How to Calculate the Ideal Economic Threshold for Your Scouting Efforts

In order to make smart decisions regarding the control of pests and diseases found on your crops, you need to be able to effectively estimate damage boundaries.

A good integrated pest management program is indispensable, and the center of any IPM strategy is scouting.

Scouting provides a greater awareness of pest populations, their activity, and the progression of diseases. It also addresses the real needs of the crop, reduces routine application of chemicals, and assures that chemical treatments are applied at the proper time, for maximum effectiveness.

Most importantly, scouting allows for proper planning so you can take action before crossing the “economic injury level” of a pest or disease. This can be tricky because the right time to take action is based on various factors. Your goal should be to avoid significant economic loss from pests or from unnecessary action (aka, your economic threshold). It’s worth noting that the economic threshold can change along the growing season as crop sensitivity changes.

These are the factors to take into account for calculating economic threshold:

  • Expected yield loss due to the scouted pest
  • Yield market value
  • Total expected yield
  • The estimated success rate of the planned action (pesticide application, the release of a natural enemy, changing environmental conditions, etc.)
  • Action cost

How often should you scout?

Frequent scouting is essential for successful pest and disease management. Learn your greenhouse and determine how often to scout based on your particular needs.

Note, frequent scouting can increase the chance of detecting an early infestation but also can lead to the introduction of pests and the transmission of pests and disease within the greenhouse.

Graph showing the optimal time based on infestation severity to treat pests

Scouting Tools

  • Sticky cards are used for scouting flying insects. When using sticky cards, the target pest will determine the number of cards used. For most pests, placing 4 to 5 cards per acre of growing area is sufficient. However, if whiteflies are present, up to 1 sticky card per 1,000 square feet is recommended. Sticky cards are available in both yellow and blue. While yellow is the all-around color of choice for general-purpose trapping, blue cards are more effective in attracting thrips.
  • Visual inspection is necessary for wingless insects such as mealybugs and spider mites, where the use of sticky cards is irrelevant. With visual inspection, it is important to monitor the various plant parts (leaves and stems) to determine the presence of non-flying insects and various life stages (eggs, larvae, nymphs, and pupae).
  • Underside monitoring of leaves Most pests are located on the underside of leaves at different stages, making routine inspection critical.
  • Loupes are recommended for getting a closer look at leaf symptoms or pests, when surface monitoring of plants proves to be ineffectual. Use white paper to observe small pests like thrips after tapping the plants.
  • Immunostrips are recommended for verifying bacterial or viral infections.
  • Maintaining records is helpful to locate “hot spots” of insects or disease development in the greenhouse.