Cutworms

Table of contents

    Important Tips for Best Management

    • The key to minimizing damage is early detection. Scout the fields and inspect seedlings every three to four days during the first few weeks of crop development, looking for bare areas, holes or notches in foliage, and plants that are wilting, toppling over or completely cut off.
    • Small larvae (12 - 18 mm; 0.5 - 0.7") pose the greatest potential for damage as they will still feed and grow.
    • Knowing the species of cutworm can be important because some species complete the larval stages earlier in the year than others, and some species are more likely to feed on and clip stems than others.
    • After the second or third year of infestation, beneficial organism populations such as parasitic insects, viral diseases and bacterial infections usually build enough to begin bringing cutworm numbers down.
    • A nominal economic threshold may be reached at 25 to 30% stand reduction.
    • Determine if the population is patchy or evenly distributed throughout the field. High populations are often localized leading to expanding bare patches in fields as they consume all the canola plants in an area and move outward in search of more food. Spot spraying the affected patches and a pass or two in the surrounding crop can often be effective in limiting outbreaks.


    About cutworms

    Background

    Cutworms can be a significant pest in canola.  Five economically significant cutworm species can be found on the Prairies, with some species more dominant in some geographic areas than others.  Cutworms are usually present in most cropland, but at levels well below economic thresholds.  However, outbreaks of cutworms seem to be occurring with greater frequency, and from 2007 through to 2010, were a serious insect pest in canola. The pale western and redbacked cutworm continue to be the most common species affecting canola crops, but reports suggest other species are also contributing to this increased frequency of outbreaks.

    Pale western = greatest concern in the southern, more open prairie areas.

    Redbacked cutworm = greatest concern in the parkland belt and northern agricultural areas of the prairies.

    Both cutworm species feed on practically all field crops, vegetables and home garden plants. They are most destructive when feeding on cereals, flax, sugar beets, canola and mustard.

    Life Cycle

    Cutworm Life Cycle

    Cutworm species in western Canada produce only one generation per year. Adult cutworm moths may lay several hundred eggs on vegetation or in the soil in August and September. Many cutworm species overwinter as the tiny eggs that are laid in fall. In April or early May, the eggs hatch and the young larvae feed mainly at night on weeds and volunteer plants until host crops begin to emerge. Army cutworm eggs hatch in August and the larvae feed until freezing temperatures occur, when they overwinter as larvae. Dingy cutworm also overwinters as larvae.

    The young larvae of most species pass through 6 stages (instars), each separated by a shedding of skin (molting). Late May and the first 3 weeks in June are the most likely times for cutworm activity (seedling to rosette stage). The larger cutworms are usually easy to find in the soil beside damaged or freshly cut plants. Younger larvae may require more careful searching. When disturbed, the cutworms will curl up. This is characteristic of all cutworms and armyworm species. 

    How long cutworms spend in their potentially damaging (larvae) stages depends on the species and the temperatures they are exposed to. For example, redbacked cutworms kept at 15ºC took 65 days on average to complete their 6 larval stages, while at 25ºC they completed their larval stages in 29 days. Darksided cutworms took an average 87 days to go through their 7 larval stages. [1],[2]

    At maturity, the larvae range in size from 30 mm (1.2") to 38 mm (1.5") depending on species.

    After the cutworms complete their larval growth, usually in late June, they burrow deeper into the soil where they make a small soil chamber in which to pupate. The reddish brown torpedo-shaped pupae of the redbacked cutworm are similar in size and shape to those of other cutworm and armyworm pupae.

    Following the pupal stage, adult moths emerge from the soil in August to early September. The moths are night fliers and not usually seen. After mating, the moths lay eggs on or just below the surface of loose, dry soil. 

    Influence of environment

    Alberta Agriculture information indicates a hot, dry August provides the best conditions for moth feeding on flowers. Egg production and egg laying depend on the nutrition obtained from flowers. The same weather conditions promote the loose, dry soil surfaces preferred by some species for egg deposition. Warm, dry spring weather can increase the severity of damage from cutworm attack.

    Wet or very dry soils during the larval stage impact cutworm behaviour and feeding habits.  The larvae of the pale western cutworm, for example, usually remain in the soil unless forced to the surface by rain or hard soil, where they are subject to attack by parasites and predators. Wet soil conditions during the larval stages can promote fungus diseases among cutworms.


