 20 17 15 14 13 12 10 10 9 9 8 8
9 26 22 19 17 16 14 13 12 11 10 10 9 9
10 29 25 22 19 17 16 15 13 12 11 11 10 10
11 32 27 24 21 19 17 16 14 13 13 12 11 11
12 34 30 26 23 21 19 17 16 15 14 13 12 11
13 37 32 28 25 22 20 19 17 16 15 14 13 12
14 40 34 30 27 24 22 20 19 17 16 15 14 13
15 43 37 32 29 26 24 22 20 18 17 16 15 14
16 46 39 34 31 28 25 23 21 20 18 17 16 15

ET depends on price of grain and cost of spraying.

For bertha armyworms, it is also the larvae that causes crop damage. They will chew holes in leaves or feed on stems and pods. Larvae will drop off the plant and curl up when disturbed. When scouting, beat plants together, and look for them on the ground- be sure to check under leaf litter and even under soil surface. The ET for these insect pests is dependent on the price of the grain and the cost of spraying. Use t Table 1 to determine your ET.

Aphids in peas and lentils are common and may also cause economic damage. Aphids are soft bodied, so they are very sensitive to physical disturbance; heavy rains and strong winds can dislodge aphids and reduce their numbers in a crop. Aphids suck plant sap and will feed at the bases of flowers and developing pods. If there is sufficient moisture, the crop may be able to compensate for some of the fluid losses. The ET for aphids in peas is two to three per 20 cm of plant tip at late-flowering to early-pod stage (or nine to 12 aphids per sweep). The ET we use in lentils is based on the data from North Dakota: 30 to40 aphids per 180-degree sweep, and few natural enemies are present and aphid numbers do not decline over a two-day period. Beneficial insects play a major role in managing insect pest populations so be sure to scout for them as well, especially before spraying pulse crops for aphids. Scouting over a period of a few days for insects is also recommended to ensure that the insect pests are not already being controlled by viruses or beneficial insects. More economic thresholds of insect pests can be found on the Economic Thresholds of Insects page.

This is also a good time to monitor all crops for diseases. Diseases fall into two categories: monocyclic or polycyclic. Monocyclic diseases are those that have only one disease cycle per year, which means that we must assess the conditions for these diseases rather than watch for disease symptoms. Once we see symptoms, it is too late for a fungicide application. Examples of monocyclic diseases include sclerotinia stem rot and fusarium head blight (FHB). Polycyclic diseases mean that the diseases have multiple cycles per year, so scouting for and monitoring disease symptoms and applying fungicide as needed is key. Examples of polycyclic diseases include stem rust, anthracnose in lentil and ascochyta blight in pulses. Remember the disease triangle and that all three factors, the host, the pathogen and the environment, must be present for disease to develop.

Sclerotinia Infographic
Determining percent bloom in canola when determining when apply fungicide for sclerotinia stem rot.

Start scouting pulse crops as the crop canopy begins to close. Different pulse crops are susceptible to different diseases, and they can be infected with more than one. Peas are most susceptible to ascochyta leaf and pod spot, mycosphaerella blight, downy mildew, powdery mildew and white mold; lentils are vulnerable to ascochyta blight, anthracnose, white mold, grey mold and stemphylium blight. It should be noted that high levels of group 11 fungicide insensitivity used for anthracnose in lentils has been identified in Saskatchewan. If you suspect you may have a field that could be affected, consider rotating fungicides this summer. Both peas and lentils are very susceptible to root rots, often caused by a root rot complex. Root rots can infect the plants at any stage, and, there is not much that can be done to control it beyond using a seed treatment. Chickpeas are mainly susceptible to ascochyta blight, and white mold. When scouting all pulse crops, check more than one spot in the field. Be sure to check plant stems and roots as sometimes the top of the plant looks fine but there is more to see under the canopy. More information on each of these diseases can be found on the Saskatchewan Pulse Growers website.

In canola, the main concern this time of year is sclerotinia stem rot. Because sclerotinia is a monocyclic disease, scout for the environmental conditions that favor this disease starting at early flower. The optimum time to apply a foliar fungicide for sclerotinia in canola is at 20 to 50 per cent bloom. The best way to determine this is to count the open flowers on the main stem (photo 2). In addition to environmental conditions, there are several other factors that go into assessing risk such as crop rotation, history of disease in the field, and crop density. The window for fungicide application for FHB is very short, so scouting must take place regularly once the wheat heads start emerging. The spraying window is from when 75 per cent of the heads on the main stem are emerged to when 50 per cent of the heads are in flower (Photo 3). It's also important to remember that fungicides for FHB are only registered for suppression rather than control. Provincial risk maps for FHB found on Saskatchewan Wheat Development Commission's website are updated daily during the growing season.

When evaluating crops for weed control after in-crop herbicide applications, it's important to look for both sprayer misses, and herbicide resistance. Sprayer misses will have a geometric pattern to the weed patches in the field. You will see a sharp, defined edge with straight boundaries and parallel lines the width of the sprayer wheels. You will also see multiple weed species within the missed area. Patches of herbicide resistant weeds won't have the sharp lines indicating a missed application. The boundaries will be less defined. You also may see a general pattern following the combine or other disturbances (e.g. kochia follows the direction of the wind). It is almost impossible to have more than one weed species developing herbicide resistance at the same time.

To get more tips on scouting and visuals on what to look for, register now for crop diagnostic week. For more information on any insects, diseases or weeds, contact your nearest crops extension specialist.

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