Story Posted: August 10, 2011

Canola Watch: Insects, Disease, Pre-harvest

Issues of the week: Insects are the hottest topic this week, with managing uneven maturity at swathing and harvest a close second. Before spraying those insects, make sure the target insect is properly identified and check that it has reached economic control thresholds throughout the field. If you need to spray, evening spraying will avoid high temperatures that could adversely affect insecticide performance. It will also reduce the impact on bees that are more actively foraging during the day.

While scouting for insects, take some time to check the disease situation. Clubroot, sclerotinia and blackleg are showing up at high levels in some regions. A least one field near Vermilion, Alberta, has heavy clubroot infection, indicating the continued eastward movement of the disease. Properly identifying what diseases are present and at what levels will help with management in future years.

Know the pre-harvest interval for any insecticide or fungicide you plan to use between now and cutting. "Cutting" is the key word. If the pre-harvest interval for a product is 21 days, it needs a minimum of 21 days between application and cutting (swathing or straight combining) to break down or dissipate sufficiently to avoid residues on or in the seed. Click here for a list of products and their pre-harvest intervals.

Crop and weather update
Peace (B.C. and Alberta): Rain fell across the whole region in the past week, with accumulations of three tenths to half an inch. Many areas still have standing water in fields. Earliest canola in the north is about three weeks from cutting. Late fields in the south are just coming out of flower and will need frost to hold off until mid to late September.

Alberta: Early crops will be ready to swath in a week or two. Late crops are still flowering. Most of the province has enough moisture, except pockets in the south that could use rain. Alberta crop report.

Saskatchewan: Swathing has started in southern regions, but the swathing period could last for six weeks across Saskatchewan given the wide range of crop maturity. The south has been fairly hot and dry. The north has had variable precipitation, with severe hail in some northwest locations. Saskatchewan crop report.

Manitoba: Rain ranged from less than a tenth to over an inch across the province. Over 2" of rain and hail hit the Deloraine area. Less than 5% of the crop is swathed but this should be a busy week of swathing. 15 Aug 8 Crop Report.

Which worm is it: bertha, diamondback or cabbage?
Many growers are spraying for bertha armyworm, even though a forecasting program based on collecting adult moths in pheromone-baited traps had reported low bertha armyworm moth counts in most areas. This shows the benefit of field scouting in addition to forecasting models. Forecasting programs can alert us to areas of higher risk, but often the levels of larvae in individual fields can be higher or lower than expected based on these regional predictions. For example, female moths are attracted to flowering canola when laying their eggs. Even if overall moth counts are low, they could be concentrated in the earliest canola crops that were flowering during egg laying. Presence of predators is another reason. Lady beetle adults can consume large numbers of bertha armyworm eggs, and if they're not present, more eggs will hatch.

Before spraying, make sure the worms are actually the species you're trying to control. Here's how to tell the difference:

Bertha armyworm. Berthas are shinier and the yellow-orange stripe along each side is more pronounced, even on younger ones. Mature larvae are up to 1.5" long, making them considerably larger than diamondback moth larvae at late stages. Being part of the cutworm family, they also curl up when agitated. While cabbageworms tend to stay on the leaves as long as leaves are available, bertha will move up into the pods earlier. The threshold for bertha is 17 per square meter when canola is $12 and application costs are $12 per acre. Click here for more.

Diamondback moth larvae. When agitated, diamondback moth larvae will wiggle backwards and dangle from the plant on a thread. Diamondback moth larvae are spindle shaped - fatter in the middle than at the end. They also rarely grow beyond 12 mm (0.5") in length, versus bertha larvae which can reach 4 cm (1.5"). The threshold for diamondback larvae is about 200 per metre square at today's canola price.

Imported cabbageworm (larvae of cabbage butterfly). These worms are light green with a lemon-yellow stripe down the middle of the back (dorsal side). They are covered with short hairs giving it a velvety appearance. They grow to about 30 mm. Cabbageworms look more like diamondback moth larvae than bertha armyworms, but are more docile than diamondback moth larvae and less spindle shaped. As long as there is good vegetative growth, they feed mostly on leaves and it is not usually economical to spray them. Some growers have reported 8-10 per plant, but before spraying, check regularly to see if damage ceases over the next week. Cabbageworms have 2-3 generations per year and will go into pupal stage quicker than the other two larvae. Most cabbageworms in southern Alberta are infested with parasitoids, so they feed less and die before pupating.

Scout throughout each field. Insects may be concentrated only in certain areas in the field, while the rest of the field is not seriously affected. A blanket spray of the whole field may not be warranted. Larval numbers can also vary substantially between fields in close proximity, depending on crop staging and the attractiveness of the field at egg laying.

	Cabbageworms (top) and diamondback moth larvae (bottom) can be found in the same field, but cabbageworms tend to stay on the leaves while diamondback moth larvae will feed on pods.

	Diamondback moth larvae feeding on pods.

	Mature bertha armyworms come in various colour shades but will all have the yellow stripe down the side. Source: Saskatchewan Ministry of Agriculture.

