Canola Watch #22
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Issues of the week
Bertha armyworm larvae have started to feed in some locations, and the second generation of lygus has arrived in the southern Prairie. Watch pods and upper foliage for signs of feeding, and scout all fields.
While scouting, also look for signs of blackleg, clubroot and foot rot. Late season foot rot has shown up in some fields, with stems pinched off at the soil surface. The window to apply sclerotinia stem rot fungicide has closed for most canola fields. Click here for last week’s article on late applications.
Finally, we say good bye this week to Doug Moisey, who has moved on to a new opportunity. Doug was with the Canola Council of Canada’s agronomy team for 18 years, and has been our eyes, ears and flag bearer in north central Alberta.
Doug Moisey leaves the Canola Council of Canada after 18 years.
Photo quiz of the week
Insect update: Lygus, bertha
Include lygus nymphs, which have telltale black dots, when doing sweep net counts for lygus. Source: Roy Ellis
Lygus: Second generation lygus have arrived. Young nymphs will reach late instar and adult stages in 5 to 14 days. When scouting to see if lygus numbers are near thresholds, include later instars — the ones with black dots on their back (see the top photo) — as well as adults in the count. Economic thresholds outlined on the tables will be in the range of 7-10 per 10 sweeps. Click here for a comprehensive article from Alberta Agriculture.
To scout for lygus:
- Lygus thresholds are based on sweep net counts. Sample several locations in the field.
- Sweep when conditions are pleasant (e.g., sunny, low wind, above 15 C and between 10 a.m. and 6 p.m.) to ensure lygus are active within the canopy.
- Sweep using a 180° arc into the canopy and aim to sweep the buds, flowers and pods while moving forward.
Bertha armyworm: Mature larvae are green, brown or black with a light brown head and an orange stripe along each side. They are 1” to 1.5” long. (See the photos below.) The most damaging larval stages are when worms are 0.5” long and larger. Small larvae feed on the underside of leaves, but usually cause little economic damage even when population levels are high.
To scout for bertha armyworm:
- Go out in early morning or late evening when larvae are most active.
- Mark out an area a quarter-metre square (50 cm by 50 cm) and beat the plants growing within that area to dislodge the larvae.
- Count the larvae that have fallen to the ground and multiply by 4 to get the number per metre square. Larvae will hide under leaf litter and in cracks, so check closely.
- Sample at least three locations a minimum of 50 metres apart. Do not sample headlands and areas within the crop that are not representative of the field.
- Use the average number of larvae at the sites surveyed to determine if the economic threshold has been exceeded. (See below for a bertha thresholds table.)
- Scout each field. Adjacent fields may have very different larval densities, depending on how attractive the crop was when the moths were laying their eggs. Adjacent fields may also have different-sized larvae, depending on when the eggs were laid.
For best results, apply an insecticide as soon as economic thresholds are reached. A single well-timed application of any registered insecticide is usually effective. Check the product’s pre-harvest interval before applying. Apply insecticides early in the morning or late evening when the larvae are actively feeding. Do not apply during warm afternoons when larvae tend to be in the bottom of the canopy, where contact will be reduced.
Click here for more on bertha armyworm.
Zebra caterpillars: As noted last week, zebra caterpillars (see very bottom for photo) tend to be isolated to small areas within a field. Zebra caterpillars will feed on pods, but do not seem to feed as aggressively as bertha armyworm. If growers decide to spray, consider spraying only the patch.
These are likely all bertha armyworms, showing the variation in color that can make identification a challenge.
Mature bertha armyworms come in various colour shades but will all have the yellow stripe down the side. Source: Saskatchewan Ministry of Agriculture.
Lots of green worms. What are they?
Bertha armyworm at a developing stage. Source: Saskatchewan Ministry of Agriculture
Diamondback moth larvae. Notice the spindle shape — wider in the middle than at the ends. Source: Saskatchewan Ministry of Agriculture.
Imported cabbageworm. Source: Wes Anderson, Pioneer
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.
Alfalfa loopers can grow almost as long as berthas, but they have a paler head than berthas. They have a light stripe down each side and two light stripes along the back.
