What makes a corn field high risk for black cutworm?

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Tillage and crop rotation

The overwintering cutworm species lay eggs based on soil type and previous year’s vegetation. Black cutworm moths arriving in Minnesota seek out areas with crop debris, sheltered areas and low spots in the field to lay eggs. Early season weed growth is very attractive to the moths and winter annuals (e.g. shepard’s purse) and early spring broadleaves (e.g. common lambsquarters) patches in fields are often infested.

The moths are not attracted to spring tilled fields.  Unworked fields, or fields with reduced tillage where more crop debris is on the surface, attract more egg laying moths. The higher risk of black cutworm attack in conservation tillage can be seen in tillage plots at the Southern Research and Outreach Center in Waseca in 1985 (Figure 1) and 1986 and the Southwest Research and Outreach Center in 2001 (Table 1).

Table 2. Black cutworm damage amon tillage systems

Waseca, 1986
Lamberton, 2001
Fall MB plow/Spring Field Cultivator 5.0
Fall Chisel Plow/Spring Field Cultivator 10.1 1.4 c
Spring Field Cultivator 3.0 c
Ridge Till 14.7 7.9 ab
No Till 10.2 1.0 c
Fall Strip Till 4.0 bc
Spring Strip Till 9.2 a

Fall tillage that buries crop residue and spring tillage eliminates early spring weed growth before the flight arrives reduces the risk and severity of black cutworm attack. Historically, soybean residue is more attractive than corn but this may be partially due to the amount of fall tillage or to species and numbers of broadleaf weeds in the seedbank between the two crops. Tillage after egg laying generally has little impact on either egg or larval survival, unless the field is kept black for a couple weeks after egg hatch, long enough to starve the larvae but a yield avoiding planting strategy.

Figure 2. Influence of tillage and previous crop on black cutworm damage in 1985

Influence of tillage & previous crop on black cutworm damage in 1985

BT Hybrids, seed applied and at-plant insecticides

Some folks are adding a soil insecticide to Bt-RW corn in areas with Bt-resistant rootworm populations. That is an entirely separate issue than cutworm management.

Soil applied at-plant insecticides can provide control of cutworm larvae. However,  they are not recommended as insurance applications for two reasons. At planting, it is difficult to predict which individual fields will have economically damaging cutworm infestations. Secondly, post-emerge insecticide rescue treatments work very well.

T-band applications for granular insecticides, if so labeled, are sometimes more effective on cutworm than in-furrow applications. However, the banded insecticides are not necessarily more effective on corn rootworm. Be cautious of potential interactions between organophosphate (counter is on example) and some herbicides. Always read the pesticide labels and use the appropriate rates. Incorporate the insecticide bands as indicated on the label. Windy planting conditions reduce the accuracy of banded applications when not incorporated. Later blowing of loose dry soils can also reduce efficacy of non- incorporated bands.

High rates of neonicitinoid seed treatments (e.g. Poncho, Cruiser, Gaucho) are very effective on many seed and seedling insects and they can provide some protection against black cutworm. They may not always provide satisfactory control.

Bt hybrids containing the Cry 1F protein (Herculex /HX1) or Vip3a protein (Viptera), alone or in stacks, are labeled as controlling black cutworm. While they reduce risk, they might still be damaged as observed in some 2012 fields. An at-plant insecticide is probably not that helpful for cutworms when added on these hybrids. Remember, the Cry 34/35 Ab1 (Herculex RW protein) is not the same as the Cry 1F above-ground protein.

Large numbers of late-instar cutworms moving from weeds to take a bite of corn can overwhelm the neonicitinoids or Bt in corn tissues.

There has been no evidence of black cutworm zombies. Therefore, you should only need to kill a cutworm once. A Bt hybrid with the insecticide treated seed and with an added insurance at-plant insecticide would be trying to kill a cutworm three times - even before you knew if moths landed nearby. A little over prepared, perhaps?

Scouting and rescue insecticides applications are still the best defense against yield loss from black cutworms if problems occur.