Cutworms: How knowing the species and biology can help you better scout and mange them. John Gavloski, entomologist, Manitoba Agriculture, Food and Rural Development, Carman, MB, R0G OJO E-mail: [email protected]

Common crop feeding cutworms in Manitoba include the redbacked cutworm (Euxoa ochrogaster), dingy cutworm (Feltia jaculifera), and darksided cutworm (Euxoa messoria) (Turnock et al. 1993, Ayre and Lamb 1990). Although many aspects of the lifecycle and feeding behaviour of redbacked and darksided cutworms are similar, they are different in dingy cutworm.

Redbacked cutworms Dingy cutworms

Some attempts have been made to group or categorize cutworms based on the feeding habits of the larvae. An understanding of the feeding behaviour can have implications for how cutworms may be managed. One such grouping (Walkden, 1950) based on feeding habits, categorises cutworms as:

Subterranean cutworms: Species that feed almost exclusively beneath the surface of the soil. Surface feeders: Species that feed at or near the surface of the soil. Tunnel makers: Species that make tunnels opening at the surface of the soil, and after cutting off portions of the host plant, drag their food into the tunnel to consume it there. Climbers: Species that climb the host plants and feed on the stems, buds, foliage, flowers or seeds.

Subterranean cutworms include pale western cutworm (Agrotis orthogonia) which is not common in Manitoba, and the glassy cutworm (Apamea devastator) which occasionally will be found associated with grassy crops. Because larvae of these species feed under the surface of the soil, they are difficult to control with insecticides.

Surface feeding cutworms include dingy and army cutworms. Redbacked cutworm generally feed at the soil surface or slightly below the surface, although on fruit crops they may climb plants to feed on leaf and blossom buds.

Climbing cutworms include the darksided cutworm, variegated cutworm (Peridroma saucia) and clover cutworm (Anarta trifolii). When feeding on older plants the feeding can occur well up the plant.

Fortunately, all of our more common species in Manitoba will come above the soil to feed, and can be successfully managed with insecticides. Thorough scouting and appropriate timing of applications can maximize the success of the application and targeted applications can help ensure the most economical returns.

There are few cultural controls of cutworms that would ensure a high level of protection of crops in large-field situations. There are some potentially very effective natural enemies, although populations of natural enemies can vary.

Predators and Parasites of Cutworms

Numerous beneficial insects, including predatory ground beetles and parasitic wasps and flies prey upon cutworms and can reduce the population. Some species of birds also prey upon cutworms. A study in Alberta found 7 species of ground beetles fed on larvae or pupae of redbacked cutworms in the field (Frank, 1971). Take steps to preserve such beneficial organisms in and around fields. Reducing unnecessary spraying for other pests and providing refuges may help to encourage beneficial organisms.

Moulting of Larvae and Activity Level

Larvae normally have 6 stages of development (known as instars) before they become fully grown. Between each instar stage, the larvae “moult” or shed their skin. During moulting the larvae are inactive and remain several centimeters below the soil surface. Some studies have found 20 to 50% of the cutworm population were in a pre-moult or recent post-moult stage and were not feeding (Byers et al., 1992). Laboratory studies have shown the moulting period during which the larvae are inactive comprised approximately 33 percent of the entire larvae life stage. Feeding cutworms can be identified by the presence of greenish mid-gut contents visible through the semi- transparent underside. A moulting cutworm will not have food in its gut.

Not all cutworms will be killed immediately after an insecticide is applied, and control will increase over a few days as more cutworms are exposed to or ingest the insecticide. Moulting larvae will not be feeding, and that can cause a delay in control of those individuals when insecticides are used as a control strategy. So if you still find some living cutworms after an insecticide application, don’t necessarily conclude the insecticide is not working. Give it some time for moulting larvae to resume feeding.

How deep do cutworms generally go in the soil?

