Cutworms generally don’t like wet soils, but entomologist John Gavloski says farmers should be on the lookout early in the crop-growing season anyway.
Scouting can start before seeding. Farmers should look for feeding notches on weed and volunteer seedlings, Gavloski, who is based in Carman with Manitoba Agriculture, Food and Rural Initiatives, told webinar participants April 20.
Cutworms can be found in almost every field if one looks hard enough, Gavloski said. The most common species in Manitoba are the redbacked, darksided and dingy. Less common are the glassy and pale western.
Dingy cutworms, characterized by V-shaped markings down their backs resembling tractor tire tracks, overwinter as partly developed larva so they are bigger early in the spring and easier to find than the redbacked and darksided cutworm.
The redbacked cutworm has two red stripes down its back that range in colour from pink to red.
The darksided cutworm has a grey stripe down its back and two brown stripes down its side.
Cutworms feed at night. They’re usually in the soil or under debris in the daytime. Where there are signs of suspected cutworm feeding, dig around in the soil to find the larva.
When looking for cutworms early in the spring, it’s easier to dig out some soil and spread it in a pan, Gavloski said.
Often cutworms are concentrated in “hot spots.” If a patch of crop is missing, it could be cutworms.
How many cutworms it takes to warrant spraying with an insecticide is a bit of a guess, Gavloski said. It depends on the crop and the stage of the cutworms.
Some crops such as wheat can make up for some destroyed plants, whereas corn can’t.
“If just the odd seedling has a notch out of it your field is going to be fine,” he said. “If most of your plants are showing notching then you want to look very carefully to see what the population is like there.”
Also, if the cutworms are large or have started to pupate the worst of the feeding has passed.
“At that point it’s nothing but revenge spraying, but it’s not really economical,” Gavlsoki said. “Again, you want to be on top of the problem early.”
A MAFRI fact sheet suggests the cutworm threshold for wheat, oats and barley is five to six larvae per square metre. Wellestablished fall-seeded crops or spring-seeded crops with good moisture conditions can tolerate higher numbers.
The suggested nominal threshold for canola is a 25-to 30-stand reduction.
No seed treatment is currently registered to control cutworms, but several foliar insecticides offer good control. The most effective are the pyrethroids, which include Decis, Matador and Ripcord.
Because cutworms are nocturnal, spraying an insecticide in the evening or night will be more effective than in the daytime, Gavloski said.
There are around 30 species of wireworms across Canada, but except for a couple of fields, they aren’t an economic pest in Manitoba, Gavloski said. Several fields in the Teulon area have had a problem for three years resulting in some reseeding.
“Most other fields, even ones not too far away from those problem fields, you can barely find a wireworm,” Gavlsoki said. “Most farmers probably do not have an economic population in their fields. We’re certainly not seeing a resurgence the way we thought we might.”
When Lindane-based insecticidal seed treatments were phased out, some believed wireworms would become a big problem. The Lindane treatments Little research has been done to develop economic thresholds for cutworms in field crops. However, nominal thresholds, which are based on experience rather than research quantifying the impact of the insects on the crop, have been suggested for several field crops. Some suggested nominal thresholds are:
killed the wireworms when they fed on the seed. The new neonicotinoid insecticides just make the wireworms sick. They will survive and can attack the crop again next year.
A wireworm survey has also revealed the most common species is not the cereal wireworm as had been believed, but a species researchers know little about.
“It might not even be a very economical one,” Gavloski said.
With spring seeding delayed by wet soils in some places and flood waters in others, farmers might consider a backup plan should seeding be delayed further, said MAFRI farm production adviser Lionel Kaskiw. For example some canolas mature as many as 10 days sooner than later-maturing cultivars.
Consider planting crops that can withstand a bit of frost in the fall.
Another option, he said, is growing feed grains, which are less subject to downgrading.
Keep an eye on weeds too. It has been a cold, wet spring but some weeds are showing up. A pre-plant burnoff should be considered.
Ideally crops should be emerging a week after planting, Kaskiw said. For many crops that means the soil should be at least 4 to 6 C.
Seed treatments are also a benefit when planting into cool, wet soils, he said.
A year ago seeding had already started in many parts of the province. While progress varied, it was quite advanced in the Carman to Morris area with MAFRI estimating in its April 26, 2010 crop report that 80 to 90 per cent of the cereals were in the ground. The earliest-sown crops were starting to emerge.
Then it started to rain, delaying seeding in many areas and preventing it in others.
The above-average rain that followed in much of Manitoba and central Saskatchewan, is contributing to flooding along the Assiniboine, Souris, Pembina and Red rivers and in the Interlake. [email protected]
– JOHN GAVLOSKI
Wheat, Barley, Oats
4-5 per square foot (new or thin stands -2/sq. ft.)
1 cutworm or more per metre of row and the larvae are still small (less than 2 cm long)
A suggested nominal threshold is 25-30% stand reduction
When 3-6% of plants are cut and small larvae less than 1 inch present
2 to 3 cutworms per square metre
1 cutworm or more per square foot (30 by 30 cm) or if there is a 25 to 30% stand reduction
Redbacked and army cutworms: 5-6/m2. Well established fall-seeded crops or spring-seeded crops with good moisture conditions can tolerate higher numbers