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Economic thresholds

By Kaeley Kindrachuk, B.App.Sc., AT, Crops Extension Specialist, Outlook

May 2019

Flea Beetle damage on Canola seedlings

Photo Credit: PPMN blog - Soroka & Underwood,
AAFC-Saskatoon

With the start of seeding, brings the start of field scouting. Each year is different in that producers have different insect pests to battle. Unfortunately, there is no way to predict exactly what the upcoming year will bring, but we can try to be prepared if action against a pest is required.

When it comes to making a pesticide application decision, scouting becomes very important. Usually it is the first signs of crop damage, or missing plants early in the spring that will trigger scouting efforts, but not every insect will damage a crop, so it makes sense that we know what is in the field. We need to know which insects that we are seeing will cause damage, which are beneficial insects and which won't harm anything. Methods of scouting for insects will vary- sometimes you will need to use a sweep net (if we are scouting for cabbage seedpod weevil), sometimes just walk a field (if we are looking for grasshoppers), sometimes dig in the soil (if we look for cutworms), or sometimes shake plants (when we scout for diamondback moth larvae). Knowing how to scout, how to identify a pest versus a beneficial insect, and the economic thresholds can assist in decision making during the already stressful growing season.

Economic thresholds for insect pests are the populations an insect pest can get to in a field before a further increase in population will start affecting yield or quality of the crop which results in financial losses for a producer that exceed the cost of controlling the pest. While we have established economic thresholds for many pests, there is still a lot of research going into thresholds for others (for example aphids). There is a cereal aphid app available for producers to use now. It is a free download on the Apple App Store and Google Play and was developed by Agriculture and Agri-Food Canada (AAFC) researchers in Saskatoon. There are some insect thresholds that take into account the price of a commodity and the cost of a pesticide application, so the threshold can fluctuate from year to year (for example bertha armyworm).

Some of the thresholds we will have to keep in mind early this spring include cutworms and flea beetles. While we use seed treatments that have some protection against these pests, the treatment only gives us so many days of protection. With the cooler soils we've had in the past few weeks in some areas of the province, seedlings were growing slower than usual, leaving them susceptible to insect damage after the seed treatment protection had worn off. The threshold for insecticide control for flea beetles in canola is when 25% of the canola cotyledon surface is destroyed and the flea beetles are still present and actively feeding (Photo 1). Cutworms can damage more than one crop, and the threshold will vary depending on the crop and the type of cutworm. In canola, a nominal threshold of a 25-30% stand reduction or 4-5/m2 is used. In cereals, the threshold of 3-4 pale western cutworms/m2 is used and 5-6 redbacked and army cutworms/m2 is used. In peas, the threshold is a bit lower at 2-3 cutworms/m2.

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