Insects and Mites

Western Bean Cutworm

Scientific name: Striacosta albicosta (formerly Richia albicosta and Loxigrotis albicosta)
Order: Lepidoptera
Family: Noctuidae

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Info

Biological Description
  • Moths: 1.5 inch wingspan; forewing has cream-colored stripe along leading edge; circle and crescent shapes outlined against black triangle on forewing
    • Can be confused with: Dingy cutworm, Spotted cutworm
  • Larvae:
    • Newly hatched: Dull orange body; black head; black pronotum (plate immediately behind head); 8-10 black spots per body segment
    • Mature larvae: Tan or brown body with pale broad stripe down center of back; orange head; pronotum has two broad, dark brown stripes; no distinctive spots;
      1.5 inches long
  • Pupae: Orange-brown color
  • Eggs: Round with small ridges from top to bottom; laid in clusters of 20 to 200; white when first laid then turn purple a day or two before hatching
Economic Importance
  • Western bean cutworm is a new pest to Wisconsin and damage to corn is sporadic.
  • Susceptible crops:
    • Corn
    • Dry beans --larvae feed on leaves and blossoms, and chew holes in pod walls and developing seeds.
    • NOT a pest of soybeans
Life Cycle
  • Western bean cutworm overwinters in Wisconsin as a full-grown larva within a soil chamber. Spring development begins when temperatures exceed 50°F.
  • Larvae pupate in May.
  • Moths begin to emerge from soil chambers in late June. Peak emergence in the Upper Midwest is typically between the second and third weeks in July.
  • Adult females lay eggs just before corn tassels. Eggs are laid primarily on the upper surface of the flag leaf (top-most leaf on the plant).
  • Eggs hatch in 5-7 days. Larvae feed on pollen in the whorl until the tassels emerge, at which point larvae switch to eating green silks.
  • By early September, mature larvae leave the ear and drop to the ground. They burrow into the soil and create a chamber for overwintering.
  • Generations:
    • There is one generation per year
Damage/Symptoms
  • Western bean cutworm is a late-season pest. Damage becomes evident between early August and early September.
  • Two types of damage:
    • In pre-tassel corn: Larvae feed on pollen in the developing tassel.
    • In tassel-stage corn: Larvae feed on shed pollen, leaf tissue, silks, and corn kernels.
  • Ear feeding:
    • Kernels at ear tip, middle, and near shank are fed upon.
    • Some larvae enter via the silks, but other larvae enter through the corn husk. Multiple larvae can be found in a single ear.
  • Secondary damage can caused by fungal pathogens.
  • Larvae do NOT tunnel into stalks.
  • Damage can be confused with:
    • Corn earworm
    • European corn borer
Scouting Procedure and Economic Threshold
  • When to begin scouting (two methods):
    • Degree-days (DD)
      • To calculate DD for a single day, take the average daily temperature (the high minus the low, and divide that number by two), then subtract the base temperature from that number. For more information on DD click here.
      • Starting May 1, add up daily DD using a base temperature of 50°F.
      • Begin scouting when 1,320 total DD have accumulated.
    • Pheromone traps
      • Pheromone traps are easy to make. Click here for directions on how to construct one.
      • Traps should be set by July 1 and monitored weekly or more frequently until trap captures decline in August.
      • Begin scouting when the first moths are detected.
  • Examine 20 consecutive corn plants at five locations in the field.
  • Check the upper three or four leaves of each plant for egg masses and small larvae.
  • Continue scouting for 7-10 days after peak flight.
  • Thresholds:
    • Field corn: foliar insecticide treatment should be considered when 8% of 100 corn plants have egg masses and/or small larvae.
    • Sweet corn: foliar insecticide treatment should be considered when 4% of 100 corn plants have egg masses and/or small larvae.
  • Corn in different stages need to be scouted separately.
  • Remember to scout non-Bt refuges for western bean cutworm when the rest of the field is planted with a western bean cutworm-resistant Bt variety.
Integrated Control
  • Natural control: Heavy rains, cold weather, and high winds can cause extensive mortality of early instar larvae. Cold winter temperatures will kill western bean cutworm larvae overwintering near the soil surface.
  • Cultural control: Tillage has not been proven to be effective against western bean cutworm. Although plowing or disking soil may reduce survival of overwintering larvae in soil chambers, the effectiveness of tillage as a management tool requires further evaluation before it can be recommended as a cultural control.
  • Biological Control: Several predatory insects feed on western bean cutworm larvae including adult and larval lady beetles, as well as damsel bugs and spiders.
  • Chemical Control: Application timing is critical for foliar insecticides to be effective. Once larvae enter the ear, control is nearly impossible.
    • If eggs have hatched and corn is tasseling: Apply insecticide after 95% tassel emergence but before larvae enter ears.
    • If eggs have not hatched and corn has already tasseled: Apply insecticide as close as possible to expected egg hatch (when egg masses have turned purple).
    Some insecticides for western bean cutworm control can flare spider mites, if present. For current Wisconsin recommendations, consult University of Wisconsin-Extension Bulletin #A3646, Pest Management in Wisconsin Field Crops for field corn and bulletin #A3422 Commercial Vegetable Production in Wisconsin for sweet corn.

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