The weather has been hot and with the heat, comes the potential for insect problems in our canola fields. I have found bertha armyworms, lygus bugs and diamond back larvae in the fields of the area. If levels get high enough, acres will need to be sprayed to protect yield. The first step to determine whether to spray or not is to find out what is in a field. Each bug has a different technique to doing counts for determining its economic threshold. Knowing the techniques is critical. To determine economic threshold for each insect, I used $13 per bushel for canola. With bertha armyworms, take a scoop shovel out in the field and gently place it under the canola canopy. Take about a meter square of canola plants that are over the shovel and shake the plants vigorously. If there are larvae in the canopy, they will fall out onto the shovel. The economic threshold for this insect is between 15 and 20 bertha armyworm larva with our present spray costs and canola price. With diamond back moths, a producer should take plants from about a 0.1 meter square area. Pull them gently from the ground and take them to the edge of the field. Find something to bang them on. I usually use the hood or endgate of my truck. Count the number of larvae that are present after the plants are banged on the surface vigourously a number of times. If you are using a 0.1 meter square, multiply your number by 10 to get the meter square equivalent. In the early podding stage, the economic threshold is between 100 and 150 plants per square meter. As a plant goes to late podding, the economic threshold goes to 200 to 300 per square meter. Lygus bugs are the most common insect in our area this year. We did have some high numbers before the rain but they probably have dropped. To check for lygus bugs, a sweep net is required. The sweep net needs to be one of a standard design so that the field scouting numbers that are obtained match the numbers of the people that developed the economic thresholds. Also the sweeping technique must be standard. Ten sweeps are taken in a field. The economic threshold for this bug is about 10 to 15 at early podding and moves to about 25 to 30 at late podding. If the economic threshold is found in a field, it should be sprayed. Using the proper insecticide at the proper time will improve the performance of the products in your fields. With any of the techniques that are described, if you are uncertain about how to obtain the economic threshold accounts, find someone who knows how to do it right. Your local agrologist should be able to take you through the steps required to ensure that you reach the appropriate levels.
By Dave Cubbon, P Ag