Manage Healthy Crops - Look Out for Cutworms
As we go further into the 2018 growing season, growers and agronomists are on the lookout for pests in their crops. Cutworms have been a problem in the prairies in recent years. Good scouting and early control are key to preventing significant yield loss from these insects. Young seedlings are often the target of cutworms; however, cutworm populations are difficult to predict, and each species leave different symptoms on the plant. South facing slopes on lighter land are more popular for cutworm moths to lay eggs on. This typically occurs in late August to early September of the previous year due to the hot and dry weather. There are several species of cutworms that are of economic significance in Saskatchewan, but only four are responsible for severe damage.
These four species of cutworms begin damaging seedlings from May to late June. These species include the dingy cutworm, pale western cutworm, redbacked cutworm and army cutworm. All four are predators of major crops including oilseeds, cereals, forages and pulse crops. Dingy cutworms’ mature larvae are about 25-32 mm in length and possess a distinguishing broad grey strip on their back. The pale western cutworm mature larvae are approximately 40 mm in length and have a partly grey and green body, with a yellow-brown head while redbacked cutworms are about 38 mm in length, with broad reddish-brown stripes ranging down its back. The army cutworm is about 37-40 mm in length and its larvae has a greenish-grey body and a light brown head. All four pests tend to be hairless, long and smooth. They are most active during the night or late evening, sheltering in the soil or under debris during the day.
Only the larval stage of their lifecycle causes damage to crops. The pale-western and redbacked cutworms overwinter in the top one cm of soil, hatching and beginning their larval stage from early April to late June. Due to their early activity in the spring, they pose a great risk to early seeded crops. The dingy cutworm larvae will begin to pupate by mid-June, whereas the army cutworm larvae undergo pupation by mid-May.
Each species has a different feeding pattern. Redbacked cutworms and pale western cutworms are considered below-ground feeders. Young larvae of redbacked cutworms and pale western cutworms begin feeding on the crop before its emergence, damaging the plant shoots and furled leaves. In early June, the larvae mature and begin feeding on the plants just below the soil level. Dingy and army cutworms are considered above-ground feeders as young larvae chew holes and notches in emerging leaves. As the larvae reach maturity, they tend to strip the plant stem. Damage can range from notches among leaf margins to full consumption of the leaf. The young larvae pose an increased threat over the adult since they eat large amounts of foliage when undergoing maturation. A good reference for more information on cutworms is “Cutworm Pests of Crops on the Canadian Prairies-Identification and Management Field Guide” (Agriculture and Agri-Food Canada).
Scouting on a weekly basis throughout mid-May to mid-June is critical. It is important to keep an eye out for wilted or collapsed plants, plants that have notched leaves or bare patches which are all giveaway symptoms of cutworm activity. Growers need to determine whether the bare patches in their fields are due to poor germination, wireworms or cutworms. The top 5-7 cm (2-2.8’’) should be examined in order to determine if the damage caused by cutworms is above the economic threshold. The economic threshold for these cutworms is generally 3-5 larvae/m2, with 25-30% stand reduction in canola from dingy cutworm activity, assuming that it is not one block in the field. Spraying should be done before the damage exceeds the economic threshold.
The type of cutworm species may have a small impact on spray timing decisions, but ultimately the control methods will be the same. Insecticides are available to control cutworms and should be applied in the evening when cutworms emerge to the soil surface to feed. Spraying at night also helps to lessen the damage of insecticide on beneficial insects such as bees. Only infested areas are required to be treated, as the larvae that are missed will eventually come into contact with treated residues and will be exposed to the insecticide. Other control methods for cutworms involve natural predators. Many predaceous insects, parasites and birds reduce the cutworm population during their lifecycle. These include ground beetles, parasitic wasps and tachinid flies. Keeping uncultivated fields weed-free reduces the likelihood of female moths laying eggs. Crops seeded later in the spring are less susceptible to losses from cutworm. If volunteer cereals show signs of cutworm infestation, cultivating the soils and keeping it black for 10 days prior to seeding will starve the larvae. For further information, please refer to:
• 2018 Guide to Crop Protection available from the Saskatchewan Ministry of Agriculture. www.saskatchewan.ca/agriculture
• Agriculture Knowledge Center at 1-866-457-2377 or aginfo@gov.sk.ca