Story Posted: August 17, 2011

Canola Watch: Insects, Swath Timing Tips, Harvest Management

Issues of the week: Lygus populations continue to be high and exceed thresholds in parts of Alberta and Manitoba. Many young nymphs are being found and crops are nearing the end of the susceptible stage so management decisions are being made on a field-by-field basis. Pay attention to pre-harvest intervals (time between application and cutting) this late in the growing season. Once a crop is within 7 days of being swathed, no insecticides can be applied. Click here for more information on product pre-harvest intervals.

Hail events throughout the growing season in parts of Alberta and northwestern Saskatchewan have resulted in significant regrowth as fields recover. Many fields now have two or three distinct stages (e.g. some plants beginning to show signs of seed colour change while others are still in full flower) making time-to-swath decisions tricky. No blanket recommendation can be given and growers will have to evaluate on a field-by-field basis.

As we pass the middle of August, the length of frost free days remaining in the growing season comes to mind. Late crops and late stages within fields are being assessed for production potential to help with management decisions, such as insect control and swath timing.

Crop and weather update
Peace: Crop is progressing nicely. Rainfall for the week ranged from a couple of tenths to an inch in the east. Crops continue to deteriorate (rotting) in the west. Swathing is about three weeks to a month away.

Alberta: Warm weather over the past week was needed for crop development however, there are still lots of late crops. Scattered hail fell in north and south central areas. Significant hail damage (as much as 100%) from Barrhead to Vermillion. Crop in the south is about two weeks away from swathing. Drowned out areas in south central Alberta are as high as 15 to 20% in a field based on infrared photos. Alberta Crop Report.

Saskatchewan: Virtually ideal temperatures last week for canola development. Also conducive to thunderstorms with hail (marble sized hail reported around Saskatoon on Monday night). Hard to find a field in the extreme northwest that does not have some level of hail damage. Swathing is well underway in the southwest and will become general across the rest of the province in a week to two weeks. Latest crops are just finishing flowering. Saskatchewan Crop Report.

Manitoba: Rainfall amounts ranged from nothing to an inch with some reports of hail. Approximately 10 to 20% of the canola is swathed with the majority ready to cut this week or next. Latest fields still need a month before they are cut. Report of first fields combined in the Beausejour area which were seeded in April and combined 10 days after swathing.

Lygus – race between insect and crop development
Quite high populations of lygus are being found in parts of the Peace and southern and central Alberta (finding as many as 80 lygus per 10 sweeps with approximately 25 to 30% of those being adults with the remainder a range of instar stages). Sweep net results also seem to be variable. For instance, if fields under irrigation are sampled after the pivot has watered then lygus numbers are significantly lower than if sampled before the pivot watered. Seems the application of water might drive the insect deeper into the crop canopy. Sweeping net sampling is still the best method of determining the potential economic impact of lygus bugs in canola. Thorough scouting on a field-by-field basis must be done to adequately gauge the insect threat. Take a close look to confirm it is a lygus instar and not alfalfa plant bug (since the two can look quite similar). Alfalfa plant bug nymphs will be larger in size and have more angular body shaped (lygus are more compact with rounded body shape). Click here for a photo of both of these insects.

Lygus juveniles usually become numerous at the time canola is mature (so there tends to be a late increase every year). However, this flush seldom requires spraying because the crop is often advanced and the bugs feed on the immature pods at the top of the raceme that do not contribute as much to yield. In a year like this one, if canola is taking longer to mature it may be more susceptible. Lygus development is also dependent on temperature. Research has shown that it would take about 3-4 days for each instar to proceed to the next stage when temperatures are at 25⁰C with cool nights. So if there are mostly young nymphs with no spots on the back (1^st to 2^nd instar) it would take about a week for them to damage the crop. Growers who are finding high numbers of adults and late-instar stages with crops that are still three weeks from cutting may need to spray and may consider spraying earlier. Each field should be scouted and assessed individually since crop staging and lygus staging both need to be determined to assess the risk. Regular scouting will help capture the population of young instars as they mature and move up higher in the crop canopy. Click here for more information on proper sweep net technique.