    Identify cutworms

    Pale Western Cutworm Larvae

    Redbacked Cutworm Larvae

    The most economically important species of cutworm in western Canada include:

    • redbacked
    • pale western
    • darksided 
    • army
    • dingy 


    Scouting techniques

    Most cutworm species feed at night and hide during the day, making detection difficult.  Determine whether bare areas with no seedlings resulted from poor germination or cutworm damage.

    When: Inspect seedlings every 3 to 4 days during the first few weeks of crop development.

    Where: Look for bare areas, holes and notches in foliage, and plants that are wilting, toppling over or completely cut off. Bare spots will begin to appear within the field, typically on south facing slopes and hilltops where the soils are warmer and soil texture is lighter. 

    How: Check the edges of bare areas for cut-off plants and search the top 5 cm (2") of soil around such plants for larvae. The key to control is early detection. When notched, wilted, dead or cut-off plants (weed or crop seedlings) are seen, dig around the roots of the plants for cutworms. 

    Larvae detection:  To collect cutworm larvae, a garden trowel and a soil sifter are useful tools. Cutworms may be found down to about 10 cm (4") below the soil surface, particularly if the soil is dry and larvae are in their later stages.  Larvae often curl up or attempt to hide in the debris when exposed. Pupae may also be collected in this way. 

    Action Threshold:  A nominal threshold of 25 to 30% stand reduction has been suggested. Scout throughout the field to determine the distribution of larvae, as patch spraying is sometimes sufficient to control outbreaks. Check for green material in the gut of larvae to determine if they are actively feeding, as they will cease feeding temporarily during molting between instars. This could affect the ideal timing for control measures depending on the proportion of larvae that are not actively feeding, although residual activity of most registered products should allow for adequate control.

    The larvae and pupae can be reared to adult moths if necessary for species determination.

    A study from Ontario on darksided cutworms found older larvae at a depth of 8 to 10 cm (3 to 4 inches) from the soil surface. Most younger larvae rested in the soil at 6 to 13 mm (0.25 - 0.5") near the base of the host plant. [2] 

    It is important to determine the staging of cutworms when scouting, as some stages of larval development hold more potential for crop damage than others. When larvae are small, 12 to 18 mm, (0.5 - 0.7") they pose the greatest potential for damage as they still must feed to grow. Once larvae have reached lengths of 30 to 35 mm (1.2 - 1.4"), most of their feeding has already occurred and chemical intervention may not be warranted.

    Small larvae = greatest damage potential.

    Because cutworm moths, like most moths, are nocturnal and attracted to light, the adult population can be monitored using light traps. Sex attractants also can be used to trap adult cutworm moths in commercial or homemade pheromone traps. However, these have not been found to be a reliable means of predicting the level of cutworms the next season. [3] 

    Damaging species and staging

    Knowing the species of cutworm can be important because some species complete the larval stages earlier in the year than others, and some species are more likely to feed on and clip stems than others. [4]

    In the southwest prairies, the pale western cutworm tends to be the dominant species, whereas in the parkland and northern regions, the redbacked cutworm is of the greatest concern.

    Information from the Saskatchewan Ministry of Agriculture indicates that in the past few years, other cutworm species have become more common in Saskatchewan crops. Dingy cutworms have been found in eastern regions and army cutworms have been reported from western regions. Several of these less common species have also been reported in Manitoba. Dingy and army cutworms feed above ground, consuming the plant foliage. Typically, these cutworms are more likely to cause damage on hilltops and in drier areas of a field. [5]

    Redbacked (Euxoa Ochrogaster Guen)

    Larvae of redbacked cutworms have two broad dull-red stripes along the length of their back. The head is yellowish-brown. Mature larvae are about 38 mm (1.5") long. Young redbacked cutworms chew holes and notches in leaves, while older larvae eat into the stems and usually sever them at or just above the soil surface. Cut plants can be found drying up and lying on the soil surface. 

    Adult moths of the redbacked cutworm have 4 colour forms and are light fawn to brick red in colour. The flight period for moths of the redbacked cutworm in Manitoba extends from mid-July to October. The peak flight period occurs during August. [6]

    Pale Western (Agrotis Orthogonia Morr.)

    The pale western cutworm is a more serious pest in Saskatchewan and Alberta, with less impact in Manitoba.  New larvae are almost colourless and about 3 mm (0.12") long.  Fully grown larvae vary in length from 30 to 36 mm (1.2 - 1.4") long, and are grayish-white to green in color, and lacking any stripes or distinguishing marks.  The head capsule is usually yellow-brown.

    The larvae feed mainly underground, only surfacing when the soil is hard or very wet.  The first signs of damage are holes cut into emerging leaves.  These holes are cut when the leaves are still underground. Larger larvae move down crop rows, cutting off the plants underground.  Feeding damage is similar to redbacked cutworm.  

