Key takeaways
- Yellow is the general card: fungus gnats, whitefly, winged aphids, leafminers, shore flies and leafhoppers. Blue is the thrips card.
- Monitoring runs on one to four cards per 1,000 square feet. Mass trapping runs on about eight large cards per 1,000 square feet — same card, different job, different count.
- Hang the bottom edge level with the top of the canopy and raise it as the plants grow. Fungus gnats are the exception: lay the card flat, just above the media.
- Change cards weekly. A card thick with dust and insects stops catching, and if you leave it up the counts become cumulative and the trend disappears.
- A card reads the adults flying through the air. It will not catch spider mites, it will not catch larvae, and on its own it is not a control measure.
A yellow card and a blue card cost about the same, hang the same way, and catch very different insects. Picking the wrong one is the quietest failure in a scouting program, because nothing looks broken — the card fills up, the counts go in the spreadsheet, and the pest you were actually worried about never appears on it.
The short version is that yellow is the default and blue is the thrips card. Yellow pulls in a wider set of flying adults, which is why extension programs reach for it first. Blue is narrower and sharper: it is the card to reach for when thrips are the thing keeping you up at night, particularly on a crop susceptible to the viruses they carry.
That is the headline, and it is most of what people come looking for. The rest of this is the part that decides whether the card earns its place: how many to put up, how high, which way round, how often to change them, and the handful of things a sticky card will quietly fail to tell you no matter what color it is.
Video transcript about 2 minutes
0:00Regular monitoring of sticky traps makes early detection of pest and insect infestations possible. It's not only a way to help keep insect numbers down, but it is a great preventative and early warning device. Detecting invasions early and precision monitoring of populations are crucial parts of integrated pest management, otherwise known as IPM, the commonly used phrase for holistic approaches to pest control.
0:22Sticky traps are a great solution for cultivators who wish to control pests while minimizing the amount of harmful pesticides they need to use. Common Culture sticky traps and rolls are manufactured in Europe to their high commercial agricultural standards, with the best science available behind them. Scientific studies have proven that particular translucency and yellow wavelength colors have had a significant impact on reducing whitefly populations in gardens. These studies have guided the development of Common Culture yellow sticky traps to make them the most optimized and effective color wavelengths to enhance the catch rate for whiteflies and other pests, making them much more effective than competing brands. Our yellow wavelength is even designed to reduce attraction to beneficial insects and predators that will help keep your garden safe.
1:02Yellow cards and rolls are great for attracting a range of leaf-feeding species including whiteflies, thrips, fungus gnats, aphids and leafminer adults, and many more pests.
1:12Blue cards and rolls are particularly great for thrips, including western flower thrips, and capsid bugs, commonly known as plant bugs, leaf bugs or grass bugs. And scientific studies at Keele University in the United Kingdom have shown that the use of blue sticky rolls reduced the adult thrip numbers by 61% in commercial strawberry farms.
1:28For usage, if you can, apply the traps even before you put your plants in the area to get an idea if there are any pests already present. Once plants are in your environment, place the sticky traps anywhere around your plants, whether on branches, where the stem goes into the substrate, or right above the canopy. And sticky rolls are efficient for large gardens and farms with rows of crops.
1:49Our rolls and cards are safe to use with bumblebee or honeybee pollination, with the release of natural enemies where biological control programs are used. All cards and rolls can be used in conjunction with predatory insects including mites, that are a great way to reduce unwanted pests while reducing the usage of harmful pesticides.
Which color catches which pest
If you only read one part of this, read the table. Every flying pest a grow room or greenhouse routinely produces responds more strongly to one of the two colors, and the split is consistent enough across extension guidance to be treated as settled.