The depth of cutworms in the soil during the day may depend on the age of larva and how moist the soil is. For example, in a study from Ontario on darksided cutworm, older larvae were observed just under the soil surface in wet weather, however in dry weather they were found at a depth of 8 to 10 cm (3 to 4 inches) from the soil surface (Cheng, 1973). Most younger larvae rested in the soil during the day at a depth of 6-13 mm (¼ to ½ in.) near the base of the host plant.

Can some crops compensate for feeding by cutworms?

Flax plants can have some increased growth in response to a reduction in plant density because of cutworm feeding (Ayre, 1990), although this may not fully compensate for the plant loss. Research on black cutworm in corn also showed some degree of yield compensation by uncut plants (Showers et al. 1983), however this study also demonstrated that under dry conditions less cutting was needed to reach a given level of yield loss compared to when conditions were damp (28 cm of accumulated precipitation).

Seeds per pod and seed weight of individual canola plants can in some instances increase with reduced plant density. This has been demonstrated in a study where canola stands at the early vegetative stage was hand thinned to reduce populations from 100-200 plants per m-2 to 40 plants m-2. This reduction in plant stand resulted in less than a 20% loss in seed yield (McGregor 1987). Compensation was attributed predominantly to an increased number of pods on the remaining plants, which increased from 20-90 to as many as 600.

Looking for Cutworms

During the day cutworms are usually below the soil or debris and come to the surface at night to feed. So during the day you may see damage left on plants, but no cutworms. Watch for plants cut off near the ground or plants that are noticeably wilting (when cutworms chew on the stems but do not sever the plant). You may also see droppings on the ground, which can indicate cutworms were there. Digging around the immediate area of a damaged plant can confirm whether cutworms likely caused the damage.

Also pay attention to the stage of larvae and how many plants are being damaged or destroyed. If you are seeing a lot of damage and most larvae are still young, about 12 mm (½ inch) or less, that means you have quite a young population of larvae and they will be doing a lot more feeding. Many of our common species can be up to 38mm (1.5 inches) long.

Once cutworms start pupating their damage is done for the year. So if the damage is not too severe and larvae are large, it may be best to just wait for them to pupate. Once the larvae complete their last instar stage, they burrow deeper into the soil and form earthen cells where they pupate.

Literature Cited

Ayre, G.L. 1990. The response of flax to different population densities of the red-backed cutworm, Euxoa ochrogaster (GN.) (Lepidoptera: Noctuidae). The Canadian Entomologist 122: 21-28.

Ayre, G.L. and R.J. Lamb. 1990. Life histories, flight patterns, and relative abundance of nine cutworms (Lepidoptera: Noctuidae) in Manitoba. The Canadian Entomologist 122: 1059-1070.

Byers et al. 1992. Effect of inactivity associated with interstadial molts on short-term efficacy of insecticides for control of pale western cutworm (Lepidoptera: Noctuidae). Journal of Economic Entomology 85: 1146-1149.

Cheng, H.H. 1973. Observations on the bionomics of the dark-sided cutworm, Euxoa messoria (Lepidoptera: Noctuidae), in Ontario. The Canadian Entomologist 105: 311- 322.

Frank, J.H. 1971. Carabidae (Coleoptera) as predators of the red-backed cutworm (Lepidoptera: Noctuidae) in Central Alberta. The Canadian Entomologist 103: 1039- 1044.

McGregor, D.I. 1987. Effect of plant density on development and yield of rapeseed and its significance to recovery from hail injury. Canadian Journal of Plant Science 67: 43-51.

Showers, W.B., L. von Kaster, and P.G. Mulder. 1983. Corn seedling growth stage and black cutworm (Lepidoptera: Noctuidae) damage. Environmental Entomology. 12: 241- 244.

Turnock, W.J., B. Timlick and P. Palaniswamy. 1993. Species and abundance of cutworms (Noctuidae) and their parasitoids in conservation and conventional tillage fields. Agriculture, Ecosystems and Environment 45: 213-227.

Walkden, H.H. 1950. Cutworms, armyworms and related species attacking cereal and forage crops in the Central Great Plains. Circular No. 849. United States Department of Agriculture.