"I do monitor — if I see a pest, I treat immediately." But a pest spotted on a plant is already a population that has had time to multiply and spread. Traps and scouting routes are not there to confirm that a pest is present — they are there to catch the moment it first appears, before it becomes a problem.
Quick Glossary
- Action threshold (economic threshold) — a quantitative indicator defining the point at which protective measures become economically justified; below the threshold — observe; above it — act
- Yellow sticky traps — the standard tool for detecting flying pests (thrips, whitefly, aphids, leafminers); yellow attracts most phytophages
- Scouting route — a regular walk through the greenhouse along a fixed path, inspecting a set proportion of plants and recording results
Why Traps Without Counts Are Just Decoration
A yellow trap that hangs for a month without being removed for counting is not monitoring — it is decoration. The value of a trap is not its presence but its trend: how many individuals this week, more or fewer than last week, which species dominates.
Count once a week, on the same day — this produces comparable data. Remove the trap, count individuals by species (a magnifying glass helps), record in a log, and hang a new one. Without a log there is no trend. Without a trend there is no basis for a decision.
Number of traps: at least one per 50–100 m², plus additional traps near entrances, ventilation points, and areas where pests appear first — corners, lower shelving, zones with poor airflow.
Yellow vs Blue Traps: What and For Whom
Yellow traps are a general-purpose tool that attracts most phytophages: whitefly, aphids, leafminers, and fungus gnats. Thrips also fly to yellow, but respond significantly better to blue.
For greenhouses where thrips is a known risk — use both yellow and blue traps: yellow for general monitoring, blue specifically for thrips. Place blue traps at the level of flowers and shoot tips — where thrips feed and where they are detected earliest.
Placement height matters: yellow traps at the level of plant canopy tops or slightly above. Move them as the crop grows. A trap two metres above a low-growing crop intercepts pests moving between zones but does not reflect conditions within the canopy.
Scouting: The Standard and What to Look For
A scouting route is not a casual walk-through. It is a fixed path, a fixed number of plants, and fixed inspection points.
Standard: walk the greenhouse along one route, inspect every 5th to 10th plant (minimum 10% of total), and for each plant check three zones:
- Underside of leaves — spider mite (fine speckling, webbing), thrips (silvery streaking, black frass), whitefly (eggs and nymphs)
- Growing tip and young leaves — aphids (colonies at the growing point), thrips (distortion of new leaves)
- Root zone and substrate surface — fungus gnats (larvae in the top layer), signs of soil-dwelling pests
Scout result: number of affected plants out of 10 inspected, by pest species. This is the numerical basis for decisions.
Action Thresholds: When to Watch, When to Act
An action threshold is not a universal table — it depends on crop, growth stage, and pest species. Reference values for protected cultivation:
Thrips: trap alert threshold 5–10 individuals per week; action threshold 20+ per week. During plant inspection — act when signs of damage are found on more than 10% of scouted plants.
Whitefly: alert threshold 1–5 individuals on a trap per week (any appearance warrants increased monitoring); action threshold 10+. For susceptible crops — tomato, cucumber — the threshold is lower.
Spider mite: not caught on traps (mite does not fly) — scouting only. Action threshold: signs of damage on 5–10% of plants, or an active population (moving individuals and eggs) on more than 3 out of 10 plants.
Aphids: action threshold is low due to rapid reproduction rate and ability to transmit viral diseases — act on detection of any colony at shoot tips.
These thresholds are guidelines, not absolutes. When beneficial insects are present in the system, the threshold may be higher — the predator compensates. In monoculture with a long cycle — lower.
Three Mistakes That Cost the Most
Not recording scout results. "I remember there were fewer last week" is not data. Without a log, trends are invisible. Two weeks of records already provide more information than a year of informal observation.
Changing the route and number of plants between scouts. If 5 plants were inspected this week and 15 in a different part of the greenhouse next week, the data are not comparable. Standardising the scouting route is the foundation of comparable results.
Reacting to trap counts while ignoring the scout route. Traps show what is in the air. Scouting shows what is happening on plants. Thrips may be within normal range on the trap but already actively feeding on lower leaves — and without scouting this goes unnoticed until visible damage spreads. Both tools together.
How to Know the Monitoring Is Generating Real Data
- The log is filled weekly with specific numbers by species
- There is a trend — rising, stable, or falling — and that trend explains decisions about protective measures
- The decision to treat is made before visible damage has spread to more than 20% of plants