Cutworm management in canola

Cutworms are a common crop pest of the Canadian Prairies. When scouting shows crop loss from cutworms, targeted insecticide sprays can reduce further damage.

Conditions that influence cutworm numbers

• Fields with flowering plants (crops or weed patches) in August or September may have higher cutworm counts the following year. These flowers attract the adult moths, which lay their eggs in these areas.
• Females prefer loose worked soil for egg laying.
• A long relatively dry fall allows for maximum egg laying.
• Mild winters and dry springs increase high winter survival.
• Wet springs increase cutworm mortality due to pathogens, but this tends to affect those species that overwinter as larvae more than those that overwinter as eggs. The most common species in Western Canada – redbacked and pale western – overwinter as eggs, and may not be as susceptible to saturated soils.
• Beneficials can reduce the severity of cutworm outbreaks. If spraying (more on that below), follow thresholds to spray only when necessary. This helps protect the natural enemies.
• Movement tends to be within fields, not field to field. Exceptions would be army cutworm and the cutworm relative Bertha armyworm (not discussed in this article), which can move field to field.

Common cutworm species

These are the most common species that will feed on canola in Canada. Content is from here.

Dingy

Key features: Grey colour with light markings down the back that look like tire tracks or a series of “V”s.
Location: All across the Prairies.
Feeding period: They overwinter as larvae, so early in the season they will be larger than species that overwinter as eggs. They can feed on canola from germination through to late June. (See the cutworm lifecycle table below.)
Typical damage: Look for lost foliage and bare patches, not clipped plants. They rarely cut plants.

Redbacked

Distinguishing features: Look for two reddish stripes along the back.
Location: Dominant species in the eastern Prairies
Feeding period: They overwinter as eggs. Larvae hatch when soils warm up in the spring and will feed through late May and June. (See the cutworm lifecycle table below.)
Typical damage: Young larvae make small holes and notches in the foliage. Older larvae eat into the stems and often sever them.
Need to know: Their populations can be isolated to patches in a field. Patch spraying may be all you need. Pyrethroids tend to work better than other insecticides on redbacked cutworms.

Pale western

Distinguishing features: Light coulored body with a dark brown head.
Location: More common in the western Prairies.
Feeding period: They overwinter as eggs. Larvae tend to hatch in May and feed through June. (See the cutworm lifecycle table below.)
Typical damage: They tend to tunnel, feeding underground near the soil surface. They often cut through stems, toppling plants.
Need to know: They tend to prefer drier conditions. Because they are less likely to surface, they can be trickier to control with insecticide.

Army

Distinguishing features: Yellowish with dark stripes.
Location: More common in Alberta and western Saskatchewan
Feeding period: They overwinter as larvae, so they will feed earlier and be larger in early spring.
Typical damage: Army cutworms will climb canola plants and feed on foliage.

Bristly

Distinguishing features: Bristly cutworms have diamond shapes down the back, and two dark stripes along their sides parallel to the diamonds. Bristles on the top and sides are more noticeable than on other species, hence the name. Its full grown length is 21-31 mm, making it one of the smaller cutworms.
Location: All across the Prairies
Feeding period: They overwinter as eggs and hatch in May, with larvae feeding in May-June.

Scouting

Scout emerged canola crops for bare patches, holes or notches in foliage, and clipped plants — telltale signs of cutworm feeding.

Thresholds

Pale western and redbacked have a threshold of four to five larvae per square metre, based on counts in the top 5cm of topsoil. For other species, the general nominal threshold for cutworm in canola is 25 to 30 stand reduction.

With a stand reduction of less than 25 per cent, canola plants may be able to branch out and compensate for some of the lost plants. However, bare patches due to a complete lack of plants does not allow for any compensation. Spot spraying in the area immediately around a bare patch may be sufficient in these cases.

Why to scout?

• Reason one: To confirm cutworms are the cause. For example, fungal seedling diseases can cause seed and seedling decay that can lead to patches of missing plants. While scouting around patches, check roots and seeds for signs of rot and infection. Cutworms often clean out all the plants and weeds in a patch, while disease may be more plant specific.
• Reason two: To identify the cutworm species present. This can determine the benefit of a spray. Species that stay underground are less likely to encounter a foliar spray. For example, pale western cutworms are primarily subterranean feeders. Fields with a history of subterranean cutworms may benefit from a cutworm seed treatment.

When to scout?

Cutworm feeding can start with seedlings and continue into early summer. For canola, the first three weeks are critical to good crop establishment – so definitely look closely for establishment issues, including cutworms, during that time.

How to scout?

Look for bare areas with missing plants or plants that are notched, wilting or completely cut off. Bare areas tend to start on south facing slopes and hilltops where the soils are warmer and soil texture is lighter. Cutworms often move through the loosest soil, which is why they tend to move along seed rows.

