Armyworms are a recurring threat to hay and silage crops in southern Victoria, with potential to cause significant crop damage if left unchecked. For farmers and harvest contractors, understanding when and how to monitor armyworms can mean the difference between a healthy crop yield and severe losses. We often know of farmers coming back to harvest ryegrass hay paddocks in late spring and early summer, only to be left with ryegrass stripped of all its leafy goodness!

This brief guide is designed to help you stay ahead of armyworms by focusing on early detection and timely intervention.


Why Armyworm Monitoring Matters

Armyworms can devastate a crop quickly, especially in the weeks leading up to harvest. Unfortunately, without regular paddock inspections, detection of armyworm infestations often only once the caterpillars have grown into large, highly destructive larvae. By adopting a proactive, regular inspection routine from November through to Christmas, you can catch infestations early, identify small caterpillars, and prevent widespread damage.


Key Points on Armyworm Inspection

Timing and Frequency:

  • Start weekly inspections from early November, increasing thoroughness from late November until the new year. The final three to four weeks before harvest are critical for detection.
  • Check early and often to catch young larvae when they’re easiest to control.

What to Look For:

  • Young Armyworms: They’re challenging to find when under 8 mm in length but cause little damage at this stage. Early detection can make all the difference.
  • Signs of Armyworm Activity: Look for chewed foliage, frass (larval droppings), and leaf skeletonizing, especially on the lower leaves in dry areas of the paddock.

Check Key Paddock Areas:

  • Drier Parts of Paddocks: Armyworms prefer these areas, so ensure multiple checks are made across different areas, paying extra attention to dry zones first.

Count Caterpillars per Square Metre:

  • To gauge the infestation level, count the number of caterpillars per square metre. High numbers suggest action may be required to avoid crop loss.


    Action Thresholds and Intervention

    Damage Thresholds: For many crops, 5-10 armyworms per square metre is typically manageable, but as numbers climb, action may be necessary to prevent severe crop damage.

    Treatment Options: If thresholds are exceeded, contact your local agricultural supplier for suitable insecticide recommendations. Always follow recommended usage guidelines to ensure effective control without disrupting beneficial insect populations.

    Continued Monitoring Post-Treatment: After treatment, continue monitoring to ensure effectiveness and prevent a resurgence. This will help ensure armyworms don’t damage crops in the critical weeks leading up to harvest.


      By implementing these regular inspections, you’ll gain peace of mind knowing your hay and silage crops are protected against armyworm damage, helping to secure your feed supply and ensuring productive, healthy yields for your farming operation.