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Managing Soil Cutworms in Corn


For further information on cutworms attacking corn, refer to NebGuide G80-501, Corn Cutworms.


Cutworm Biology

Black Cutworms
 

Some damage to field corn from cutworms occurs every year in Nebraska. Several different species of cutworms can attack corn. The severity and the area affected varies greatly, and is dependent on the species involved, previous crop history, and weather conditions.

Cutworms that attack corn can be divided into two general categories based on seasonal life cycles. Black cutworms do not overwinter in Nebraska. Dingy, claybacked, darksided, sandhills, pale western, and other species overwinter as partially grown larvae in the soil.

Since black cutworms do not overwinter in Nebraska, they are dependent on spring weather conditions, primarily southeasterly winds, to bring them into our state. Nebraska is on the western edge of the black cutworm's area of influence, and they are rarely found west of the 100th meridian. Because of their cutting habits and the possibility that large numbers can be transported to Nebraska if favorable weather conditions occur, they have the most potential to cause a widespread problem.

April and May weather patterns are the first key to potential black cutworm problems. In recent years pheromone traps and light traps have been used to monitor the flight patterns and populations of black cutworms. The presence of moths in a trap only indicates potential problems and is no guarantee that extensive damage will occur. Trap counts are more useful in alerting growers and consultants as to when to begin scouting efforts. Scouting should begin for black cutworm larvae before 300 growing degree days (50 degree F base) accumulate after significant catch.

Black cutworm moths prefer to lay eggs in green vegetation or heavy surface residue, particularly no-till soybean stubble. If food hosts are available (weeds or corn), the cutworms will survive.

Cutworms that overwinter as larvae generally prefer to lay eggs in the fall in green vegetation such as weed hosts, small grain stubble, legumes, rye, and pasture. The eggs hatch and the larvae feed on the vegetation present before overwintering. In the spring, after the previous crop is removed and the corn emerges, the cutworms will transfer their feeding activity to the corn. Damage from these species will occur as soon as the corn emerges. Recent experience has been that corn planted into alfalfa that has been killed in the spring has a greater potential for cutworm problems.

You cannot be sure that tillage will have a significant effect on cutworm populations. If fields were tilled before black cutworm migration, it may limit egg laying in those fields. Cutworms already in the field may suffer some mortality by mechanical action, but there is no guarantee that tillage by itself will eliminate cutworm problems. Many cutworm problems have occurred in fields that have been conventionally tilled. Previous vegetation is probably the most important factor in cutworm potential.

It is extremely rare to experience cutworm problems in continuous corn. Corn stubble is not a preferred egg laying site. Potential problems in continuous corn may be the result of a late season flush of weeds or an interseeding of a fall cover crop such as rye, which would attract egg laying moths.

Pest Management of Cutworms in Corn

Seedling Damage by Small Cutworms
Plant Cut Beneath Soil Surface
Several options exist for the grower who wants to manage cutworms in corn. Since a vast majority of corn acreage is not affected by cutworms, the most economically sound practice is to scout for cutworm damage as soon as the corn emerges and apply a rescue treatment if necessary. Cutworm damage may usually first appear as small holes chewed into the leaves. Use this feeding as an early indicator. Cutworms are nocturnal and will not normally be seen on the surface of the soil during the day. Scrape the soil around the base of damaged plants and look for a fat-bodied, grayish-brown to blackish colored worm curled into a C-shape.

The size may range from 3/8 to over an inch in length. They are often found near the moisture line, usually within 2 to 3 inches of the plant. Not every damaged plant will have a cutworm near it, as they will migrate from plant to plant. Survey at least 20 consecutive plants in a minimum of 5 places in the field, more if there are indications of feeding. Early detection of a problem is essential because most of the cutting occurs within 7-10 days of plant emergence. However, with cooler weather, growth of corn and cutworms will slow and activity may be extended. Generally, a rescue treatment should be considered if 5% or more (1 plant in a set of 20) feeding damage has occurred, cutting is observed, and the worms are one inch or less in length.

As the cutworms become larger, they begin cutting the plants off at or under the soil level. Corn is most vulnerable in 1 to 4 leaf stage. Corn cut off at the soil surface may recover, but the further advanced the corn is, the less recovery there will be. From a practical standpoint, scouting should begin soon after corn emergence and through the 4 leaf stage, at least every 3 days. It should be noted that late flights of black cutworm have produced damage to corn as advanced as 4 to 8 leaf stage and occasionally larger. These situations usually occur in no-till into late-spring killed alfalfa or under very weedy conditions, which attract egg-laying moths. The corn plants are too large for the cutworms to cut off, and they will bore into the stalk just below soil line, damaging the growing point, causing "dead heart" symptom.

Rescue treatments are effective in controlling soil cutworms. Ambush 2E, Asana XL, Lorsban 4E, Pounce 3.2EC and Warrior 1EC have all given satisfactory control as postemergence sprays. These materials are all registered for chemigation and should provide excellent control if chemigated. In dryland situations, if the soil is dry or crusted, rotary hoeing immediately before or after Lorsban application may enhance control. The other insecticides are pyrethroids and should not be incorporated. Most labels give a range of rates to choose from. Generally, the middle range is adequate for cutworm control. Very dry conditions may require higher rates.

There is some use of planting time treatments for cutworm control. The use of granular soil insecticides and broadcasting or banding liquids has met with mixed success. In the case of black cutworms, the material may deteriorate before the black cutworms migrate into an area. Planting time treatments may work better on cutworms that overwinter in the soil, since they are already present when treatment takes place. Excessively dry conditions may limit the activity of granular insecticides. Therefore, all fields should be scouted, even those that had a preplant or planting time application for cutworm control. The primary drawback to using planting time treatments is economic. Since there is no way to know whether or not a field is or will be infested with cutworms, most of these "insurance" type treatments are applied when nothing is present, resulting in a loss of money spent for the insecticide.

Insecticides for Soil Cutworm Control in Corn