	Bertha armyworm at a developing stage. Source: Saskatchewan Ministry of Agriculture

	Imported cabbageworm. Source: Wes Anderson, Pioneer

	Diamondback moth larvae. Notice the spindle shape - wider in the middle than at the ends. Source: Saskatchewan Ministry of Agriculture

	Diamondback moth at its various stages. Source: Saskatchewan Ministry of Agriculture

Tips to control multiple insect species at once
Some canola fields have lygus bugs, cabbage seedpod weevils, bertha armyworms, diamondback moths and grasshoppers. Economic spray thresholds are calculated based on individual species. We don't have scientific evidence to combine thresholds, but where an "additive" effect of multiple species can make sense is when more than one species are feeding on key yield-producing areas - flowers, buds and pods - at the same time.

For example, if you have lygus bug and cabbage seedpod weevil in the same field and if they are both at 50% to 60% of their economic spray thresholds, spraying may provide an economic benefit.

Cabbage seedpod weevil counts low but damage high. In southwest Saskatchewan, quite a bit of cabbage seedpod weevil damage has been reported even though sweeps have not shown high numbers. Why? Wind may have pushed the weevils down into the canopy during the day, throwing off the sweep counts. Maybe another insect was also at work. Or maybe the thresholds need to be adjusted for specific environmental or crop conditions. If you are in this situation, check out the sweep net techniques provided in this week's Canola Watch to see if any differences with your technique may have affected your numbers. If counts versus damage still have you scratching your head, a second opinion may be warranted.

How to sweep net properly
Thresholds for lygus bugs and cabbage seedpod weevil, for example, are based on a specific sweep net technique. If you're not doing a complete 180° pattern, walking while you sweep, and keeping the net near the top of the canopy, your counts could be significantly different from someone using the recommended techniques.

Here are the sweep net techniques outlined in the Canola Council of Canada's Canola Scouting and Sweep Net Insect Identification Card.

• Use a standard 38 cm diameter net.
• Sample preferably when the foliage is dry. If the foliage is wet small insects may stick to the inside of the sweep net bag making it difficult to identify them and giving you an inaccurate count.
• Hold the net with the hoop end nearest to the ground in front of you. The plane of the hoop should be perpendicular to you.
• Tilt the net opening so the lower edge of the rim is slightly ahead of the upper rim.
• Swing the net from side to side in a full 180-degree arc. Sweep one stroke per step as you casually walk through the field or down the row.
• In short vegetation, swing the net as deeply as possible. In taller vegetation, sweep only deeply enough to keep the upper edge of the sweep net opening even with the top of the plants. In general, don't let the net go more than 25 cm below the top of the plants.
• Each passage of the net is considered one sweep.

When to swath multi-branched canola
The ideal time to swath for yield and quality is when 50-60% of seeds on the main stem are turning color. At this stage, most seeds on the side branches will have reached maturity and will ripen in the swath.

This may not be a safe assumption when very low plant counts result in heavily branched canola plants. To make sure most of the pods on these side branches are mature, remove seeds from the latest pods and roll them around between your finger and thumb. If they roll, they have a chance of ripening after swathing. If they turn to mush, these seeds will not mature if swathing at that stage. Waiting longer to swath, if possible, could benefit this crop - as long as most of the yield potential is deemed to be in these later branches.

Growers with many canola acres to harvest will not likely be able to swath all canola at 50-60% seed colour change. CCC research shows that canola swathed at 30-40% seed colour change on the main stem yields about 8% less than canola 50-60% seed colour change, but 30-40% seed colour change is a good starting point for growers with a lot of canola to harvest. (See the graphs below.) Going earlier is not recommended. Starting at 10 to 20% or earlier could lead to lower yield as well downgrading or harvesting delays from green seed, especially if warm and windy conditions promote rapid dry down. This will not give the chlorophyll clearing enzymes time to complete their job.

For fields with uneven maturity. Delay swathing of the least mature areas or plants until there is at least some seed color change in the lower pods (and firm green seed in the least mature pods), provided this does not put the crop at risk of frost damage or excessive shattering of the ripest plants. Here's the key: Don't put the best and biggest part of the crop at risk of shelling in order to save a few acres of later, higher-risk and possibly low yielding canola.

Click to download this PDF: Swathing and Harvesting Multiple Stage Crops
Click here for a Canola Swathing Guide
Click here for a video

	Summary results pooled for both 3 and 5 lb/ac seeding rates at all locations based on similar trends observed. Straight cut treatments at 10 of 12 locations produced relative yield of 107% vs 30-40% SCC.

Apply pre-harvest glyphosate only after seed moisture drops below 30%
Growers considering a pre-harvest glyphosate cannot apply it to canola (RR or non-RR) until seed moisture drops to 30% or lower. This roughly coincides with 30% seed colour change. Spraying earlier raises the potential for glyphosate residue in seed. Pre-harvest glyphosate will kill green weeds and even out the crop for straight combining, but it can take up to 3 weeks to do its job under cool fall conditions.

