Updated: October 23, 2024
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IPM Threshold Guide for Vegetable 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 2018
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

COLE CROP INSECTS

Plant Emergence (or Transplanting) to Beginning of Heading or Reproductive Development
CABBAGE ROOT MAGGOT Control when planting
FLEA BEETLES > 50%of newly emerged plants infested and "shothole injury" is present.
Treatment thresholds for leafy cultivars not available
CABBAGE APHIDS AND OTHER APHID SPECIES Broccoli and Cauliflower:  infestations of all species combined reach nearly 100%
Brussel Sprouts:  > 15% of plants are aphid infested from transplanting till 3 weeks before harvest
Cabbage:  2% of plants are infested with 5 or more aphids on leaves
THRIPS Fresh market cabbage:  > 20%of plants infested
IMPORTED CABBAGEWORM, CABBAGE LOOPERS & DIAMONDBACK MOTH CATERPILLARS Sample 50 plants - treat when count is > 0.5 larval units per plant

Weighting factor for larval unit determination:
Cabbage Looper: large=1.0 small=0.7
Imported Cabbageworm: large=0.07 small=0.1
Diamondback Caterpillar: any size=0.1

Heading or Sprout Development
CABBAGE APHIDS AND OTHER APHIDS Treat when 2% of plants are infested with > 5 aphids
IMPORTED CABBAGEWORM, CABBAGE LOOPERS & DIAMONDBACK MOTH CATERPILLARS Cabbage: treat when count is > 0.5 larval units per plant
Broccoli, Cauliflower and Brussel Sprouts:  treat when > 1 caterpillar per 25 plants

CUCURBIT INSECTS

Plant Emergence to Three Leaf Stage
SPOTTED AND STRIPED CUCUMBER
BEETLES		 For wilt susceptible cucumbers and muskmelons –use systemic insecticide treatment at planting time

Provisional threshold for pickling cucumbers –foliar insecticide when 20%of plants are infested with cucumber beetles

Three Leaf Stage to Harvest Maturity
MELON APHID Provisional threshold > 20%of runners have > 5 aphids on leaves
THRIPS Heavy infestation, leaf injury, plants not actively growing
SPIDER MITES > 50%of runners show early leaf injury on crown leaves and live mites present

Three Leaf Stage to Harvest Maturity
SPOTTED AND STRIPED CUCUMBER BEETLES Thresholds not available

Treat moderate to high CB infestations levels on wilt susceptible varieties

Treat all cucurbits when high CB infestations cause excessive fruit damage
LEAFHOPPERS Severe leaf injury expected to retard fruit maturity and affect yield
SQUASH VINE BORER As soon as moths are trapped

LIMA BEAN INSECTS

Bloom to Harvest
PLANT BUGS (LYGUS) Early bloom to harvest >6-10 adult/nymphs per 20 sweeps

After mature bud set >20-40 adults/nymphs per 20 sweeps
CORN EARWORM > 50% of larvae are > 1/2"

Fordhook Lima Beans: Up to 4 weeks from harvest > 1 larvae per 6' of row
Less than 4 weeks from harvest 3 larvae per 6' of row

Baby Lima Beans: > 1 larvae per 6' of row, from late flat pod stage to harvest

PEA INSECTS

PEA APHID 50 aphids per sweep or 5-10 per plant

PEPPER INSECTS

Early Fruit Set to Harvest
GREEN PEACH APHID Before fruit formation > 2 aphids per leaf

Once fruit is present 4 aphids per leaf
PEPPER MAGGOT  As soon as flies are caught on sticky traps
EUROPEAN CORN BORER  When fruits are forming on plants > 25 moths trapped per 5 days, average  

Shorten treatment schedules if > 100  moths trapped per 5 days 
CORN EARWORM  When fruits are forming on plants, >100 moths trapped per 5 days (see sweet corn section for
MDA Pest Survey website link)

POTATO INSECTS

Plant Emergence to 12" Shoots
POTATO FLEA BEETLE >20%leaf loss
COLORADO POTATO BEETLE Overwintered >5 adults per 10 plant clusters and > 10%shoots chewed off at ground level

