Use yellow sticky traps as a monitoring tool in every greenhouse, all season, because they tell you a whitefly or fungus gnat population is rising while it is still small. Use released predators as the actual control. A closed structure is the one place biological control reliably works, since the predators stay where you put them instead of dispersing across the neighbourhood.
Traps are
Monitoring first, control second
Traps attract
Whitefly, fungus gnat, thrips
Ladybug appetite
Aphids, in quantity
Sprays and predators
Do not combine them
Researched from published specifications and verified owner reviews · Updated 2026-08-16
These are not really alternatives. One tells you what is happening and the other does something about it, and a greenhouse benefits from both.
The mistake worth avoiding is treating traps as a control method. A greenhouse full of sticky traps and a rising whitefly population is a common sight, and it means the traps did their job and the message was ignored.
The comparison
Figure 1. Sticky traps against released predators·7 rows
Figure 1. Sticky traps against released predators
Measure
Yellow sticky traps
Released predators
Primary purpose
Monitoring
Control
Catches adults
Some, of flying stages
Eats all stages it hunts
Works on fungus gnats
Yes for adults, good indicator
Specific predators exist
Works on aphids
Poorly
Yes, ladybugs eat many
Cost per season
Very low
Moderate, per release
Compatible with spraying
Yes
No, sprays kill them
Catches beneficials too
Yes, an argument for placement care
Not applicable
Researched general information rather than professional advice.
How to use traps properly
Hang Yellow Sticky Traps, 36-Pack just above the canopy and move them up as the crop grows, since the flying stages of whitefly and fungus gnat travel at canopy height. Put one near the door and one at each end of the house. Then, and this is the part that gets skipped, look at them weekly and count roughly rather than glancing at them.
A trap going from four insects to forty in a week is the signal to act. Waiting until the trap is covered means acting three generations late.
Why biological control works under glass
Released outdoors, Live Ladybugs, 3,000 Count mostly leave within a day or two. Released into a closed greenhouse with the vents meshed, they stay and eat, which is why commercial glasshouse growers rely on biological control rather than sprays. Release in the evening, after lightly misting the foliage so there is water available, and give them somewhere to land in the crop rather than tipping them onto the floor.
The obvious requirement is prey. Releasing predators into a house with almost no aphids wastes them, and releasing into one with an enormous population rarely catches up. The useful window is the one the traps identify.
Sprays and predators cancel each other out
Insecticidal soap and neem kill beneficial insects they contact just as readily as pests. Decide which approach you are taking. If you have released predators, put the sprayer away, and if you have just sprayed, wait until any residual activity has gone before releasing anything. Doing both in the same fortnight is paying twice for one result.
Fungus gnats are a media problem, not a pest problem
Traps will catch fungus gnat adults all season and never solve it, because the larvae live in the top layer of wet media. The actual fix is a dry surface: water from below with wicking trays or capillary matting, avoid overwatering, and move air across the bench. The traps then confirm the population is falling.
The decision, summarised
Every greenhouse, every season: traps, hung at canopy height and actually read.
A rising aphid population caught early: released predators, in a house with meshed vents.
Fungus gnats: change how you water, and use traps to confirm it worked.
An outbreak already out of hand: spray first, then release predators once residues have gone.
Deciding between the two approaches: exclusion with fine mesh reduces the need for both.
Monitoring, control and exclusion
The three parts of a greenhouse pest strategy, in the order they should be adopted.
+- Do yellow sticky traps actually control whitefly?
They remove some adults, which is a useful side effect, but they do not touch the eggs and juvenile stages on the leaf undersides and they will not stop a population that is already established. Their real value is early warning. A trap that goes from a handful of insects to dozens in a week is telling you to act now rather than in a fortnight.
+- Will released ladybugs stay in my greenhouse?
Far better than they stay in a garden, which is the whole reason biological control is a glasshouse technique. Close the vents for the release, or better, mesh them permanently. Release in the evening onto lightly misted foliage so there is water available, and make sure there is actually prey present, since predators released into a clean house simply starve or leave.
+- Are sticky traps harmful to beneficial insects?
They catch whatever flies into them, including some beneficials and occasionally pollinators. Placement helps: keep them at canopy height among the crop rather than near open flowers, and use the number you need for monitoring rather than covering the house. If you are running a predator release, that is a reason to use fewer traps, not none, because you still need to see what is happening.
+- What should I do first if I find pests in the greenhouse?
Identify what it is, because the treatment differs completely between aphid, whitefly, thrips, spider mite and fungus gnat. Then check whether the cause is cultural: fungus gnats mean wet media, spider mite thrives in hot dry air, and both are addressed by changing conditions rather than by spraying. Exclusion, monitoring and biological control come before a sprayer in a closed structure.
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