Sticky Traps in Greenhouses: Use and Monitoring

What is a sticky trap and what is it for?

Sticky traps are coloured cards coated with an adhesive that attract and catch flying pests in a greenhouse. They have two purposes: first and most important, monitoring (early warning), and second, a limited contribution through mass trapping. Their real value is that they catch pests before you can spot them by eye, so you can decide to intervene before the population explodes. In my field work, regular trap reading is one of the cheapest but most valuable habits in biological control. Trap counts are the data behind integrated pest management decisions; without monitoring, the call on when and what to treat is guesswork.

What is the difference between yellow and blue traps?

Trap colour is chosen by target pest, because each insect is drawn to a different colour. Yellow traps strongly attract whitefly, aphid and the fungus (sciarid) fly at the soil surface. Blue traps are more effective for thrips. For that reason it is usually best to run both colours in the same house, so different pests are monitored at once. Using a single colour risks missing a whole pest group entirely.

Where and how are traps placed?

A trap’s effect depends on correct positioning. Cards are hung just above the crop canopy, at points that catch light and airflow, and are raised as the plants grow. Greenhouse entrances, vent openings and areas that had problems in the previous season are where pests appear first, so extra traps there strengthen early detection. Distributing traps evenly across the house prevents decisions being made on the misleading density of a single spot.

How is a trap read?

Hanging traps is not enough on its own; regular reading is the heart of the job. Recording the pest count on each trap at least once a week shows the direction of the population: if counts are rising you bring intervention forward, if they are flat or low you avoid an unnecessary application. That record takes biological control out of guesswork and grounds it in data. Tracking which trap shows which pest rising also tells you where in the house the problem started, so intervention becomes local and timely.

Do sticky traps eliminate the pest?

This is the most common false expectation. Even when they catch large numbers, sticky traps will not end a population on their own; their real function is monitoring and early warning. There is a mass-trapping benefit, but real control is carried by timely beneficial insect releases and cultural measures. The trap is the dashboard that tells you when and where those decisions should be made. I cover how the release program is built in the beneficial insect release program article.

Are traps a problem when beneficials are present?

Sticky traps are not selective; in theory they can catch some beneficial organisms too. In practice, a balanced number of traps used for monitoring rarely does meaningful harm to the beneficial population, and the early warning they provide is worth far more than that risk. Even so, hanging very large numbers of traps for mass trapping needs careful thought during the period when beneficials are establishing. The right balance is to treat the trap as a monitoring tool and leave control to biological releases.

How do traps link into a monitoring system?

Sticky traps deliver their highest value not alone but as part of an integrated monitoring system: regular counting, record keeping, yellow and blue traps used together, and results tied back to release decisions. Once that system is in place the pest is caught before it explodes, and intervention becomes both cheaper and more precise. For a monitoring and biological control program tailored to your greenhouse, the other guides on the biological control hub page are a good next step.