Your sweep net technique has to match the methods used to establish spray thresholds — or else your counts won’t mean much. Follow these steps to do it right.
STEP 1. GET A 15-INCH NET
Jim Broatch, a Lacombe-based pest management specialist with Alberta Agriculture and Rural Development, says the mouth of the net must be 15 inches to match the net used to set the protocols. In Alberta, sweep nets are available at UFA stores for approximately $65. Or you can buy online at BioQuip (www.bioquip.com)and pay about US$15 for their Standard Series Insect Nets. This does not include shipping and handling. In Manitoba, sweep nets are available from the Manitoba Forage Seed Association in Arborg, and in Saskatchewan, you can get them at Regina-based Prometal Industries Ltd.
STEP 2. START SAMPLING AT THE RIGHT TIME
For cabbage seedpod weevil, sample as soon as the canola enters
Lygus bugs (left) and cabbage seedpod weevil (right) are two common canola pests that you can count with a sweep net.
the bud stage and then continue through flowering. Sampling for lygus starts when the crop is in the bolting stage and must continue through to the early pod stage.
STEP 3. SAMPLE IN 10 LOCATIONS
Sampling is best done on a sunny, dry day when temperatures are 20C or more. Growers may find it convenient to choose their 10 locations along a W or Z pattern through the field. This will ensure that all parts of the field have been covered.
For cabbage seedpod weevil, choose uniform areas. Don’t sample on hills, field edges or solonetzic areas. Populations will always be larger on the perimeter than in the interior of the field.
For lygus bug, samples can be taken near or along the field edges.
STEP 4. MAKE TEN 180-DEGREE SWEEPS AT EACH LOCATION
After every sweep move two steps forward so the previous sweep doesn’t have an impact on the population caught in the next sweep. James Tansey, PhD candidate in the Department of Agricultural, Food and Nutritional Science at the University of Alberta, describes the motion like this: “Sweeping for weevils is a little like using a hockey stick. If you shoot left, grasp the net handle at its end with your right hand and about half-way down the handle with your left hand. Begin the swing as far to the left as you can comfortably reach and end it as far to the right as possible. Turn the net to face left and swing the other direction like a backhand. Righties reverse the process.”
Jim Broatch says that as you swing, the net should be perpendicular to the ground, and it should be held at a consistent height in the crop. Usually the upper edge of the net is lined up with the top of the canopy. Tansey recommends swinging the net hard enough to collect a small amount of plant material. He also suggests giving the net a little shake at the end of each swing to knock the insects captured down into the net bag.
STEP 5. COUNT YOUR CATCH
After your 10 sweeps, you need to count the bugs. A simple way, Tansey says, is to dump the contents of the bag into a large, light-coloured pail. A pad and paper are useful at this point, considering you’ll be counting 10 times. After you’ve counted at one location, dump out the insects completely before you go on to the next location.
STEP6.KNOWWH ATTHE PESTS LOOK LIKE
The cabbage seedpod weevil is ash-grey and three to four mm long with a curved snout.
Lygus bugs are about three mm wide and six mm long. Their col-our can be uniform or mottled, and ranges from pale green to reddish brown to black. They have a clearly marked V shape on their backs.
STEP 7. KNOW THE ECONOMIC THRESHOLDS FOR EACH PEST
While economic thresholds vary according to moisture levels, type of crop, and price of crop, generally three to four cabbage seedpod weevils per 10-sweep field sample of canola will indicate a need to spray. For lygus bugs, the threshold is higher and as the crop matures, it rises. At the end of the flowering stage, thresholds are 10 to 18 lygus bugs per 10-sweep sample. In the early pod ripening stage, the threshold increases to 15 to 22 lygus bugs per 10-sweep sample. Alberta Agriculture has good economic threshold charts on its website at www1.agric.gov.ab.ca.Search for “Economic thresholds of insects attacking oilseeds.”
Accurate, regular sampling gives the grower a clear indication of the level of infestation of the crop and whether or not treatment is warranted.
Patty Milligan lives on a farm near Bon Accord, Alta.