| Pest | Card color | How it goes up | Note |
|---|---|---|---|
| Fungus gnat adults | Yellow | Horizontal, just above the media | Flat beats vertical, especially early in the crop cycle |
| Whitefly | Yellow | Vertical, at canopy height | Add cards near the hosts whitefly favor |
| Winged aphids | Yellow | Vertical, at canopy height | Aphids on a card in cold months usually mean an indoor population |
| Leafminers | Yellow or blue | Vertical, at canopy height | Caught on both; blue cards are commonly sold as thrips and leafminer cards |
| Thrips | Blue first, yellow still works | Vertical, just above the canopy | Bench level catches more than floor level or hanging-basket level |
| Capsid bugs (plant, leaf and grass bugs) | Blue | Vertical, at canopy height | Caught alongside thrips on the same blue card |
| Shore flies | Yellow, blue also catches them | Horizontal, just above the media | They feed on algae, not on the plant |
| Leafhoppers | Yellow | Vertical, at canopy height | Also caught on blue thrips and leafminer cards |
| Fruit flies | Yellow | Vertical or horizontal | Usually an indicator of something fermenting rather than a crop pest |
Two things in that table do most of the work. The first is that yellow covers six of the eight rows, which is why UC IPM treats it as the default and says to use it unless something specific points elsewhere. The second is that thrips are the one genuine exception, and they are exceptional enough — small, hard to see on a leaf, and the vector for tospoviruses — to justify running a separate color for them.
If you want a single sentence for what a blue card is for: it is a thrips card, with leafminers and leafhoppers as a bonus. That pairing is consistent enough that blue cards are usually sold under both names at once, which is also how the leafminer range is arranged.
There is a cost to blue that nobody mentions until you are standing at the bench with the card in your hand. UC IPM puts it plainly: insects are harder to see and count on a blue card. A pale thrips on a blue background is a much worse contrast problem than the same thrips on yellow. If you are the person doing the counting every week, that is not a small consideration, and it is the main argument for running yellow everywhere and adding blue only where thrips matter.
What the color is actually doing
The card is not bait. Nothing on it smells of anything, and there is no lure in the glue. What a colored card does is imitate a signal the insect is already hunting for, and the two colors imitate two different signals.
Yellow, to a flying insect, reads as foliage — a bright, healthy, slightly yellowed leaf, which is a reasonable thing for an aphid or a whitefly to fly toward. The greenhouse whitefly has a sensitivity peak at 520 nm, in the yellow-green, with a second peak down at 340 nm in the ultraviolet. A yellow card reflects very little below 500 nm and a great amount above it, which puts almost all of its output on the side of the spectrum the insect is tuned to.
Thrips see from about 400 to 635 nm, and a blue card sits squarely inside the lower half of that window where a yellow card has almost nothing to offer. That is the mechanical reason blue works on thrips and does comparatively little for whitefly: it is loud in a band one insect is listening to and quiet in the band the other one is.
Why a card you can see light through catches less
This is the finding that changes how you look at a thin card. A 2018 study in PLoS ONE varied nothing but the translucency of a yellow roller trap and measured the catch. Doubling the thickness of a moderately translucent trap raised the greenhouse whitefly catch 7.0 times in one season and 3.5 times in the next. A less translucent trap caught 2.3 times more than a moderately translucent one of the same color.
The reason is contrast. A card with light coming through the back of it is a dim card against a bright sky, and the insect is responding to a ratio rather than to an absolute. The practical test takes two seconds: hold the card up to a window before it goes in the room. If you can read your hand through it, it is working at a fraction of what it could.
The same study found that a printed black pattern covering roughly 30 percent of one side raised the catch 2.3 times on large 10 × 25 cm cards — and reduced it on the least translucent cards. Pattern helps a card that needs edges; it gets in the way of one that is already dense.
One honest footnote on yellow. Cornell research found that red cards outcaught yellow for fungus gnat adults, and other work points the same way for black. Red and black cards are not what the industry manufactures at scale, so yellow remains the practical choice for gnats — but if you have ever wondered whether yellow is optimal for that one pest rather than merely available, the answer is that it is available.
The glue changes the answer
Here is the part that should make anyone cautious about a flat yellow-versus-blue verdict, including the one at the top of this article. The adhesive on the card is not a neutral layer. It has its own optical properties, and in at least one well-controlled study it was enough to flip the result.
A 2021 paper in Scientific Reports tested western flower thrips against four plate colors with two glues: one clear, one with a whitish haze. With the clear glue, yellow caught the most and the difference from every other color was significant — the textbook result. With the hazy glue, in the wind tunnel, attraction to yellow fell 58 percent and attraction to blue rose 65 percent.
With the hazy glue, catches on yellow fell 35 percent and catches on blue rose 32 percent.Greenhouse trials in rose and chrysanthemum, Scientific Reports, 2021
That was the wind tunnel. In two working greenhouses, over four weeks in June and July, the same swap moved yellow down 35 percent and blue up 32 percent. Thrips turned up on 90 percent of blue plates and 87 percent of yellow ones in the rose house, and on 77 percent of blue and 91 percent of yellow in the chrysanthemum house — so both colors were finding thrips. What the glue changed was which one found more.