    Pale western cutworm moths are mottled greenish grey with distinct pale lines on the forewings.

    Army cutworm (Euxoa auxiliaris)

    Army cutworms are more commonly found in Alberta and Saskatchewan, but rarely at economic levels in Manitoba. The larvae are pale greenish-grey to brown with a pale-striped back, and finely mottled with white and brown, but without prominent markings. There is usually a narrow, pale mid-dorsal stripe, and the head is light brown (tan) with small dark spots. [7]

    Feeding is done entirely above ground and the larvae move from one plant to another after all the leaves are stripped.  The larvae move en-mass from one depleted area to new areas, giving them the name 'army' cutworm.  Larvae generally feed from late afternoon to early evening, making scouting easier. [7] Because of their early emergence, army cutworms are more of a threat to early seeded canola crops.

    The adult moth is 40 - 45 mm (1.6 - 1.8") and grey-brown with two prominent spots on the forewing. [7] The female moths lay about 1000 eggs in soft soil in late August, and the eggs hatch and larvae feed on plant foliage during the fall.  Development stops when the ground freezes.  The larvae begin to feed again in April, and continue to feed until pupation in May to early June.

    Darksided cutworm (Euxoa messoria)

    Darksided cutworms are soft, thick-bodied, dull-colored worms with shiny heads, at maturity reaching a body length of 3.75 cm (1.5"). Each larva has a prominent white stripe on the side above the legs and dark coloring on the head resembling spots or freckles. During the day the larva remains curled in the soil at the base of the plant on which it feeds at night. The larvae chew emerging plants at or below the soil surface. They may completely consume the above-ground portions of small plants, and especially the succulent ones that have been transplanted recently. [8]

    Darksided cutworm is often found in mixed populations with redbacked cutworm, and may be easy to confuse. [4]

    Dingy cutworm (Feltia jaculifera)

    Larvae of dingy cutworms (Fig. 6) have a thin light line down the very middle of the back. On either side of this thin line there is a broader series of somewhat diagonal markings that look like tire tracks, or to form "V's" on the back. They also have 4 equal-sized black dots on the back surface of each abdominal segment. Dingy cutworms are primarily leaf feeders, and rarely cut plants. [4]

    Type of Damage

    Patches of bare soil characterize infestations where the crop has started to disappear. These patches gradually enlarge until the area of destroyed crop reaches anywhere from 0.5 to 1 ha (1 - 2 acre) to the complete field. First signs of damage usually appear on hilltops, south facing slopes or in areas of light soil that normally warms faster and shows damage early. 

    Can be confused with: In many cases, bare hilltops are attributed to poor germination rather than cutworms.


    Minimize infestations

    Natural enemies and biological control

    Natural enemies to cutworm species may increase as cutworm populations rise. These natural enemies are mainly comprised of parasitic insects, viral diseases and bacterial infections. Typically, after the second or third year of infestation, beneficial organism populations build enough to begin bringing cutworm numbers down. These larger populations of naturally occurring biological controls will usually keep cutworm outbreaks from occurring again for at least a couple of years.

    Many predaceous insects, parasites and birds prey upon cutworms and reduce their populations. A study in Saskatchewan found larvae of redbacked cutworms parasitized by 4 species of bee flies, at least 4 species of tachinid flies, and several species of parasitic wasps. Ground beetles can be important predators of cutworms. Disease and parasites were found to be the most important factors causing the rapid decline of redbacked cutworms from the peak of an outbreak. [4]

    Field Management

    Cultural control is an important part of managing cutworm activity. 

    Summerfallow weed control: If fields are being summerfallowed in areas infested with redbacked cutworms it is important to keep them free of weeds in August (egg laying time). Summerfallow fields that have a protective crust through August and the first half of September are much less attractive for egg laying by pale western cutworms. Therefore, work fields in late July and allow them to harden by summer rains.

    Delayed seeding: In the spring, a delay of 10 to 14 days between cultivation and seeding may help reduce populations. Studies with pale western cutworm showed that larvae that have already fed will die if deprived of food for several days. Cold weather after cultivation and seeding will have a similar effect.

    The benefits of these cultural control options for cutworm control need to be balanced against the potential negative effects of delayed seeding, drying and /or crusting of the seedbed and increased soil erosion. Often these negative effects may outweigh the benefits in areas where cutworm populations only occasionally reach numbers sufficient to cause significant losses.

