Below certain levels, pests do not cause enough damage to warrant the cost of control. It is important, however, to know what levels of pests are acceptable and what levels should trigger immediate control method application. Thresholds are frequently defined by members of the academic community. Researchers derive threshold values through studies that determine the amount of damage caused by different pest levels. Unfortunately, threshold development in nursery crops has lagged. However, over time, growers can develop specific threshold guidelines for operations.

The use of thresholds allows for decreased use of pesticides over the traditional scheduled spray method (where pesticides are applied on a schedule regardless of pest presence). Thresholds help reduce development of resistance, reduce reentry intervals related production disruption, improve plant growth in cases where pesticides are slightly phytotoxic, and reduce pesticide expenditures (chemical cost and labor cost).

illustration of thresholds

Thresholds are established at the level where the increased revenue derived from pest control is sufficient to offset the cost of the control method to be applied. These factors are most dependent upon the specific crop and its intended use. Many factors should be considered when determining how much damage can be tolerated:

  • What kind of pest is causing damage
  • What is the crop or cultivar that is being damaged
  • How much damage can be tolerated in the final product
  • Is the plant part that is being damaged the part that is marketable (root damage in cut flower production?)
  • How fast acting are the control methods?
  • Time until harvest
  • Stage of plant development
  • Regulations for pest load levels
After determining how much damage and pest presence can be tolerated, thresholds can be developed. Monitoring is a critical component because it leads to good records. Pest records and the resulting crop value can allow development of "acceptable pest levels." Thresholds must be flexible to allow adaptation to new crops or altered conditions. Thresholds should be quantitative and can be adapted to all production operations.