The authors are careful, and so is this: the mechanism is not proven. The haze raised reflected blue light by 18 percent and violet by 21 percent on the yellow plates, which is a plausible explanation, but the paper says it is not yet clear whether that shift is what drives the change. One thrips colony, one set of conditions.
What it means at the bench is narrower and more useful than a verdict. It means the glue is a variable you have to hold constant. If you compare this month's counts against last month's using cards from a different supplier, you have changed two things and learned nothing. Pick a card, keep picking it, and let the numbers mean something.
It also explains why the literature looks contradictory. Across the thirteen field and greenhouse studies the authors reviewed, the three run in winter, spring and autumn favored blue, while the six summer studies gave yellow the advantage in half of cases, blue in a third, and found no difference in the rest. Season, crop, glue and colony all move the result. The sensible reading of the whole body of work is that blue is reliably good for thrips and yellow is reliably good for everything else — which is where section one started, and now with the caveat attached.
How many cards, and where they go
Card count is the number most people guess at, and the guess is usually low. Extension guidance varies by a factor of eighty depending on what you are asking the cards to do, so the first question is not how many — it is what for.
| What the cards are for | Cards per 1,000 sq ft | Source |
|---|---|---|
| Bare minimum coverage | 0.1 — one card per 10,000 sq ft | UC IPM default where nothing else applies |
| General pest monitoring | 3 to 4, minimum 1 | UConn IPM |
| Fungus gnat and shore fly monitoring | 1 to 4 | UMass Extension |
| Whitefly-susceptible crop | About 1 | UC IPM |
| Monitoring, manufacturer's own figure | 0.5 — one 10 × 25 cm board per 200 m² | Common Culture Impact Board |
| Mass trapping thrips | About 8 large cards (40 × 20 cm) | ONfloriculture, 2026 |
| Mass trapping, manufacturer's own figure | 4.6 — one board per 20 m², plus one every 2 m² in hot spots | Common Culture Impact Board |
The jump from the monitoring rows to the mass-trapping rows is the whole point. Eight large cards per 1,000 square feet is roughly eighty times the UC IPM minimum, and it is not a stricter version of monitoring — it is a different activity, covered properly in section seven.
The manufacturer's own numbers are worth reading next to the extension figures, because they are written for these exact boards rather than for cards in general, and because they land in the same territory from a different direction. One board per 200 m² for monitoring works out at about half a card per 1,000 square feet — above UC IPM's bare minimum, below UConn's three to four. One board per 20 m² for mass trapping is 4.6 per 1,000 square feet, in the same range as the eight large cards ONfloriculture recommends once you account for card area. Four independent sources, no outlier.
One timing point that is easy to act on and almost never mentioned: put cards up before the plants go in. An empty room is not necessarily a clean room, and a week of cards in an empty space tells you what is already living there before you have a crop to confuse the reading. It costs four cards and it is the cheapest information in the whole program.
Placement matters at least as much as count, and there are four rules worth following regardless of density:
Where the cards go
- At every door and vent — pests arrive from outside, and a card at the entrance tells you a week before a card in the middle of the room does. For side-vent houses, two to thirteen feet from the opening.
- In every management unit — if you treat two benches differently, they need their own cards, or you end up with one number describing two situations.
- Extra on the susceptible cultivars — whitefly and thrips pick favorites. Put cards where the pressure concentrates, and keep the most susceptible crops away from the doors if the layout allows it.
- Fewer when you are releasing winged biologicals — parasitic wasps get caught the same as anything else, and a dense card grid works against the release you just paid for.
Size is the quiet variable in all of this. The published densities assume a standard card — 3 × 5 inches for the UConn and UMass figures, 40 × 20 cm for the mass-trapping figure. A pack of small cards at a density written for large ones is not the same program, and the plant care range carries both formats for that reason.
Hang height, and the one time it goes flat
A card at the wrong height is not a weaker version of a card at the right height. It is in a different air layer from the insect you are trying to count, and it will tell you there is no problem right up until there obviously is.
