Updated: April 4, 2024
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: Updated 2017.
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) 1

2-18” - > 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 per100 sweeps or 0.2 per sweep
4-6”stem ht. – 50 per100 sweeps or 0.5 per sweep
7-10”stem ht. – 100 per100 sweeps or 1.0 per sweep
10-14”stem ht. – 200 per100 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 per ft. of row or 3 per 25 sweeps
Wide rows - 2 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/linear row ft. & larvae < ¾ inches
Barley - 1/linear row ft. & larvae < ¾ inches

WEEDS OF FIELD CROPS

ANNUAL WEEDS # per 25 sq. ft. to cause 10% loss: Drilled Row
  Cocklebur 1 3
  Jimsonweed or Velvetleaf 1.5 3
  Pigweed, Lambsquarters or Morningglory 3 5
  Annual grasses 5 20
PERENNIAL WEEDS % of field infested:

Light 30%
Moderate <10% 
Heavy <30%
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

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