With lygus, it is the pod that needs to be protected. Research has shown that late-instar nymphs can feed on young developing pods but if pods are mature with ripening seeds then only adults are able to cause damage. Once pods become leathery (a rule of thumb is about two weeks before swathing), then even adult lygus cannot cause damage and control is not economical.

Economic control thresholds for lygus bugs are based on adult and late instar counts. 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 insecticides. This late in the season, a product with a long pre-harvest interval (e.g. 21 days or more) should likely not be used. It is vital that all of Canada’s Canola is Export Ready. Click here for more information on lygus. Click here for more information on product pre-harvest intervals.

Bertha Armyworm still spraying in some areas
Bertha armyworm can be found in most fields in the central Peace with about 25% of the fields exceeding threshold and requiring spraying (around Lacrete and Fort Vermillion). Spraying is also happening in pockets in eastern Saskatchewan (south of highway 16 and east of highway 11) and in northern areas around Delisle and North Battleford. Other areas are approaching threshold and continue to be monitored closely. The threshold for bertha is 17 per square meter when canola is $12 and application costs are $12 per acre. Click here for more information on bertha armyworm.

Click here for more information on how to identify insect larva such as bertha armyworm, imported cabbage worm, and diamondback moth.

Diamondback moth at threshold in some areas
Parts of the Swan River Valley were spraying for Bertha last week and now are contending with diamondback moth. Second generation larva and cocoons are being found in other areas (Arborg in Manitoba and parts of north and eastern Saskatchewan) and these populations continue to be monitored. The threshold for diamondback larvae is 200 per meter square at today’s canola price.

Cabbage seedpod weevil
Numbers of adults in southern Alberta range between 3 to 4 per sweep but the majority of fields are completely out of flower. Spraying this late in the season is not economical because the adults that are caught in sweeps now are 2nd generation and they have already done their damage.

Imported Cabbageworm some larva, lots of adults
Cabbageworm larva populations are quite heavy in the central corridor on the western side of Saskatchewan and unprecedented amounts of adults can be found around Elstow. As long as there is good vegetative growth, they feed mostly on leaves and it is not usually economical to spray them.

Grasshoppers migrating to canola
Parts of the Peace are seeing grasshoppers migrate from other crops into field edges of canola. Economic threshold for grasshoppers in canola is 7 to 12 per square meter.

Effect of cooler temperatures on canola
Cooler overnight temperatures (in the neighborhood) of 1 to 2 C occurred overnight Monday into Tuesday in parts of Alberta. These cooler temperatures will not affect canola crops significantly. Growth chamber research shows that seed enzyme effects could be impeded with temperatures at 1 C which resulted in higher green seed. Any impact these low temperatures may have in field conditions will be insignificant and no management decisions should be based on these temperatures. Lowest temperatures tend to occur in low lying areas which are often the areas with highest production. This year is an exception since many low lying areas are the areas with poorest production.

How long from 30 to 50% seed colour change
Seed colour change is an indication of moisture dry down. The following is an estimate of canola seed dry down rates during ripening based on research of weather conditions during mid-August to early September in different areas across the Prairies. Under cool, wet weather moisture loss in the seed will be less than 1% per day, and may even gain moisture with rain. On an average early fall day, moisture loss may be about 1 to 2% per day. In areas with cool nights (e.g. foothills, Parkland transition zones), and later in September, more typical dry down would be 1 to 1.5% per day. Under warm to hot and windy conditions, moisture loss can be as high as 2 to 3% per day.

A rough guide is that a 10% increase in seed color change occurs with about 5% moisture loss. So the period from 30% seed color change to 50% seed color change would need a 10% drop in moisture. This may take 10 days in cool weather to only 3 days in hot windy weather. Click here for more information on canola maturation and swathing.