    Minimum tillage: An 8-year research study from Manitoba has shown that minimum tillage practices were associated with greater diversity of cutworms and their parasitoids, including some non-pest species of cutworms, than fields under conventional tillage. Min-till was not related to increased crop loss from cutworm, and the study suggested the increased diversity suggests a more stable ecosystem in which outbreaks of cutworms would be less common. [9]


    Reduce economic loss

    Economic Threshold

    Cutworm control may only be necessary in small areas of the field, when bare patches appear and large numbers of cutworms are still actively feeding. Canola may be more susceptible to cutworm damage than cereals because no regeneration and tillering occurs to compensate for loss of plants, although the ability of canola plants with minimal damage to produce more yield in thinner stands needs further study.  

    Action Threshold: A nominal threshold of 25 to 30% stand reduction has been suggested. 

    The nominal thresholds is based on experience rather than research quantifying the impact of the insects on the crop, as little research has been done to develop economic thresholds in field crops.

    Chemical control options

    As cutworms tend to be primarily nocturnal, insecticide application may be difficult to time correctly. 

    Application timing: Apply insecticide in the evening to maximize exposure to the cutworms and to minimize effects on beneficial insects. 

    Spot treatment:  Spot treatment with insecticides may sometimes be sufficient, since cutworms may at times be a problem only in patches of fields(Canol@Fact)). 

    Assess larval size:  Most feeding has already occurred and insecticides may not be economical if larvae have reached lengths of 30 to 35 mm (1.2 - 1.4").

    Molting affect on insecticidal control

    AAFC research showed that control with insecticides may be delayed in those larvae that are molting or shedding their skin. In the study, the effectiveness of sprayed insecticides was delayed by up to 3 to 5 days in that portion of the population that was molting. The researchers found natural infestations sampled before application had 20 to 50% of the cutworms in a premolt or recent postmolt stage and were not feeding. [10]

    To check if the majority of cutworms are feeding, cut open the gullet and look for green material. If no food is found, the cutworms may be in a molting phase and chemical control of molting individuals may be delayed. Insecticides registered for cutworms in canola do have sufficient residual effect that desired results should occur eventually. 

    Insecticides registered for cutworm control in canola: Check your provincial crop protection guide for registered control products and up to date registrations.

    Insecticide products for control of cutworms

    Product Chemical Group
    Name
    Chemical Group
    Number
    Active
    Ingredient
    Preharvest
    Interval
    Application LD50 (Mammalian
    Toxicity)1
    Pounce; Perm-UP: Ambush Pyrethroids 3A Permethrin Treat prior to 6-leaf stage      Ground 1276
    Lorsban; Pyrinex; Nufos; Citadel      Organophosphates 1B Chlorpyrifos 21 days Aerial or ground      205-418

    1 LD values represent the relative toxicity of a pesticide. They represent the dose (in mg/kg body weight) that will kill 50% of the test animals. Thus the lower the number the greater the toxicity. Values given are for oral LD.

    http://www.agriculture.gov.sk.ca/Guide_to_Crop_Protection

    http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex32

    References

    [1] Jacobson, L.A. 1970. The Canadian Entomologist. 102: 85-89.

    [2] Cheng, H.H. 1973. The Canadian Entomologist. 105: 311-322.

    [3] Ayre, G, L, Turnock, W, J, & Struble, D, L. 1982. The Canadian Entomologist. 114: 993-1001.

    [4] Cutworms in Field Crops, MAFRI 2011.

    [5] SAF News, Hartley. S, 2008

    [6] Ayre, G.L. & Lamb, R.J. 1990. The Canadian Entomologist. 122: 1059-1070.

    [7] Insect pests of the Prairies, U of A, 1989. Philip, H., and Mengersen, E.

    [8] Agriculture and Agri-Food Canada Insect Identification Sheet No. 35 December 1979.

    [9] Species and abundance of cutworms (Noctuidae) and their parasitoids in conservation tillage fields. Turnock, W.J. (Agriculture Canada Research Station, Winnipeg, Man. (Canada)); Timlick, B.; Palaniswamy, P. Agriculture, Ecosystems and Environment (Jul 1993) v. 45(3-4) p. 213-227.

    [10] Effect of Inactivity Associated with Interstadial Molts on Short-Term Efficacy of Insecticides for Control of Pale Western Cutworm (Lepidoptera: Noctuidae) BYERS, J. R.; HILL, B. D.; SCHAALJE, G. B. Journal of Economic Entomology, Volume 85, Number 4, August 1992 , pp. 1146-1149(4).