Start scouting at the edge of patches or around the last living plants within patches. Cutworms, if present, will be in these areas. Dig up the soil around the base of living plants at patch edges, collecting soil from a 50cm by 50cm area to a depth of 5cm. Put the soil into a basin, loosen and shake it up to activate cutworms. Larvae often curl up or attempt to hide in the debris when exposed.

Count the number of larvae then multiply by four to estimate the number of larvae per square metre. Concentrate on damaged areas if planning a spot spray. If checking the whole field for a general spray, repeat this process five to 10 times at 50m intervals, including at low, side-slope and high parts of the field.

Cutworms are often just above the moisture in the soil, so if it has been hot and dry, dig to 7.5cm (~3”). Note that flocks of birds in a field are often feeding on cutworms.

Scout late in the day or extremely early in the morning to look for above-ground feeding cutworms. During the day, cutworms usually go back underground.

Wireworms. While scouting in the soil, also look for wireworms – another below-ground feeder that can cause similar bald patches to appear in canola fields. All cutworms will curl into a ball. Wireworms do not. Wireworms are yellow and do not have a prominent head. Note that no products are registered (or effective) for wireworm control in canola.

Spraying – considerations before action

Answer the following questions before deciding the spray:

Are cutworms still actively feeding?

Put flags in the ground at the edges of a patch. Check the next day to see if the bare patch of missing plants has expanded beyond that flag.

How big are the cutworms?

Larger cutworms (1 to 1.5 inches long) will be closer to the end of their larval stage. When they pupate (go into their cocoon-like brown shells), they’re done eating. The risk in trying to estimate this lifecycle stage is that cutworms do most of their eating in that final instar. In warm conditions, it can take 4 to 6 days for cutworms to grow through the final instar. They can do a lot of damage in that time.

How much damage can they do in a day?

A cutworm can kill a young canola plant in a few minutes by cutting through the stem at or below the soil surface. Species of cutworm that crawl up on plants to feed may eat leaves, but not kill the plant unless plants are very small or cutworm numbers are very high. Weather also influences how much they’ll eat. While metabolism slows in cool weather, they will still feed a bit. But that also could mean they live longer between instar cycles.

What colour is their gut content?

Cutworms stop feeding while molting between instars and as they are about to pupate. Squeeze a bunch of them. Green insides indicate active feeding. Brown insides indicate they’re not feeding.

While insecticides work through contact or ingestion, cutworms do have to surface to be controlled. If they’re not eating, they may stay underground. However, residual activity of most registered products should get them when they start feeding again. For example, in a study where 50% and 34% of pale western cutworms were in a molt at the time of application, by 7 days later the pyrethroid that was used killed 81% and 84% of the cutworms (Hill, Byers, and Schaalje. 1992: J. Econ. Entomol. 1387-1392).

Do some follow-up scouting, and continue with thresholds. A few living cutworms found in the few days after an application may not indicate a need to spray again.

Do you see beneficial organisms?

Most cutworm species are native to North American so have many natural enemies – including insects such as cotesia and diseases caused by fungi, bacteria and viruses – that provide population control. Letting nature take its course when numbers are below threshold is usually the best decision.

Stick to threshold-driven decisions to protect the crop only. As former Alberta Agriculture entomologist Scott Meers once said, “Lots of things control population. Spray is not very good at it.” Also, spraying now will not reduce populations for next year because that population is based on adult egg-laying later in the season and adults (moths) are very mobile.

Spraying – steps to improve results

For specific requirements on water volume, rates, timing, restrictions and product activity, read the label for the chosen product. Some products are systemic insecticides taken up into the plant’s tissues and work when the insect eats portions of the plant. Contact products work best when the insect is sprayed directly. Products have varying amounts of residual activity – some very little, some up to two weeks.

Timing

Cutworms surface at night to do their above ground feeding. For most insecticides registered for cutworms, spraying as close to nightfall as possible will improve results. Synthetic pyrethroids (Decis, Matador, Silencer, for examples) further benefit from late-day spraying because they should be applied when temperatures are below 25°C. A systemic product with a long residual, such as Coragen (which has “translaminar” movement into the leaf or stem), can be applied during the day. Since not all cutworms will surface on a single night, it may take several days before full effect of a residual insecticide is achieved.

Water volume

For all products, 10 gallons is the recommended minimum water volume and higher is often better for coverage of leaves and stems. The more plant surface area covered, the more likely that cutworms will take in the insecticide.

Provincial guides to crop protection include foliar insecticides registered for cutworms in canola. Find them here: Saskatchewan Alberta Manitoba

More…

Cutworms section in the Canola Encyclopedia
Cutworm Pests of Crops on the Canadian Prairies
Manitoba cutworm factsheet
Canola Watch cutworm quiz
Clover cutworms section in the Canola Encyclopedia
Research on cutworms in canola