By Shannon Chant M.Sc. PAg., Regional Crops Specialist, Southwest, Regional Services Branch
In the past number of years my colleagues and I have spent a couple weeks in late May to early June surveying for pea leaf weevil damage in the western and southern parts of Saskatchewan.
When we were out surveying this year we noticed that there was a lot more damage than usual and the insects were unusually easy to find in some fields. Pea leaf weevils are nocturnal and will fall off the plant and play dead if they detect people nearby so they are normally difficult to find when out surveying. During surveying, staff counts the number of notches on the plant and if there is damage to the clam leaf on 50 plants per field.
Pea leaf weevils overwinter in Saskatchewan and the milder winter this past year is likely a big part of the reason for the higher numbers seen this year. The economic threshold for applying an insecticide is three out of 10 plants with a notch in the clam leaf (top leaf) of the pea plant before the six node stage. Unless defoliation on a plant is severe, the plant can usually compensate and outgrow the damage to the leaves. The major damage comes from pea leaf weevil larvae that feed on the nodules on the pea roots and disrupt or stop nitrogen fixation.
Most peas are past the six node stage now but if you noticed a lot of damage to your pea crop and the crop is showing signs of nitrogen deficiency you can top-dress nitrogen to help meet the pea crop’s nitrogen needs. Pea crops require an average of 3.1 pounds of nitrogen per bushel. To calculate the amount of nitrogen your pea crop needs, multiply the expected yield for pea on your farm or in your area in bushels and multiply by 3.1. Some nitrogen will be supplied from the soil and you subtract the nitrogen that you had in your soil test from the total nitrogen needed to determine the amount you need to apply. Remember that different fertilizer sources have different levels of efficiency and that will need to be accounted for when calculating the amount of fertilizer to apply.
Table 1. Average pea yields (bu/ac) in a few southwest Saskatchewan RMs (2005-2014) and the nitrogen required (lbs/ac) to obtain that yield.
| RM number | Average yield (bu/ac) | Nitrogen needed (lbs/ac) |
| 78 | 28 | 87 |
| 106 | 25 | 78 |
| 111 | 36 | 112 |
| 135 | 29 | 90 |
| 137 | 32 | 99 |
| 166 & 168 | 31 | 96 |
| 228 | 33 | 102 |
| 229 | 34 | 105 |
For more information on pea leaf weevil and other cropping questions, contact your local agronomist or Saskatchewan Agriculture Regional Crops Specialist or call the Agriculture Knowledge Centre at 1-866-457-2377.
Shannon Chant M.Sc. PAg.
Regional Crops Specialist
Southwest, Regional Services Branch