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 come in brown, black and green shades. The green berthas are shinier than other green worms, 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.
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 square metre 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.
Alfalfa looper. Loopers are 24 mm long (1”) with light green or olive color, and a paler head than berthas. They have a light stripe down each side and two light stripes along the back. Mature larvae have a swollen abdomen. There is no economic threshold available. Delaying insecticide application as long as possible may allow viral diseases to reduce populations.
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.
Combining thresholds for multiple insects
Many canola fields have a combination of insects feeding on the crop, including lygus bugs and cabbage seedpod weevils, or bertha armyworm and zebra caterpillars. 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 or pods — at the same time.
For example, lygus and cabbage seedpod weevil will both feed on pods. If they are in the same field and if they are both at 50% or more of their economic spray thresholds, spraying may provide an economic benefit. Bertha armyworm and zebra caterpillar are also pod feeders, but they will also feed on other parts of the plant.
Beneficials to look for while scouting
Lygus, bertha armyworm and diamondback moth have a number of natural enemies that will keep populations in check. These beneficial insects may not control an immediate pest threat that has already exceeded thresholds — growers will still have to spray in that case — but beneficials can keep a lid on populations. The key to preserving beneficial insects is to follow thresholds and spray only when necessary. Here are some beneficials to watch for while scouting:
Cotesia wasps lay eggs in diamondback larvae. When the cotesia eggs hatch, the larvae eat diamondback larvae from the inside out. You may notice parasitized diamondback larvae while scouting. Source: Lloyd Dosdall.
Lacewing larvae are beneficial insects that eat lygus nymphs and aphids in canola crops. Insecticide sprays will take them out.
Banchus is a parasitoid of bertha armyworm. Source: Lloyd Dosdall
The hover fly is a beneficial pollinator and its larvae are predators.
Lady beetle (ladybug) larvae eat lygus nymphs. Source: Lloyd Dosdall
Lygus enemies. Laboratory studies show that lacewing larva consumed seven Lygus nymphs every 24 hours. Damselbugs and crab spiders also consumed large numbers of lygus nymphs. Lady bugs (a.k.a. ladybird beetles) consumed low numbers of third, fourth and fifth instar Lygus nymphs. This suggests to us that these general predators all play a role in suppressing Lygus in canola within the canopy, and may have “feeding niches,” with each predator preferring specific nymphal instar stages as prey.
Diamondback larvae pressure seems to have eased off the past few weeks, and it may be because of two natural enemies — Diadegma and Cotesia. These wasps lay eggs inside diamondback larvae. When the larvae hatch, they feed on the diamondback moth larvae from the inside out, killing them.
Bertha armyworm has a few natural enemies. They are an orange wasp called Banchus, which lays eggs in the bertha armyworm, and Tachinid flies, which lay eggs on the backs of bertha armyworms. Tachinid eggs hatch within minutes, and the tiny larvae bore into the bertha’s body. When these two natural parasitoids reach levels where they’re attacking 60% of the bertha population, we can expect to see bertha populations drop the following year and enter the downward side of their cycle.
Click here to download a PDF on insect scouting and insect identification.
Swede midge identified
In the northeast this year the racemes appeared normal except that some flowers had fused petals that did not open, suggesting a later infestation that should limit any impact on yield. Source: Julie Soroka, AAFC
Swede midge larvae in flower. Source: Julie Soroka, AAFC
Swede midge larvae. Source: Julie Soroka, AAFC
Swede midge larvae have been found in misshapen canola flowers from fields in the northeast region of Saskatchewan. Swede midge is not new to this region, as Canadian Food Inspection Agency personnel caught small numbers of adult swede midge in pheromone traps in the northeast (Nipawin and Melfort) and east central (Yorkton) areas of Saskatchewan in 2007. However, prior to now plant symptoms produced by their feeding activity have only been observed in canola fields in eastern Canada.