All stages

Chemical treatments: Defoliation 20%and density per 10 plant clusters > 5 adults or > 40 small larvae or 15 large larvae or combination of any 2 stages, at 1/2 above levels

Bt treatment: 10%plant infestation, > 30%eggs hatched (trigger date)
Greater Than 12" Shoots to Bloom
GREEN PEACH APHID, POTATO APHID Prior to bloom - 2 per leaf
During bloom - 4 per leaf
Within 2 weeks of vine kill - 10 per leaf
Greater Than 12" Shoots to Bloom
MELON APHID  Prior to bloom >1 per leaf
During bloom > 2 per leaf
Within 2 weeks of vine kill > 5 per leaf
POTATO LEAFHOPPER > 3 adults per sweep or > 1 nymph per 10 leaves
EUROPEAN CORN BORER 100 moths trapped per 5 days (reduce if host plants unavailable) or 5% of leaves are infested with egg masses
Bloom to 50% Leaf Yellowing or Vine Kill
COLORADO POTATO BEETLE Defoliation > 30%and potential for further damage

 

SNAP BEAN INSECTS

Plant Emergence to 3rd Trifoliate Stage
SEEDCORN MAGGOT 5 to 10 maggots per seed
THRIPS > 6 per leaflet with leaf injury
SPIDER MITES > 20 live mites per leaflet
BEAN LEAF BEETLE AND MEXICAN BEAN
BEETLE		 Pre trifoliate stage 6 or more per row foot
Post-trifoliate stage 20% leaf loss, > 2 per plant
Prebloom Stage (3rd Trifoliate to Bud)
POTATO LEAF HOPPER > 5 adults+nymphs per sweep
MEXICAN BEAN BEETLE > 20%defoliation
BEAN APHID 50%or more have 5 or more aphids per terminal, distributed throughout
GREEN CLOVERWORM >20%defoliation and >15 larvae < 1"per sweep
Bloom to Harvest
LEAFHOPPERS During podset >25 per adults/nymphs per sweep
During bloom >12 adults/nymphs per sweep
MEXICAN BEAN BEETLE Defoliation > 10%during podding and population present
EUROPEAN CORN BORER >25 moths trapped per 5 days
CORN EARWORM > 100 moths per 5 days

SWEET 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
FLEA BEETLE For Stewart’s wilt susceptible varieties from spike stage to silking >5% of plants infested
CORN EARWORM At tassel emergence >15% tassel infestation
From tasseling to harvest -1st spray at 30% silk and apply subsequent sprays according to 5-day trap catch

 

TOMATO INSECTS

Plant Emergence or Transplant to 10" Plants
COLORADO POTATO BEETLE Overwintered CPB:
At plant emergence - adults reducing plant densities below recommended levels for maximum yield

Actively growing > 15 adults per 10 plants
10" Plants to Early Fruit Set
COLORADO POTATO BEETLE All stages

Chemical Treatments: Defoliation 20% throughout and > 20 adults and/or larvae per 10 plants
Bt Treatment:10% plant infestation with egg masses and > 30% eggs hatched (trigger date)
10" Plants to Early Fruit Set
POTATO APHID, GREEN PEACH APHID Natural controls not present and > 20% of terminals are infested
SPIDER MITES No specific threshold - treat during hot dry weather when damage is noticed due to heavy infestations
HORNWORMS 20 % defoliation and further damage potential
Early Fruit Set to Fruit Maturity or Vine Kill
COLORADO POTATO BEETLE Defoliation potential > 10%or > 2% of plants have at least 1 freshly-injured fruit
TOMATO PINWORM Active leaf mines > 0.7 per trifoliate leaf
TOMATO FRUITWORM > 5 damaged fruit in sample of 200 (2.5% damage)

WEEDS OF FIELD CROPS

ANNUAL WEEDS # per 25 sq. ft. to cause 10% loss:

Cocklebur ------------------------------------------------ 3
Jimsonweed or Velvetleaf ---------------------------- 3
Pigweed, Lambsquarters or Morningglory ------- 5
Annual grasses ----------------------------------------- 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 equals the 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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