What if frost does happen?
As crops near maturity, the risk of frost is watched closely especially this year with late crops and late stages within fields. If a grower wants to cut a relatively late crop to avoid frost damage, consider the following:

• A forecast for damaging frost that could justify early swathing is in the neighborhood of -3 to -4 C. Forecasts for light frosts may not actually occur or affect the majority of the field, and thus, would not justify early swathing.
• It is important to cut at least two days before the damaging frost occurs.
• The weather during those two pre-frost days needs to be conducive to adequate dry down.
• Freezing temperatures cause ice crystals to form in water-containing tissues of the plant. As plants mature, there is less moisture within the plant, so less frost damage occurs. Click here for more information on average first fall frost in Manitoba, Saskatchewan, and Alberta.

Disease Update
Relatively low levels of sclerotinia infection can be found in most areas except in heavy and late maturing crops in northwest Saskatchewan where disease pressure is high due to frequent rains through the flowering period. Higher amounts of alternaria are occurring in B. juncea and B. rapa in western Saskatchewan and in B. napus fields in eastern Manitoba. In eastern Manitoba, more blackleg is appearing.

Scouting and identifying disease(s) present can be a valuable tool for assessing this year’s production practices and preparing for next years. Here’s what to look for with blackleg: symptoms showing up at this stage of the season are greyish lesions on the stem with the black picnidia spots, and also basal lesions (cankers) — 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 blackened infection. This infection will eventually grow through the stem, cutting off nutrient flow. If you see plants drying up, cut a few open at the base of the stem with clippers to check. With sclerotinia look for white lesions or fuzzy rot on the main stem or on branches. Sometimes the plant may look healthy at the base, but there will be dead branches. Infected stems and branches will have white-grey tissue (not green) and will often start to shred apart easily. Click here to see photos of sclerotinia and blackleg infection.

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.

Tips to time swathing
Swathing is about to become general in Manitoba and proper timing is tricky in parts of Saskatchewan and Alberta because of variable staging. The following are some tips to help determine optimal time to swath:

• Hail damage causing crops to mature unevenly. When swathing a hailed crop, assess the seed-colour change at the highest-producing parts or stages within the field. Consider the yield potential of the earliest stage, especially if the delayed maturity creates a high risk for fall frost damage. Click here for more information on how to handle variable staged crops.
• Scout fields individually and often. Scout swath timing by breaking open pods and assessing the level of seed colour change. Scouting based on field colour change is not a good indication of seed maturity. Maturity can change quickly and it may help to open up the field with a swath cut around the perimeter to grasp where the field is at. Scout fields individually since varieties may respond differently and different plant populations will influence the amount of branching. Click here to watch a video about assessing crops for swathing time with Canola Council agronomist Jim Bessel.
• When to swath. The best timing for yield and quality is to cut when 50% to 60% of seeds on the main stem are turning from green to brown. Click here to view a copy of the Canola Council of Canada’s Canola Swathing Guide.
• If need to swath earlier. Fields that have extremely late stages may benefit from being cut earlier (e.g. 20 to 30% seed colour change) to prevent green seed issues later on. Remember that these fields will take longer for the swath to dry down. If possible it is better to leave the crop ripen longer and swath at night to reduce shattering losses.
• Avoid swathing in mid-day heat. Swathing during the heat of the day may result in green seed issues because the enzyme that breaks down chlorophyll can be interrupted under high temperature conditions. Be prepared to swath in the evening, overnight, and early morning if high daytime temperatures occur. For more information, click here for a pod cast on time of swathing with Canola Council agronomist Dan Orchard.
• Lay swaths parallel to the direction of prevailing winds. This reduces the risk of swaths blowing and pods shattering.
• Careful how the swath roller is adjusted. Swathing widths have increased in recent years to 30 feet and beyond. A heavy crop cut at thirty feet or more needs only gentle downward pressure on the middle of the swath to be anchored properly. Air movement within the swath is important during dry down. The swath roller should be adjusted to tuck swath edges into the stubble.
• Large dense swaths take longer to condition. Large swaths (wide swath width with heavy crop canopy) with a large volume of material take longer to cure than smaller, narrower swaths. Thin swaths lying close to the ground may also take longer.

If you are in the Fairview area, click here to learn about a time of swathing crop walk with Canola Council agronomist Greg Sekulic.

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