Cutworms

Cutworms

With the 2017 growing season on the horizon, growers and agronomists are preparing to keep an eye out for pests in their crops. Cutworms have become a problem in our trading area in recent years. Good scouting and early control measures are key to preventing crop losses from these crop pests.

Life Cycle

Scouting

It’s important to take note of areas that will be more likely to see cutworm pressure, and be vigilant about scouting in these areas. South facing slopes with lighter land are typically more inviting for cutworm moths to lay eggs on in late August-early September the previous year. Hot, dry weather in this period creates more favorable conditions for egg laying. Anecdotal evidence suggests that cutworms may be more likely to lay eggs on pea stubble, which tends to be warmer soil in the fall.

Cutworms will feed on most crops, but canola seems to be most susceptible to damage. Unlike wheat, canola does not have the capability to tiller and recover from feeding.  Canola may fill in somewhat if the stand has been thinned, but bare spots in the crop will not recover, and will create issues for weed control further in the season.

Key scouting time for cutworms is late May to early June.  Watch for wilted or toppled over plants, or plants that have notched leaves, or leaves with holes in them.  A tell-tale sign of cutworm feeding is bare patches in the field (although this must be differentiated from poor seed germination).   Scout the edges of bare patches to find worms, as they will not stay in areas where there is no food source available.  Use a small shovel or trowel to look for worms 1-2 inches from the seed row, and about a half inch deep.  A small screen can be used to sift the worms from the soil. Cutworms will curl up as soon as something touches them. This is a characteristic of most species of cutworm and army worm. Cutworms are more active during the night, but can be scouted for during the day.

If cutworms are found in a field, leave them undisturbed until they uncurl, and assess the length of about 10 worms. Larvae that are less than ½ – 1 inch long have the most potential to grow, and cause more damage due to feeding. Larvae that are 1 – 1 ½ inch long are close to maturity, and have already caused the majority of damage that they are capable of.  To assess whether the cutworms are actively feeding, cut open the worms to see if there is green material in the gut. If there is no green material in the gut, the worms are probably resting between developmental stages (instars), and it may be more effective to wait until worms are actively feeding to spray.

Most species of cutworms go through six larval stages, called instars. The speed at which the cutworms   develop through each instar is temperature dependent. For example, in one study, Red Backed cutworms took 65 days to move through 6 instars at 15° C. At 25° C, they moved through their development twice as fast. This means that feeding activity may be accelerated to support their growth, and crop damage may also increase in proportion to the feeding pressure.

Determining Species

There are several different species of cutworms of economic significance in Saskatchewan, but there are only two species in the north west that typically cause any real damage.  Red-backed cutworms create most of the problems in the park land zone, but Pale Western cutworms also cause damage to a lesser extent. Pale western cutworms are the major culprit for damage in the southern part of the province.

Red backed cutworms have two broad, dull red stripes along their back.  The head is yellowish-brown.  Pale western cutworms are almost colorless in early instars, and become greyish white-green as they mature.  They have no striping, and the head is also yellowish brown.

Each species has different feeding patterns.  Red backed cutworms tend to chew holes and notches in leaves while they are still below ground when the larvae are young. When the larvae are older, they tend to strip the plant stem at or just above the soil surface.  Pale Western cutworms feed mostly underground, and will cut holes in emerging leaves. Pale western cutworms tend to move down seeding rows in the field.

The species of cutworms may have a small impact on spray timing decisions, but ultimately control methods will be the same regardless of species when dealing with red-backed or pale western cutworms.  This is however, different in other areas of the province where different species cause damage, and different species’ behavioural patterns will dictate different methods for control.

Control Methods & Deciding When to Spray

There are mainly two different control methods for cutworms – insecticidal seed treatments and in-crop insecticides. Seed treatments to control cutworms in canola are Fortenza and Lumiderm. There are no seed treatment products available to control cutworms in cereal or legume crops.  For these products to work, the cutworms must feed on plants that have the seed treatment applied, so the grower must be willing to accept some damage to the crop (similar to products that control flea beetles).  In most cases, if the seed treatment products are overcome by high cutworm pressure, the companies that manufacture the seed treatment will cover a portion of the cost of in-crop insecticide. If a grower is seeding canola on ground that has seen previous cutworm pressure, or has characteristics that make it attractive to cutworms (south facing slope, light, dry land), then a seed treatment is a good choice to prevent cutworm damage.

A threshold of 25-30% damage in canola crops has been suggested as a guideline for spraying.  However, if cutworms are detected in heavy enough numbers prior to seeing that level of damage, and worms are small, the decision to spray can be made before the damage threshold is reached.  The best control of cutworms is achieved when spray applications are made at night when worms are actively feeding. Spraying at night also helps to mitigate the impact of insecticide on beneficial insects.

Products available for cutworm control include chlorpyrifos (Lorsban, Pyrinex, Nufos, etc.), Matador/Silencer, or Coragen. Consult with your agronomist to make the choice that best fits your farm. Each of these products works in different ways, and all have varying levels of residual control. If spray timing is less than ideal, or cutworm larvae are going through a molting stage (and may not be actively feeding), choose a product that has a longer residual activity to achieve better control.

Most cultural control methods for controlling cutworms (such as tillage, delayed seeding, etc.)  have larger agronomic drawbacks than the advantage they would give for cutworm control. Following practices that allow canola plants to reach the two leaf stage as quickly as possible helps to reduce the window of risk where cutworms can cause damage. The greatest risk is associated with cutworms that are feeding by snipping off entire plant stems. When the plants are too big to snip through in one bite, they have a greater chance of survival. When plants can grow to this stage in two weeks rather than three, the window of risk is shortened. Practices that give the plant a head start include applying Jumpstart to the seed, applying seed placed phosphorus fertilizer and using good fertility in general, seeding shallow, and encouraging quick germination with appropriate seed treatments.

 

Written by: Amber Bernauer