The default is straightforward. Set the bottom edge level with the top of the canopy, or a few inches above it on a fast-growing crop, and raise it as the plants grow so it stays there. UConn puts the number at four to six inches above the canopy, clipped to a stake with a clothespin so moving it takes a second rather than a rebuild. Common Culture's own instruction for the Impact boards is a base about 10 cm above the top of the crop, moved upwards as the crop grows — the same number in metric, arrived at separately.
Orientation default: the longest side vertical. A vertical card presents its face to insects arriving from any horizontal direction, which is most of them, and UC IPM recommends it as the way to catch the widest range.
The exception that catches people out
Fungus gnats and shore flies do not behave like the rest. They emerge from the growing media and spend their time very close to it, so a card hanging at canopy height is above the traffic. For those two, lay the card flat, just above the media surface or on top of the pot. UConn is explicit that horizontal placement works better than vertical for fungus gnats, and that the gap is widest early in the crop cycle — exactly when you most want to know.
Thrips have a height preference too, and it is not the one most people guess. Bench level catches more than hanging-basket level or floor level, and counts tend to run higher at the ends of a house than in the middle. Under mist propagation the catch drops, which is a property of the mist rather than of the thrips population — worth knowing before you conclude a prop area is clean.
The practical arrangement in most rooms is both at once: vertical cards at canopy height for the whitefly and aphid traffic, and flat cards on the media for the fungus gnats. They are counting two different populations, and one card cannot do both jobs from one position.
How often to change a card, and how to read it
A sticky card has a working life measured in days, not months, and the failure is gradual enough that a card can look like it is still doing something long after it has stopped.
The guidance is consistent across sources: inspect once or twice weekly and replace weekly. UC IPM notes that replacing the card at every inspection is simply the easiest approach, because it removes the arithmetic. UConn suggests spot-checking twice a week once temperatures rise and insects start developing faster, and allows less frequent replacement through winter.
Two things end a card's life. Dust and debris bury the adhesive, and a full card has nowhere left to catch. Both are visible, and neither is a judgement call once you have seen one.
Why the card itself says six to eight weeks
Read the instructions on a box of Impact boards and you will find a different number: replace after six to eight weeks, or sooner if covered in insects. That is not a contradiction, and it is worth understanding rather than picking a side on.
The two intervals are measuring two different things. Six to eight weeks is how long the adhesive keeps working — it is the card's physical service life, and for mass trapping, where the job is holding open sticky area, that is the number that matters. Weekly is how long a card can serve as a measurement, because a count is only readable if it covers a known period. Leave a monitoring card up for seven weeks and you have one cumulative number where you needed seven points on a line.
So: if the card is there to tell you something, change it weekly. If the card is there to remove insects, run it until it is fouled or the manufacturer's interval is up, whichever comes first.
If you do leave a card up across inspections, remember that the counts are cumulative. Subtract the previous count or you will record a rising population that is simply the same insects counted twice, which is the most common way a card-based trend is read backwards.
Counting a card without losing an afternoon
- Count each pest type separately — one number for everything on the card tells you nothing you can act on.
- When the card is crowded, count a one-inch vertical strip on both faces and multiply by the card's width in inches. A printed grid makes this quick.
- Never cut the card down to make counting easier. A smaller card attracts fewer insects, so a trimmed card is no longer comparable with the ones around it.
- Write it down the same way every week — counts per card per week, into a sheet, turned into a graph. The trend is the output; the single number never was.
- Chill the cards before handling them. Refrigerated cards are markedly less sticky and far easier to separate and carry.
One timing rule worth putting on the wall: after releasing winged biological control agents, wait a day or two before changing cards. Parasitic wasps get caught, and swapping cards straight after a release both removes the release and distorts the count.
Monitoring is not control
This is the sentence that most sticky-trap writing avoids, so here it is without a cushion: UC IPM states that sticky traps are generally not effective for direct control, and may not be good for deciding whether treatment is needed at all.
The reasoning is about which part of the population lands on the card. A card catches airborne adults, and adults often are not the stage doing the damage. Fungus gnat larvae feed on roots and tunnel into crowns and stems; the adults are a nuisance. Thrips feeding damage comes from insects on the plant, not from the ones in the air. The card is an early-warning instrument and a trend line, not a census and not a treatment.
Pesticide timing makes this worse in a way that is easy to misread. An application can temporarily depress adult catches, and an adulticide can temporarily raise them, so a count taken shortly after a treatment may say the opposite of what happened. UC IPM is blunt about it: large numbers on a card do not necessarily mean control is needed. Pair the card with plant inspection, every time.
