How to Manage Pests

UC Pest Management Guidelines


Scheduling Defoliation

(Reviewed 5/13, updated 5/13)

In this Guideline:

Harvest cotton as early as possible to avoid yield and quality losses due to insects, weathering of cotton fiber, moisture, and poor drying conditions. Harvest operations should start only after an acceptable percentage of bolls are mature and open, and after effective defoliation is achieved.

Defoliation is the last operation where management decisions can have a large impact on profit. Defoliants are applied to cotton to increase the rate of leaf loss and desiccation, allowing for timely and efficient harvesting. An earlier crop termination is often suggested when populations of whiteflies or aphids are threatening. Be sure to control threshold and greater-than-threshold populations. An additional consideration of timing defoliation and harvest is the regulation of root dislodging, residue shredding, and residue incorporation for the pink bollworm control program in the San Joaquin Valley.

Make defoliation decisions on a field-by-field basis. You will see a wide range of crop maturity and impacts of weather and crop conditions among fields. Note that fields with even vegetative cutout and good boll load will be far easier to defoliate than fields with a non-uniform fruit set. Remember that there is substantial late vegetative growth due to factors like late plantings, late irrigations, or high late-season soil nitrogen. Plan for early crop termination where whitefly or aphid infestations are severe.

To time application of defoliants, count the nodes above cracked bolls (NACB) and follow these guidelines:

  • Generally, if the percent open boll is 60% or higher, it is safe to defoliate.
  • For Upland or Acala, 4 to 5 NACB is the target for the first harvest aid.
  • For Pima, 3 to 4 NACB is the target for defoliant timing.

Best Conditions for Effective Defoliation

  • Moderate to high air temperatures (daytime greater than or equal to 80 F, nighttime greater than 60°F)
  • Relatively low plant and soil nitrogen levels
  • Moderate soil water levels (plants not water stressed)
  • Relatively uniform crop development; plants at vegetative cutout with limited or no regrowth
  • Weeds, insects, and diseases under control
  • Ability to get good chemical coverage and penetration of the chemicals into the plant canopy

Pay attention to the calendar, the weather, and consider how much risk you want to take when choosing a final harvest date. Use these steps:

  1. Keep an eye on predicted trends in the weather.
  2. Consider your own experience when determining the number of days needed for harvest.
  3. Determine the last harvest date you consider to be an acceptable risk.
  4. If using nodes above cracked boll (NACB) to determine readiness for harvest, recommended values are 3 NACB for Pima and 4 NACB for Acala and Upland varieties.

If the crop is late and it is not possible to wait to reach the recommended NACB values, count back 21 to 28 days from the desired harvest date to determine when to start with the defoliation program. In this case, the start of defoliation is independent of the maturity stage of the crop.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Cotton
UC ANR Publication 3444

General Information

  • L. D. Godfrey, Entomology, UC Davis
  • P. B. Goodell, UC IPM Program and Kearney Agricultural Research and Extension Center, Parlier
  • E. T. Natwick, UC Cooperative Extension - Desert Research and Extension Center, Imperial County
  • D.R. Haviland, UC Cooperative Extension, Kern County and UC IPM Program
  • V. M. Barlow, UC Cooperative Extension, Riverside County and UC IPM Program

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