Scouting for Black Cutworm

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BCW size comparison

Scouting for cutworms can be combined with stand evaluations and scouting weeds for herbicide selection and application timing.

The first sign of black cutworm is usually leaf feeding on emerged corn or weeds. Sometimes, the larvae will cut weeds before they move to corn. Look for wilted, partially cut plants. 

Be wary when lambsquarters and ragweed patches begin to disappear without the aid of an herbicide. Post emerge herbicide applications may cause cutworms to switch from feeding on weeds to corn.

BCW scouting

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 and determine size and species. This can be frustrating so why bother?

Knowing which species is present is important to understand the extent of the threat. Black cutworms are more damaging to corn than some other species. For example, dingy cutworms are a very common cutworm species in Minnesota that feed 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, increasing the difficulty of detection by crop scouts and other predators, most cutworm 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. All cutworm species have a distinct head capsule and three pairs of true legs near the front with fleshy abdominal prolegs at the back.

Take 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 the field and return the next day to determine if damage is ongoing.

Cutworm infestations in small corn (3 leaf or less) require more aggressive management than large corn. In a worst-case scenario, large cutworms can cut corn and other plants before they have a chance to emerge. Do not give up on scouting too early. Late-instar black cutworms can kill up to 6-collar corn by burrowing into the growing point. 

While not recommended as insurance treatments, corn rootworm and seed treatment insecticides labeled for cutworm can provide some control.  Make sure cutworms are still present!

How to prioritize your black cutworm scouting efforts

  • Look at the timing of cutworm flights in Table1. Focus your initial efforts on cornfields that were not yet tilled and planted when the moths arrived. The end of cutting column provides a guideline for when larvae will begin to stop feeding.
  • Fields without fall tillage and ridge/strip till fields can be higher risk. The later a field was worked and planted with respect to moth arrival, the greater the potential for large cutworms to cause stand loss.  
  • In corn, focus scouting efforts on fields where a hybrid without a Herculex I or Viptera aboveground Bt trait were planted. The Handy Bt Trait Table shows which Bt trait packages have some efficacy on black cutworm.
  • Small corn is at greater risk for yield loss. Large corn (> 5 collar) is usually safe from black cutworm damage.
  • Very small cutworms of any species can cut sugarbeets. Pay close attention to emerging seedlings and oat cover crops for signs of damage.
  • Fields with a history of winter annual and early spring emerging weed (e.g. common lambsquarters) are higher risk. Cutworms are forced to concentrate on crop seedlings after  weeds or cover crops are killed.
  • Egg laying is often concentrated in lower and moister areas of the field.
  • If you start finding economic populations of black cutworm in these higher risk situations. It would be worth the effort to look at other fields in the area if you have not already done so. Bt hybrids and insecticides can be overwhelmed when faced with very high populations of large cutworms.

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, with 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. Corn response to plant population (Lamberton, Waseca, and Morris, MN 2009-2010)
Final Stand (plants/acre) Expected Yield (%)
44,000 100
41,000 100
38,000 100
35,000 100
32,000 100
29,000 99
26,000 96
23,000 92
20,000 87
17,000 81
Source: Manage corn planting decisions to optimize yield and economic return; Jeff Coulter, U of M Extension Corn Agronomist


The reason for cutworm size 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 1½ 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. Approximate sizes for black cutworm larvae
Body Length head capsule width
Instar inch mm (range) mm (range)
3 1/3 7-9 0.6-0.8
4 1/2 12-25 1.1-1.5
5 1 25-37 1.8-2.4
6 1.5 30-35 2.5-3.3
7 31-50 3.6-4.3

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. Worksheet for black cutworm management in field corn. Rescue treatment yield loss factors and formula for projected crop loss

Avg. Moisture Conditions Number of Leaves (collars)

Inadequate Moisture Conditions Number of Leaves (collars)
Avg. Instar Approx. days left to feed 1 2 3 4 5 1 2 3 4 5
3 21 2.4 1.8 0.8 0.7 0.7 1.6 1.2 0.4 0.4 0.2
5 14 1.2 1.4 0.6 0.3 0.3 0.8 0.9 0.3 0.2 0.2
6 5 0.7 0.2 0.1 0.1 0.1 0.5 0.2 0.1 0.0 0.0

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.