Canola fields in the Codette, Nipawin and Carrot River areas surveyed on July 18-20 were found to contain midge larvae. Symptoms included aborted flowers or flowers with petals apparently fused or glued together, and misshapen, stunted or sometimes missing pods. Based on the experience in Ontario, where the pest was first found in the year 2000, damage symptoms depend on the growth stage of canola when feeding occurs, and the intensity of feeding. If larvae attack pre-bolting canola, the growing point may become necrotic and bolting might not occur. Damage to a bolting stem may cause a “palm tree” effect, with a shortened raceme crowned with a bouquet of pods radiating out from one point rather than along a typical raceme. After full flowering swede midge does not impact canola yields. In the northeast this year the racemes appeared normal except that some flowers had fused petals that did not open, suggesting a later infestation that should limit any impact on yield. When dissected, the flowers contained small larvae. Small, misshapen, or missing pods occasionally were found below the infested flowers.
Disease update: Blackleg, sclerotinia, foot rot, clubroot
Foot rot caused by rhizoctonia.
Rhizoctonia foot rot can cause fields to thin out as plants topple.
It could be a bad year for clubroot. Pull up plants and look for galls. If found, make plans to contain the spread.
Growers can’t do much to prevent disease at this stage of the season, but scouting and identifying disease is easier in the weeks leading up to harvest. Identifying disease now means growers will be able to take measures to avoid spreading this disease this fall, and makes better management decisions with regard to crop rotation and variety selection for next year.
Sclerotinia stem rot: It could be another big year for this disease. Fields not sprayed may have high rates of infection. Nothing can be done at this point. The most costly infection has set in, and fungicide does not have curative effect. The best you can hope for is dry weather that causes infected leaves to drop before infection spreads to stem. Click here for more on late applications.
Foot rot and brown girdling root rot: Canola plants pinched off at ground level likely have rhizoctonia foot rot. Another possibility is brown girdling root rot (BGRR), a similar infection caused by the complex of rhizoctonia, pythium and fusarium, producing a characteristic brown root. Growers have no management options for this season, but if they’re seeing high rates of disease, note that these diseases can be much higher in tight canola rotations.
Blackleg: Growers in some parts of Manitoba are seeing fairly high incidence of blackleg. Infection that occurred at seedling to rosette stages is what’s causing stem lesions and cankering now. Fungicide applied now will not help these plants, and fungicide applied now would be too late to have any benefit to plants that have not been infected. New blackleg infections at this stage of the season will be superficial and not reduce yield. Assessing how much yield loss will occur based on presence of stem cankers is difficult, but if you see premature ripening over the next week or two, that’s a good sign yield loss will occur.
Clubroot. Above-ground symptoms, including wilting and pre-mature ripening, should be evident soon in severely-infected plants. Even if you don’t see above ground symptoms, pull plants and look for galls. Plants with small galls will not show above ground symptoms, but light or severe infection both have essentially the same risk for spreading the disease with equipment. If galls are present, the best management at this stage is to prevent spread within the field, and certainly from field to field. Harvest those fields last to prevent field to field spread. Consider no-tillage in those fields to prevent spread throughout the field and from field to field by the tillage tool. Click here for more on clubroot. Inspect other weeds that can host clubroot as well, especially in non-canola crops. The most common clubroot-susceptible weed species of the Prairies are wild mustard, stinkweed (a.k.a. French weed or pennycress), flixweed (a.k.a. tansy mustard) and shepherd’s purse. Click here for photos of these weeds and other less susceptible weeds.
Lots of aster yellows
Aster yellows in canola.
Fields all across the Prairies are reporting aster yellows. The phytoplasma infection causes misshapen pods and flower buds. Infection looks awful and stands out, but rarely causes economical loss. Most fields don’t go above 1% infection rates. However, some fields are reported this year with rates at 5% to 8% of plants infected. This would be similar to 2007, which had Prairie canola fields at 5% to 20% infection.
A 10% infection rate results in 3% to 7% misshapen seeds or no seeds at all.
Note that plants infected with aster yellows can have misshapen seeds and yield loss without showing any of the obvious signs of infection — such as the puffed bladder-like pods. The plant looks normal but some pods are empty or contain misshapen seeds.
Aster yellows phytoplasma is carried by aster leafhoppers, which come up each year on south winds from the U.S. Canola varieties are not known to have any difference in infection rates. If aster yellows is found in one variety, it will be found in all varieties.
contact the Canola Council agronomist in your area