Top Crop Manager

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Cutworm review


November 30, 1999
By Top Crop Manager

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Cutworms, belonging to the family of moths called Noctuidae, naturally occur across the Prairies at varying levels. In recent years, cutworms have caused economic losses in localized areas in seemingly greater numbers. Although 2011 saw relatively fewer problems than the past few years, the best advice of researchers is that cutworms are always present to some degree, and that farmers should be out scouting for them every year as part of their early-season activities.

“The question is not if cutworms are there, the question is will they cause damage to the crop,” says John Gavloski, an entomologist with Manitoba Agriculture, Food and Rural Initiatives at Carman, Manitoba. 

Gavloski explains that there are many different species of cutworms that occur naturally on the Prairies but only a few cause economic damage to crops. Of the few pest species, the most common are redbacked, pale western, darksided, dingy, glassy and army. Proper identification of the species is important: some complete their life cycle earlier than others, some are surface feeders and some are subterranean feeders. Each factor affects insecticidal control.

Pale western and glassy cutworms spend much of their time as larvae below the soil surface, whereas species such as redbacked, dingy and army cutworms come above the soil surface in the evening to feed. The pale western and army cutworms are more common in Saskatchewan and Alberta, and less abundant in Manitoba. Larvae feed underground and can move down seed-rows, cutting off the plants underground and pulling seedlings below ground to feed.   

Surface feeding cutworms, such as redbacked, dingy and darksided, chew holes in stems and leaves, and older larvae usually clip plants off at the soil surface.  Look for dry leaves lying on the surface. 

Key characteristics
The redbacked cutworm is most common on the eastern Prairies in the Parkland belt and more northerly areas. It has two red stripes on the back of the larvae and may be very localized in a field. Young larvae notch leaves; older larvae clip and defoliate the crop. 

Darksided cutworm is fairly common across the Prairies and has a dark brown strip down the sides of its larvae. It has a similar feeding pattern to the redbacked, and the two species are often found together in the same field. 

The dingy cutworm has a light stripe on its back, and a series of diagonal markings that look like a tire track pattern or a series of Vs. The dingy causes a lot of defoliation and the larvae may consume the entire aboveground plant. It overwinters as larvae, thus feeds earlier in the season, and may be larger early in the season than species that overwinter as eggs.  

Glassy cutworm prefers grassy crops and is less likely to be found in broadleaf crops. The glassy cutworm is almost translucent and has an orange head capsule.  It only occasionally causes economic damage.

The pale western cutworm is partially translucent and has a darker head capsule. It is more common in the western part of Saskatchewan and Alberta, preferring drier soils. The first sign of damage is holes cut into emerging leaves – the holes were cut when the leaves were still underground. 

Army cutworm migrates out of the Rocky Mountain regions through Alberta and western Saskatchewan.  It overwinters as larvae, and begins feeding in April until pupation in May to early June.

Scouting suggestions
Because cutworm infestations are sporadic, and usually localized, scouting every three to four days after crop emergence is recommended for early detection. Look for notched, wilted, dead or cut-off plants and weeds and bare patches in a field. Because cutworms are nocturnal, detection is difficult. Check the edges of bare areas by digging up the top two inches of soil around plants and looking for larvae. 

Gavloski says small larvae are of the greatest economic concern, because they will be feeding longer than larger larvae.

“Take note of the species, if possible, and estimate the level of damage that is occurring. Size and stage are important. When you start digging up larger larvae and pupae, they are at a stage where it is probably no longer economical to control,” explains Gavloski

No scientifically established economic threshold
Unlike some insect pests with well-defined economic thresholds, little research has been conducted on thresholds for cutworms on the Prairies. Instead, nominal thresholds have been suggested, based on experience and field observation. 

“The nominal thresholds were put in place for guidance. They are based on experience but we don’t have good data to back them up,” explains Gavloski.

Some crops also have the ability to compensate more than others. Canola, for example, can compensate for some loss of plant stand, and five percent feeding damage doesn’t necessarily equate to five percent yield loss. Research has also shown that flax and peas can compensate for some feeding damage.

Foliar insecticide application can be effective. Best control is achieved when the insecticide is applied in the evening because the cutworms feed during the night. 

AAFC research at Lethbridge by Bob Byers showed insecticidal control is affected by the life cycle of the larvae. They found that 20 to 50 percent of the natural populations of pale western cutworm were in a premolt or recent postmolt stage and were not feeding.  However, currently registered insecticides have sufficient residual to control cutworms for several days after application.

Gavloski says another reason to scout fields and only apply insecticides when warranted is to minimize the impact on beneficial insects. Many species of predator beetles, parasitic wasps and flies, birds, and disease, can impact cutworm and help to control damaging species.

An eight-year study in Manitoba also found that minimum tillage practices were associated with a greater diversity of cutworms and parasitoids. The research suggested that min-till created a more stable environment that reduced cutworm damage. Conversely, once cutworm larvae are present, tillage in the spring to control plant growth for 10 to 14 days can starve cutworms.

During the late winter of 2011, the Prairie Pest Monitoring Network recognized the lack of research on cutworms in the Prairie region, and set research priorities to better understand the pest. Species identification, economic thresholds, duration of feeding, egg-laying behaviour, trap monitoring, environmental impact and insecticide control were all identified as areas requiring further work.