Mass trapping, which is the exception
There is one situation where cards genuinely reduce a population, and it is a question of scale rather than of principle. Put up enough card area and you are removing insects faster than they arrive. That is mass trapping, and the card count in section four — about eight large cards per 1,000 square feet — is what it takes.
The numbers are real, and they are measured rather than modelled. A Keele University trial on everbearer strawberry in open-sided UK polytunnels lined the plots with blue roller traps and compared them against untreated controls. The result: 61 percent fewer adult thrips per flower, 63 percent fewer larvae and 55 percent less fruit bronzing. Adding a thrips aggregation pheromone to the same traps took those three figures to 73, 90 and 68 percent. The authors put the return at £2.2k per hectare and called that a conservative estimate.
Scaled to a typical house, one grower's arithmetic runs like this: at high pressure a large card may take up to five onion thrips a week, so 120 large cards in a 15,000 square foot house removes up to 1,200 thrips a week, or around 12,000 across a ten-week crop. That figure is an estimate from card counts rather than a measured result, and it is worth treating as a rough scale rather than a number.
Timing is seasonal and tighter than you would expect. Cards go up in the last week of May or the first week of June to catch the start of fly-in, and the program is pushed hardest through July and August when outside pressure peaks. Cards added in response to a problem in August are already late.
One honest caveat on the monitoring side, from the same source: card counts alone can mislead even for thrips. A large fly-in of eastern flower thrips can mask other species on the card, and the author's own view is that plant taps and walking the crop for damage beat card counts as the primary monitoring method. The cards are the early signal and the removal tool. The crop is still the evidence.
What else lands on the card
Glue does not discriminate, and a program that runs cards alongside biological control has to decide how those two things share a room. Pretending there is no tension is the wrong answer, and so is treating one as a replacement for the other.
The specific conflicts are known and manageable. Bumblebees are attracted to blue, and UConn advises against blue cards in greenhouse tomatoes where bumblebees are doing the pollinating. That is a direct trade: blue is the thrips card, and in a bumblebee-pollinated house it is also a bumblebee card.
The manufacturers do not read it the same way, and both positions belong here. Common Culture states that its cards and rolls are safe to use alongside bumblebee or honeybee pollination and alongside natural enemy releases, and that the color matrix on the Impact boards is tuned to maximize pest catch while reducing attraction to beneficial insects. Those are the maker's claims about its own product; the UConn caution is extension guidance written across every blue card on the market, including ones with no such tuning.
Nothing here settles it for you, and pretending otherwise would be dishonest. What is reasonable: in a house with managed pollinators, treat blue as the color that needs a reason, check the maker's statement for the specific card in your hand, and keep a yellow card or a clear roll as the default where a reason is not there.
Parasitic wasps are the other one. Winged biological agents — Encarsia, Aphidius and the rest — fly, and anything that flies lands. The guidance is practical rather than absolute: reduce the number of cards while a release program is running, and wait a day or two after each release before changing cards out. The beneficial insect range and a card program coexist perfectly well; they just need the card density turned down rather than left at scouting levels.
Clear film is the third route, and it is the one most people have not considered. A clear roll has no color signal at all, which means it catches what blunders into it rather than what it has summoned. That is a weakness for monitoring and a genuine strength around pollinators — the clear roll in the range is stated safe alongside bumblebee and honeybee pollination and alongside natural enemy releases, while still working for mass trapping.
The same logic applies to how you read a card. Not everything on it is a pest. Predatory midges, aphid parasitoids and hoverflies all turn up, and counting them as pest pressure will push you toward a spray you do not need. If you cannot tell them apart at arm's length, magnification settles it in seconds — and that is the cheapest instrument in the room.
What a sticky card will not tell you
The limits are as useful as the capabilities, because a scouting program built on a card alone has three blind spots and every one of them is a pest that will happily occupy the space.
Spider mites do not fly. They walk, and they travel on clothing, tools and leaf contact. No amount of card density will detect them, and a clean card is not evidence of a clean room. Mites are found by turning leaves over and looking, which is what a hand loupe is for, and dealt with from the mite control range once found.
Larvae do not fly either. Fungus gnat larvae are the stage that damages roots, and they are in the media where no card reaches. The standard method for those is a potato disc: raw potato, peeled, cut into pieces about an inch across and half to one inch thick, laid on the media surface. Ten discs may be enough to cover a 10,000 square foot house. Lift them after a couple of days and count the larvae on the disc and on the media underneath.
