Agriculture & Natural Resources

IPM – Processing Snap Beans Scouting Guidelines

I. Pre-planting Decisions

Use a combination of cultural practices to reduce problems from seed corn maggot: plow down cover crops 3-4 weeks before planting; completely bury cover crops or previous crop residues to reduce adult fly attraction to rotting organic matter; reduce use of heavy manure applications and let manure age before incorporating; and use a set of drag chains behind the planter during seeding to reduce the moisture gradient.

  • Use a seed treatment containing chlorpyrifos or thiamethoxam if a combination of factors favoring seed corn maggot exist.
  • Obtain access to weather-station information for white mold forecasts
  • Rotate fields with non-legume crops and plow under crop residue to avoid root rot
  • Select varieties with rust resistance

II. Emergence to Third Trifoliate

A. Thrips:

  • Sampling: Sample for thrips from plant emergence through bloom. Collect 5 leaves in each of 10 locations throughout the field and count the number per leaflet. Leaflets should be selected from the middle and top half of non-consecutive plants.
  • Thresholds: An insecticide treatment may be needed if thrips populations exceed 6 per leaflet. If plants are drought stressed and other insects are present, the threshold may need to be reduced by one third to one half.

B. Spider Mites:

  • Sampling: Field should be scouted on a weekly basis from early July through early August, especially during hot, dry seasons. Begin sampling along field borders and look for signs of white stippling near the base of leaves. Examine 10 leaves in 5 to 10 locations throughout a field and count the number of mites per leaf.
  • Thresholds: Treatment is recommended when white stippling is first noticed and you find 20 mites per leaflet. Rainfall and high humidity can help to reduce mite development and survival. However, rain will not provide enough suppression when heavy populations are developing under high temperatures.

C. Mexican Bean Beetle and Bean Leaf Beetle

  • Sampling: Begin sampling fields at plant emergence especially along field margins next to overwintering sites. Record the percent of stand reduction, estimate the percent defoliation and count the number of beetles per plant.

  • Thresholds: (a) Before the First Trifoliate: 6 or more beetle per row foot and 75% stand reduction (b) First to Third Trifoliate: 2 or more beetles per plant and 20% defoliation

III. Prebloom Stage: Third Trifoliate to Pre-Bud

A. Potato Leafhopper

  • Sampling: Using a standard 15-inch diameter sweep net, take 10 sweeps in 10 locations throughout a field. Count the number of leafhopper adults and nymphs and calculate the number of leafhoppers per sweep.
  • Thresholds: Treatment should be applied if populations exceed 5 or more per sweep.


B. Mexican Bean Beetle

  • Sampling: At this stage, counting the number of beetles per plant is too time consuming. Instead, estimate the percent defoliation to the nearest10% on 5 plants in 5 to 10 locations throughout a field. It is also important to determine the predominant life stage present (eggs, adults, larave or pupae).
  • Thresholds: A treatment should be applied if defoliation exceeds 20%. Controls should be delayed if the predominant life stages are eggs and/or pupae.

C. Bean Aphid

  • Sampling: Aphids are generally found on the lower leaf surfaces and terminal buds. Sample 5 terminals in 5-10 locations throughout a field and count the number of terminals infested with 5 or more aphids.
  • Thresholds: Treatment is recommended when populations are found throughout a field and 50% or more of the terminals have 5 or more aphids per terminal.

D. Green Cloverworm

  • Sampling: Using a standard 15-inch sweep net, take 10 sweeps in 10 locations throughout a field and count the number of larvae per sweep.
  • Thresholds: Apply an insecticide treatment if defoliation exceeds 20% and the cloverworm numbers exceed 15 larvae per sweep.

E. European Corn Borer

  • Sampling: If egg hatch occurs before bloom, young larvae feed on the leaves for a
    7-10 days before boring into the stems. Sample 5 plants in 5-10 locations for the presence of small larvae beginning to bore into stems. Blacklight trap counts over 20 moths per night also indicate the potential for stem feeding.
  • Thresholds: Treatment should be applied at the pre-bud stage when ECB moth catches exceed 20 per night in local blacklight traps and/or when first small larvae can be found boring into stems.

IV. Bud Stage to Harvest

A. European Corn Borer (ECB):

  • Sampling: Sampling is based on monitoring moth catches in local blacklight traps. For the most precise decisions, traps should be placed within one mile of each field. Traps should be serviced 3 times per week, preferably daily when moth activity increases.
  • Thresholds: The most critical time for corn borer treatment occurs at the bud-early bloom and pin stages. As a general guideline, treatment should begin when trap catches average 5 ECB moths per night. However, if moths can be observed readily in a field and trap catches have not reached 5 per night, sprays should still be applied at the bud-early bloom and pin stages. After the pin spray, the following thresholds and spray intervals should be used:
Number ECB Moths/ 5 Days Spray Interval
less than 10 no spray
11 – 25 7 Day
26 – 50 6 Day
51 – 75 5 Day
76 – 250 4 Day
250 + 3 Day


B. Corn earworm (CEW)

  • Sampling: Blacklight traps, pheromone traps, and direct observation of moths in fields should be used to monitor moth activity. A standard drop cloth can be used to detect small larvae.
  • Thresholds: As a general guideline, sprays should be applied on a 3-7 day schedule when blacklight trap catches exceed 20 per night. The spray interval will vary depending on corn borer moth activity and temperature.

C. White Mold

  • Sampling: Watch for the development of white mold, especially in narrow row plantings
  • Thresholds: Treatments will be needed when the soil is moist for 6-10 days before bloom. Treatments should be applied when 70 to 80 percent of the plants have one or more blossoms open. A second treatment is needed in 5 to 6 day if the soil remains wet and blossoms are still present.


University of Delaware IPM Program
Joanne Whalen – Extension IPM Specialist
 Bill Cissel – Extension IPM Agent