But what if the grower hasn't sprayed at all? Some growers fighting excess moisture are still trying to get their first glyphosate application on their Roundup Ready canola. They'll now have to wait until after seed has less than 30% moisture - for the reasons stated above.

Glyphosate is not a desiccant. Glyphosate will even out the crop and allow weeds to die and dry naturally for easier straight combining, but it will not make the crop mature faster. Premature application of glyphosate will simply increase the risk of excess residues, which could impact acceptability of the crop in foreign markets, and terminate seed filling prematurely, resulting in shrunken seeds with higher green count. Click here for more on pre-harvest herbicide.

Cleavers. Some growers in eastern Saskatchewan who didn't get a chance to spray through the growing season because of excess moisture are seeing very large cleavers poking up through the canopy. Pre-harvest glyphosate may stop a small proportion of seeds from reaching maturity, but many of the seeds are likely already mature enough to remain viable. Unfortunately it is likely too late to avoid issues with cleavers seeds in dockage or back on the ground after harvest.

Lygus no threat when pods get leathery
Economic control thresholds for lygus bugs are based on adult and late instar counts. As an example, when canola prices are $12 per bushel and application costs are $12 per acre, the threshold is 11 adults or late instar bugs per 10 sweeps.

Before spraying, consider the pre-harvest interval for the chosen insecticide. Products with the shortest pre-harvest intervals still require 7 days between spraying and cutting, so if the field is within a week of swathing, then spraying is no longer an option. In fields that close to cutting, many pods are hardening and lygus will not be able to penetrate and cause damage anyway.

How to scout for clubroot
Clubroot infestation has been spotted in a field near Vermilion, Alberta. This suggests a continued eastern movement of the disease. Growers in Alberta or western Saskatchewan are encouraged to examine strange disease patterns to see if clubroot has arrived on their farms.

Clubroot-infected plants can look similar to plants under heat stress or infected with blackleg or sclerotinia. For that reason, proper diagnosis of clubroot should always include digging up plants to check for gall formation on roots.

• Start at field entrance. Pull up 10 random plants, remove the soil and examine roots for evidence of galls.
• Walk 10 paces into the crop and pull another 10 plants.
• Turn 90 degrees and walk another 10 paces. Sample this way in a zig-zag pattern until 100 plants have been sampled. Record the percentage of infected plants.
• When sampling, do not be selective in choosing plants. Do not look for sick plants. Make sure the plants sampled are random.

When unsure if a field has clubroot infection, then it is appropriate to sample affected plants to first identify the disease. Then use the above technique to quantify the level of infection. The top photo shows canola roots with young clubroot galls. The photo at the bottom of this article shows developed galls.

Yield losses due to clubroot are about half of the percentage of infected stems. For example, if 10% to 20% of plants are infected, yield loss will likely be around 5% to 10%.

No control products are registered for clubroot in canola. The recommendation is to rotate out of canola for four years in slightly infested fields and seven years in severely infested fields. Control volunteer canola and susceptible weeds (mustard family, dock and hoary cress) in the rotational crops. And consider a clubroot-resistant hybrid the next time canola goes on that field.
Click here for more on identification, prevention and management of clubroot.

	Canola with small developing clubroot galls. Source: Stephen Strelkov

	Developed clubroot galls. Source: Stephen Strelkov

Is that sclerotinia or blackleg?
Growers seeing stem lesions or pre-mature ripening should take a closer to identify the cause. There is a good chance it's sclerotinia. But it could be blackleg. (Or clubroot, as noted in the previous article.)

It's important to identify the cause because it will help in management for next year. Proper fungicide selection, variety selection, crop rotation intervals and weather response are different depending whether the problem is sclerotinia or blackleg. Here's how to tell the difference:

Blackleg: Symptoms showing up in Manitoba at this stage of the season are greyish lesions on the stem with the black picnidia spots, and also basal lesions - which are the most serious. Basal lesions can't really be properly identified until you cut through the base of the stem and look for greyish infection. This infection will eventually grow through the stem, cutting off nutrient flow. If you see plants drying up, cut a few open to check.

Sclerotinia: Look for fuzzy rot on the lower branches. The plant may look healthy at the base, but there will be dead branches. Infected branches will have white-grey tissue (not green) and will often start to shred apart.

	Blackleg on canola stem. Source: Anastasia Kubinec

	Cross section of stem with basal blackleg lesion. Source: Anastasia Kubinec

	Sclerotinia lesion on stem. Source: Anastasia Kubinec

	Another look at sclerotinia infection. Source: Anastasia Kubinec

Keep malathion out of canola bins
Malathion CANNOT be used to treat bins where canola will be stored or to treat canola as it goes in to storage. These applications can result in residues in the canola that are unacceptable in some of Canada's key export countries with low minimum residue limits (MRLs) or zero tolerance for malathion. If a bin was treated previously, do not store canola in the bins within six months of treatment. Detection of malathion residue in canola seed above the allowable limit will result in rejected shipments and increased monitoring.

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