Cutworms (Agrotis spp.) are a sporadic caterpillar pest that are usually only a problem in the seedling and vegetative stages. Distribution in the paddock is usually patchy or confined to portions of the paddock. While generally a pest of seedlings (1 to 5 leaf stage), cutworms can occasionally cause damage at tillering and early stem elongation in winter cereals.
Insects and damage
Cutworm larvae have darkish bodies, often with longitudinal lines or spots. They curl up when disturbed.
Damage:
Small larvae cause skeletonised or scalloped leaves (damage may be confused with that of lucerne flea or pasture web worm)
Large larvae (40-50 mm) are the damaging stage, severing seedling stems near ground level. These larvae may remain below the soil surface feeding on the stem at or below ground level.
Monitor and record
Cutworm can be difficult to detect as most species feed at night and shelter under the soil surface during the day.
Inspect crops regularly from emergence to establishment. Larvae are active from late afternoon through the night.
Look for signs of feeding on leaves – if detected, use a trowel to check around the base of freshly damaged seedlings for sheltering larvae, or where the plant stand is thinned.
Larvae may move into a crop from a neighbouring weedy fallow, particularly as the weeds start to dry off, or are sprayed. Be alert to sources of larvae from outside the field.
Beneficials
Fly and wasp parasites (particularly orange and two-toned caterpillar parasites and orchid dupe), predatory beetles and diseases may suppress cutworm populations, but are unlikely to prevent crop loss in the event of an outbreak.
Cultural control
Control weeds in and around fields prior to planting to reduce potential cutworm infestations.
Be aware of cutworm movement from sprayed weedy fallow into neighbouring crops.
Prolonged green feed in autumn allows larvae to develop to a large size by the time crops emerge.
Aim to control potential hosts at least 2 weeks prior to planting to ensure larvae do not survive to infest crops.
Thresholds (speculative)
2 larvae per 0.5 metre of crop row (visual inspection)
Chemical control
Chemical control is warranted when there is a rapidly increasing area or proportion of crop damage.
Treat the crop when seedling loss is nearing minimal plant density crop requirements
Treat older plants if more than 50% of plants have 75% or more leaf tissue loss
Chemical control is most effective when applied late in the day to maximise likelihood of larvae contacting/ingesting insecticide when they emerge at night to feed
Ground rig applications may provide flexibility to treat just affected areas, or to apply a border spray where larvae are moving into the crop from neighbouring weeds.
Multi-pest considerations
Using broad-spectrum insecticides to control cutworm will affect other establishment pests (lucerne flea, mites and early infestations of aphids) and beneficial insects.
Be aware of the impact of chemical choices on insecticide resistance in earth mites (synthetic pyrethroids can flare resistant mites)