Species get masked. When one species arrives in numbers it can bury another on the same card — a heavy eastern flower thrips fly-in will hide a western flower thrips population you actually needed to see. The card gives you a total; the identification still needs a person.
None of this makes cards less useful. It makes them one instrument in a program rather than the program. Pair them with weekly plant inspection, potato discs where fungus gnats are a live concern, and the rest of an integrated approach — and when a count does cross the line, the organic control options and the biologicals are the response, not more cards.
On thresholds, the honest position is that there is not a universal one. No extension source consulted here publishes a number of insects per card per week that triggers treatment, because tolerance depends on the crop, the growth stage, the damage potential and whether tospoviruses are present locally. Where they are, the thrips target is as close to zero as the program can get. Everywhere else, the threshold is the one you establish from your own records — which is the real reason the weekly count goes into a sheet.
The cards in the range
Twelve products, three formats and four color choices. The table below is every item currently listed in the traps collection, with what each one is suited to, so the color decision from section one maps onto something you can actually put in a room.
| Product | Color | Format and size | What it suits |
|---|---|---|---|
| Sensor Yellow Sticky Monitoring Cards, 50 pack | Yellow | Card, 3 × 5 in, double-sided | The size every published density figure is written around. General scouting. |
| Common Culture Yellow Sticky Glue Traps, 10 pack | Yellow | Card, 10 × 25 cm | The large European format. More area per card where pressure is high. |
| Common Culture Yellow Sticky Glue Roll | Yellow | Roll, 15 cm or 30 cm × 100 m | Mass trapping and long runs between posts. Cut to the length you need. |
| Sticky Whitefly Traps | Yellow | Card, 3 and 5 packs | Small rooms and spot checks near whitefly host plants. |
| Seabright Laboratories Aphid/Whitefly Traps, 5 pack | Yellow | Card, 5 pack | Winged aphid and whitefly monitoring in a small space. |
| Seabright Sticky Thrip Leafminer Traps, 5 pack | Blue | Card, 4 × 14 in | A tall blue card for thrips, with a one-inch grid for counting. |
| Grower's Edge Thrips & Leafminer Sticky Trap, 5 pack | Blue | Card, 5 pack, weatherproof | Thrips, leafminers and leafhoppers. Holds up in a humid room. |
| Common Culture Blue Sticky Glue Traps, 10 pack | Blue | Card, 10 × 25 cm | The large format in blue, for thrips pressure across a bench. |
| Common Culture Blue Sticky Glue Roll | Blue | Roll, 15 cm × 100 m | Thrips mass trapping at the density section seven describes. |
| BioWorks Yellow/Blue 2-Sided Sticky Cards, 100 cards | Both | Card, 100 per box | Yellow on one face, blue on the other. One card when you want both signals. |
| Dual Action Plant Label & Sticky Traps, 4 × 100 ct | Yellow | Pointed stake label, pots to 5 gal | Media-level catching in a pot, with the date and plant ID on the reverse face. |
| Common Culture Clear Film Glue Roll | Clear | Roll, 18 cm × 120 m | No color signal, so it is the option around bumblebee and honeybee pollination and natural enemy releases. |
Three notes on reading that table. The 3 × 5 inch card is the reference format — when a source says three to four cards per 1,000 square feet, that is the card it means. The 10 × 25 cm cards are the large European standard, and the one the black-pattern research used. And the rolls are for a different job entirely: at mass-trapping densities you are covering area, and covering that area with 100 meters of tape works out cheaper per square meter and faster to put up than hanging a hundred cards.
If the thrips row is the one that matters to you, the thrips control range carries the biologicals and sprays that go alongside the blue cards. If the gnats are the problem, the same is true of the fungus gnat range. The card is the instrument that tells you which conversation to have.
Frequently Asked Questions
The questions that come up most often about colored sticky cards, answered as directly as the evidence allows.
What is a blue sticky trap used for?
Thrips, primarily, with leafminers and leafhoppers as a secondary catch. Blue sits inside the 400–635 nm window thrips can see and yellow largely does not, which is why extension guidance recommends blue for detecting low thrips levels on susceptible crops.
Are fungus gnats attracted to yellow sticky traps?
