Scouting for Black Cutworm

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BCW size comparison

Scouting for cutworms is easily combined with stand evaluations and scouting weeds for herbicide application timing.

The first sign of black cutworm damage is leaf feeding on emerged corn or weeds. Sometimes, larvae will cut weeds before they move to corn. Be wary when lambsquarters and ragweed patches begin to disappear without the aid of an herbicide.

The leaf feeding and missing or cut plants are not hard to see but it is useful to find a few of the larvae that caused the damage to determine 

BCW scouting

size and species. This can be frustrating so why bother? Knowing the size of cutworm larvae will help determine the potential for damage. Black cutworm are more damaging to corn than some other species. Dingy cutworms are a common species that feeds at or above the soil surface. As a result, it does not cut corn below the growing point.

Cutworms are nocturnal. During the day, they hide under soil clods, crop residue and loose soil, typically at the boundary between dry and moist soil. Cutworms will likely be deeper when soils are dry. Carefully look under pieces of residue and soil clods close to cut or injured plants. If you don’t find a cutworm near the base of an injured plant, look near a couple plants on either side in the row. Using a possum-like defense strategy, most cutworm species roll into a semicircle and remain motionless when disturbed.  Unfortunately for us and other predators, most species, including black cutworms, are more or less soil colored.

Finding cutworms in high residue, cloddy or muddy conditions is especially difficult. With leaf feeding you are looking for very small larvae. Move to another area with injured plants if unsuccessful. Looking at this optimistically, you only need to find a few to make your treatment decision. Do not confuse headless, legless cranefly larvae with cutworms. 

Do stand counts in areas of the field with damage and note the percentage of plants with leaf feeding and  those cut. To help with your decision, you can flag areas of row within in the field and return the next day to determine if damage is ongoing.

Cutworms infestations in small corn (3 leaf or less) require more aggressive management than large corn. Don't give up on scouting too early. Late instar black cutworms can kill up to 6 leaf corn by burrowing into the growing point.

More on thresholds - When to treat a problem

Cutworms reduce corn yield by deceasing final stand or plant population. The generic economic threshold for black cutworm in corn is 2 to 3 percent of the plants cut or wilted when the larvae are less are than ¾ inch long. The threshold increases to 5% cut plants when larvae are larger. However, because of high corn prices these thresholds could be lowered to 1% wilted or cut and small larvae and 2 -3% wilted or cut for large larvae.

Remember to take into consideration corn populations in individual fields and adjust threshold numbers accordingly. For example, if the current plant population is at or near yield limiting levels, you can afford to lose fewer plants than in a field with a higher emerged population. The role of plant stand in determining yields can be found in Table 1.

table 1

Source: Manage corn planting decisions to optiize yield and economic return; Jeff Coulter, U of M Extension Corn Agronomist

The reason for cutworm sized based threshold is based in larval feeding. Cutworms must shed their skins (molt) in order to grow.  The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~1/2 inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are full grown and cease feeding between 1 ½ and 2 inches long. While larger larvae will cut or tunnel into larger plants, they have less time left to feed and as a result have the potential to cut fewer plants. Table 2 below gives approximate sizes in length and width of the head for the larger instar black cutworm larvae.

table 2

The rescue insecticide calculator and Table 3 are an example of a dynamic threshold that is used in several management guides. Modern corn yields and prices would indicate treatment at a very low percentage cut plants using this worksheet, perhaps leading to over-reactive treatment decisions. The yield loss factors are still useful when combined with Jeff's (Table 1) yield loss by stand reduction charts.

table 3

Yield loss = Yield Loss Factor (Table 3) X % of plants cut (as decimal) X Expected Yield X Price per Bushel

  1. Determine average instar of the black cutworm larvae and number of corn leaves.
  2. Consider soil moisture inadequate if top 3 - 4 inches are dry and rain is not in forecast.
  3. Find yield loss factor in Table 3.
  4. Calculate projected yield loss.
  5. Insecticide treatment is warranted if projected yield loss is greater than cost of insecticide + application.

Source: University of Illinois Extension & Outreach: Black Cutworm

What about other crops?

Broadleaf crops have their growing points above ground at emergence. This means a cut plant can be killed and even climbing cutworm species can be a threat.

Soybean: While black cutworm will cut soybeans they are seldom a yield limiting problem. Soybeans are seeded at a much higher plant density and can compensate (up to a point) for reduced stand much better than corn.   

Sugarbeets are at risk because of their yield and quality sensitivity to stand. In addition, they are planted early and often with an oats cover which may encourage black cutworm egg laying. Cutworms move to beet seedlings as oats and weeds are killed by herbicides.

Insecticides

Cutworms are controlled well with rescue insecticide applications. Spot treatments can be effective when combined with careful scouting. Make sure you still have cutworms present if you make a decision to treat. 

Many products provide effective control of black cutworms. In a dry spring (top 3-4 inches of soil are dry) insecticides are less effective. A rotary hoe or row cultivation can help improve efficacy if done before a pyrethroid insecticide or after chlorpyrifos is applied.

Always read and follow the pesticide label.