Agriculture & Natural Resources

IPM – Potato Scouting Guidelines

I. Pre-planting Decisions

  • Rotate field away from previous years solanaceous crop.
  • Dispose of cull piles
  • Use certified seed potatoes
  • Eliminate volunteers
  • Obtain access to weather-station information for late blight forecasts

II. Plant Emergence to 12 Shoots (scout twice weekly)

A. Flea Beetle: Damage will appear as small round holes in the leaves.

  • Sampling: if small round holes are present in the leaves, take 10 sweeps in 10 locations. Also, rate the infestation level as absent, light, medium or heavy.
  • Threshold: Although generally not a problem when soil systemic insecticides are used, the feeding injury should not exceed 20% leaf loss. A treatment should be considered if you find 2 or more beetles per sweep and defoliation is greater than 20%.

B. Overwintered Colorado Potato Beetle (CPB) Adults

  • Sampling: Begin scouting at plant emergence for adults. Fields should be sampled twice a week. Be sure to watch for movement of adults from adjacent fields that were planted in a host crop the previous season. Check for adult beetle on 5 consecutive in 10 locations. Record the number of beetles and the percent defoliation.
  • Threshold: 5 adult beetles per 10 plants and greater than 10% defoliation.

C. Colorado Potato Beetle (CPB) – All Life Stages

  • Sampling: Begin scouting fields at plant emergence for eggs, small larvae (first and second instars), large larvae (third and fourth instars), and adults. Check 5 consecutive plants in 10 locations for fields up to 40-50 acres in size.
  • Thresholds:


Number of CPB per 50 Plant /Stem Sample
Adults Small Larvae Large Larvae
Low 0-15 0-75 0-30
Medium 16-24 76-199 31-74
High 25 or > 200 or > 75 or >


  • Treat when you find the high level of any one stage present or
  • Treat when you find any two life stages at the medium level
  • A defoliation threshold of 20% should also be considered for Superior potatoes. This will vary with the variety and the producer.

D. Aphids

  • Sampling: Sample for aphids at the same ten sites sampled for CPB
    Collect 5 leaves at each of the 10 sites site but not from consecutive plants
    Early Season: collect leaves from the entire plant
    Mid-Season: concentrate on sampling from mid-lower sections of the plant
  • Thresholds
    Green Peach Aphid and Potato Aphid


Prebloom: 2 per leaf (100/50 leaves)
Bloom: 4 per leaf (200/50 leaves)
2 weeks from vine death: 10 per leaf (500 per 50 leaves)

Melon Aphid:
At all stages, reduce the threshold per leaf by one half.

E. Late Blight

  • Sampling: Look for large, black or purplish lesions on stems or leaves and the telltale white mold growth usually on the undersides of the foliage. Be sure to check the leaves and stems under the crop canopy where the disease is most likely to get established first. Scouting should be concentrated in low-lying areas, field edges along creeks and ponds, near center pivot irrigation rigs, and in areas near woodlots or any area where leaves remain wet for long periods of time.

  • Threshold: Although no threshold is available, the use of prediction systems can be used to time fungicide applications. The first fungicide should be applied when 18 severity values are accumulated.

III. Greater than 12 shoots (> 12 inches) to Harvest

A. Colorado Potato Beetle – sample as above growth stages

B. Aphids – same as above growth stages

C. Potato Leafhopper

  • Sampling: Begin sampling as soon as leafhopper adults are first active in alfalfa in the area or when adults are first noticed jumping from the foliage as you sample for other insect pests. Take 10 sweeps in 10 locations for adults as you are sampling for other pests. As you sample for aphids on the leaves, be sure to count the number of nymphs per leaf as well.
  • Thresholds:
    • Treat if you find 0.5 adults per sweep and 0.1 nymphs per leaf or
    • Treat if you find 0.5 – 1 adult per sweep and nymphs and/or adults have been present for > 2 weeks or
    • Treat immediately if you find 1 – 1.5 adults per sweep and nymphs are
      present or within one week if no nymphs are detected
    • Treat immediately if you find > 1.5 adults per sweep

D. European Corn Borer (ECB)

  • Sampling: Be sure to watch local blacklight trap catches and take note of ECB moths flushed out of the plant canopy as you sample for other pests. One moth catches in the BLT reach 10 per night, begin looking for small entrance holes of small larvae entering the leaf petioles, leaf mid-ribs, and upper nodes on 5 main shoots in each of 10 locations to determine the percent infested shoots
  • Threshold:
    • The first treatment should be applied when 10 % (fresh market) or 25% (processing) of the shoots show at least one entrance hole caused by small larvae.
    • Fields should be re-sampled within 5 days of a treatment to determine if additional egg hatch and terminal damage has occurred .
    • A second treatment should be applied 7-10 days after the first treatment if moth catches are above 10 per night and terminal injury has increased.

E. Late Blight – Sample same as above. The use of prediction systems can now be used to determine spray intervals for late blight.



University of Delaware IPM Program
Joanne Whalen, Extension IPM Specialist