Yes. Yellow catches fungus gnat adults reliably, and it is the standard recommendation. Cornell research found that red cards outcaught yellow for this particular pest, but red is not widely manufactured, so yellow remains the practical choice. The bigger gain is orientation: lay the card flat just above the media rather than hanging it at canopy height.
Do yellow sticky traps catch spider mites?
No. Spider mites do not fly, so they are never in the air stream a card samples. A card that shows no mites is telling you nothing about mites at all. They are found by turning leaves over with a loupe.
How many sticky traps do I need?
For general monitoring, three to four cards per 1,000 square feet, with an absolute minimum of one. For mass trapping thrips, about eight large 40 × 20 cm cards per 1,000 square feet. Add extra cards at doors and vents and near the cultivars that attract the most pressure.
Should a card hang up or lie flat?
Hang it, longest side vertical, with the bottom edge level with the top of the canopy, and raise it as the plants grow. The exception is fungus gnats and shore flies, where horizontal placement just above the media catches more — noticeably more early in the crop cycle.
How often should I change a sticky card?
Weekly, if the card is there to tell you something. Replacing at every inspection keeps each count independent; leave a card up across inspections and you have to subtract the previous count, or the trend reads as a rising population when it is the same insects counted twice. The six-to-eight-week interval printed on the box is the adhesive's service life rather than a monitoring interval — it is the right number for a card doing mass trapping, and the wrong one for a card doing measurement.
Will sticky cards harm bees or beneficial insects?
They can. UConn advises against blue cards in houses where bumblebees pollinate, because bumblebees are attracted to blue. Winged parasitic wasps get caught too — reduce card numbers during a release program and wait a day or two after a release before changing cards. Manufacturers take a different view of their own products: Common Culture states that its cards and rolls are safe alongside bumblebee and honeybee pollination and natural enemy releases. Both positions are on the record, and a clear film roll sidesteps the question entirely by carrying no color signal.
Does the type of glue really matter?
Enough to change which color wins. In controlled trials, swapping a clear adhesive for a hazy one moved thrips catches on yellow down 35 percent and on blue up 32 percent in a working greenhouse. The practical consequence is not that one glue is better — it is that you have to keep the same card if you want this month's counts to be comparable with last month's.
Can sticky cards control an infestation on their own?
At monitoring densities, no. UC IPM says plainly that sticky traps are generally not effective for direct control, because they catch airborne adults while the immature stages do most of the damage. At mass-trapping densities — around eighty times the minimum monitoring density — they do measurably reduce populations. A Keele University trial on UK strawberry using blue roller traps recorded 61 percent fewer adult thrips per flower, 63 percent fewer larvae and 55 percent less fruit bronzing against untreated controls. That is a different program with a different card count.
Why can I barely see anything on my blue cards?
Because contrast works against you. Thrips are pale and small, and a blue background is a much harder read than a yellow one. UC IPM names this as a known drawback of blue cards. A printed one-inch grid helps, a loupe helps more, and it is a reason to run yellow as the standard card and add blue only where thrips justify the extra counting effort.
The Bottom Line
Yellow for almost everything, blue for thrips. If you remember nothing else from this page, that sentence covers the decision you came here to make, and it is backed by every extension program consulted.
What turns that decision into a working scouting program is the four numbers underneath it. Three to four cards per 1,000 square feet for monitoring, or eight large cards if you are trying to remove a population rather than measure one. Bottom edge level with the canopy, raised as the crop grows, except for fungus gnats where the card lies flat on the media. Changed weekly, counted by pest type, written down. And a card that is opaque rather than translucent, because a card you can see light through is working at a fraction of its potential.
The last piece is knowing what the card is for. It is an early-warning instrument and a trend line. It will tell you something has arrived, roughly when, and whether the number is going up or down — which is enough to act on a week before the crop shows it. It will not tell you about mites, it will not tell you about larvae, and at monitoring density it is not going to fix anything by itself. Walk the crop as well. The card tells you where to look.
If you are putting a program together from scratch, start with yellow 3 × 5 cards at four per 1,000 square feet, add flat cards at media level wherever fungus gnats have been a problem before, and add blue only once thrips are on the list. That is a program you can run every week without resenting it, and the one that gets run is the one that works.
Questions about which format fits your room, or what the counts you are seeing actually mean? Give us a call or drop a comment below — we would rather talk it through than have you guess at a density.