"As tempting as it is to take a break once herbicide sprays are complete, growers need to continue scouting their canola fields. Otherwise, they may miss the opportunity to prevent significant damage from late season insect pests," warns Derwyn Hammond, Canola Council of Canada agronomist for Manitoba.

By the time canola reaches the bolting stage, growers have made a significant investment in the crop. Field scouting every few days from bolting through swathing will go a long way in preventing late season pests from stealing the returns on that investment, adds Hammond.

Some pests to watch for after bolting include bertha armyworm, diamondback moth, lygus bugs, cabbage seedpod weevil, and grasshoppers.

High bertha armyworm or diamondback moth numbers in late May through early July indicate the potential for a large hatch of damaging larvae from mid June through August. Track moth counts through provincial agriculture websites or representatives. However, high moth numbers do not always mean control of the larvae will be necessary, he says.

For accurate bertha armyworm larval estimates, sample at least three locations per field, a minimum of 50 m (164') apart and at least 20 m (66') from the field edge, and determine the average number of larvae. At each location mark out 1 m2 (10 ft2) and beat the plants within the area to dislodge the larvae. Then count the larvae on the ground, carefully searching the soil and leaf litter to find any larvae hiding in cracks or under leaves.

Monitor diamondback moth larvae by removing the plants from a 0.1 m2 (1 ft2) area, beat the plants onto a clean surface and count the number of larvae dislodged. Repeat this procedure in at least five locations within each field, says Hammond.

The economic threshold for bertha armyworm varies with the cost of the insecticide application and the value of the crop, but generally ranges from 10 to 34 larvae/m2 (8 to 28 larvae/yd2). For a more detailed decision table, consult page 1011b of the 2003 Canola Growers Manual. An average of 200 to 300 diamondback moth larvae/m2 (20 to 30/ft2) warrants an insecticide application. Consider spraying at 200 larvae/m2 (20/ft2) if the leaves are drying up, forcing the larvae to feed on the pods, and at 100 to 150 larvae/m2 (10 to 15/ft2) at early flowering.

Hammond advises growers to use a sweep net on canola from bud stage through to pod ripening to monitor for pests such as lygus bugs and cabbage seedpod weevil. Use a 15" (38 cm) diameter net and sample when the foliage is dry. Tilt the net, keeping lower edge slightly ahead of the upper rim, and swing the net in a full 180-degree arc with the upper edge even with the top of the plants. Then take a step, reverse the net and take a sweep in the opposite direction. Continue until 10 sweeps are completed, and then count the insects collected in the net. Repeat this procedure in 10 to 15 locations in the field.

The economic threshold for lygus bugs ranges from 10 to 21 lygus per 10 sweeps at the end of flowering, and increases to 15 to 29 per 10 sweeps at pod ripening. Consult the decision tables on page 1024b of the 2003 Canola Growers manual to determine the appropriate threshold. For cabbage seedpod weevil, an insecticide application is warranted if three to four of the small (3 to 4 mm--0.12 to 0.16" long) ash-grey adult weevils are collected per 10 sweeps.

Hammond says many areas are at high risk for grasshoppers this year, as can be seen on the forecast maps available at http://www.canola-council.org/production/2004_grasshopper.html. While grasshoppers normally favour cereals over canola, when numbers are high they will eat virtually anything green. When scouting for grasshoppers use 'foot-square' counts to rate the field. Start at one corner of the field and walk diagonally past the centre of the field, turn and walk straight out to one side of the field. "Make 20 'foot-square' counts while walking by estimating the number of grasshoppers that jump from a 1 ft2 (0.1 m2) area as you walk toward it. Divide the total number of grasshoppers from the 20 counts by 2 to estimate the number of grasshoppers per m2 (10 ft2)," says Hammond.

There is no confirmed economic threshold for grasshoppers in canola, but preliminary research suggests it's probably equal to slightly higher than the 8 to 12 m2 (7 to 10/yd2) for cereal crops. For more information on identifying pest species, refer to the website by Dan Johnson at the University of Lethbridge: http://people.uleth.ca/~dan.johnson/htm/forecast04.htm.

For more information on identifying specific pests, growers can order the 2003 edition of the Canola Growers Manual from the Canola Council or check out the insect page of the manual on the Council website at: http://www.canola-council.org/production/caninsec.html

Canola has less opportunity to recover from damage late in the season. An effective scouting program will allow growers to prevent that damage and maintain the crop's profitability, Hammond concludes.