Updated: June 1, 2026
By R. David Myers
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IPM Threshold Guide for Agronomic Field Crops

This reference was adapted from the University of Maryland and Delaware Cooperative Extension Filed Crop and Vegetable IPM Pest Management Manuals.

Compiled R. D. Myers 2000; Updated: 2009, 2017, 2026.

Compilation and layout assistance by Carol Jelich, Master Gardener, Anne Arundel County

Reviewed by Galen Dively, Terrance Patton, and Sandra Sardenelli, University of Maryland, College Park

ECONOMIC THRESHOLD

Level of pest activity when control action is suggested to prevent economic injury.

Alfalfa Insects

ALFALFA WEEVIL

Begin sampling when feeding damage is noticed

  • Record # of larvae/30 stem sample
  • Determine average plant height in inches (based on $100/ton hay value & $10.00 spray cost/A)
    • 12-18” - > 60 larvae/ 30 stems.
    • 18-24” - > 75 larvae/ 30 stems.
    • > 24” - > 80 larvae/ 30 stems or cut.

POTATO LEAFHOPPER

Sample with 15” sweep net, measure stem
height:

  • < 3”stem ht. – 20 per 100 sweeps or 0.2
    per sweep
    4-6” stem ht. – 50 per 100 sweeps or 0.5
    per sweep
    7-10”stem ht. – 100 per 100 sweeps or 1.0
    per sweep
    10-14”stem ht. – 200 per 100 sweeps or 2.0
    per sweep

PEA APHID

50 aphids per sweep or 5-10 per plant

Soybean Insects

DEFOLIATION & STAND REDUCTION 

  • Seedlings - 30% defoliation or 25% stand Reduction 
  • Vegetative stages - 30% reduction 
  • Bloom through pod fill - 15% defoliation Full green bean to 50% leaf drop - 35% defoliation 

EARLY SEASON DEFLOIATORS – BEETLES 

Defoliation threshold & > 5 larvae per ft. of row 

LATE SEASON DEFOLIATORS -- WORMS & BEETLES 

  • Defoliation threshold & 5 larvae per ft. of row 

SPIDER MITES 

  • > 50% of plants with stippling on 1/3 of leaves and 50 mites per leaflet 

THRIPS 

  • drought stressed - 8 per leaflet 

POTATO LEAFHOPPER 

  • Drought stressed - 4 per sweep 
  • Non-stressed - 8 per sweep 

CORN EARWORM 

Drop cloth or Sweep net 
  • Narrow rows - 1 larvae per ft. of row or 3 per 25 sweeps 
  • Wide rows - 2 larvae per ft. of row or 5 per 25 sweeps

Corn Insects

CUTWORM 

  • 1-2 leaf - 10% damaged plants 
  • 3-4 leaf - 5% damaged & 4 larvae per 100 plants 

WHITE GRUB 

  • Heavy soils - 2 per sq. ft. 
  • Sandy soils - 1 per sq. ft. 

WIREWORM 

  • 1 per bait station 

SLUG

  • spike to 3 leaf - 5 per plant 

STALKBORER 

  • 4%, 6% or 10% damaged at the 2, 3 or 4 leaf stage 

ARMYWORM 

  • 35% of plants > 50% defoliated & larvae < 3/4” 

EUROPEAN CORN BORER 

  • Not irrigated - 80% infested with live larvae 
  • Irrigated - 50% infested with live larvae 

CORN ROOTWORM 

  • 1 Western or 2 Northern per plant

Small Grain Insects

CEREAL LEAF BEETLE 

  • Wheat - 1 larvae per flag leaf 
  • Oats - 2 larvae per flag leaf 

GRAIN APHID 

  • Tillering - 150 aphids/row ft. & < 1 predator/50 aphids 
  • Heading - 25 aphids/head & <1 predator/50 aphids 

GRASS SAWFLY 

  • 0.4 larvae/ linear row ft. & larvae > ¾ inches 

TRUE ARMYWORM 

  • Wheat - 2-3 larvae/linear row ft. & larvae < ¾ inches 
  • Barley - 1 larvae/linear row ft. & larvae < ¾ inches

Weeds of Field Crops

ANNUAL WEEDS 

# per 25 sq. ft. to cause 10% loss: 
  • Cocklebur
    • Drilled -1
    • Row - 3
  • Jimsonweed or Velvetleaf 
    • Drilled -1.5
    • Row - 3
  • Pigweed, Lambsquarters or Morningglory 
    • Drilled -3
    • Row - 5
  • Annual grasses
    • Drilled -5
    • Row - 20

PERENNIAL WEEDS 

% of field infested: 
  • Light <5% 
  • Heavy <30% 
  • Moderate <10% 
  • Severe >30% 

NOXIOUS WEEDS 

  • No threshold, eliminate all

IPM DEFINITIONS

Economic Injury Level - EIL

“The lowest pest population density that will cause economic damage. At the EIL the Cost of Control = Benefit of Control.”

Economic Threshold (Action or Treatment Threshold) - ET

“The density of a pest at which control measures should be implemented to prevent an increasing pest population from reaching the EIL -- ET is generally 80% of the EIL.”

IPM Graph