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Pest, Disease & Weed Timing CalendarsThrips Monitoring Calendar
Thrips are small enough to be missed until damage shows, so monitoring needs tools, a routine and attention to weather.
Key points
- Thrips are tiny, so use sticky cards, tapping and a hand lens, not just a glance.
- Start early in tunnels and greenhouses, and watch warm dry spells in fields.
- Tie decisions to crop stage and local thresholds.
- Inspect incoming plants and remove weedy hosts nearby.
Why are thrips hard to monitor?
Adult thrips are often one to two millimeters long, slender and quick. Larvae are even smaller and hide in leaf folds, flowers and the growing tips of plants. By the time you see silvery scarring, stippling or distorted growth, populations have usually been building for a while.
Because they are so small, a hand lens or magnifier is a useful tool, and monitoring should rely on more than a quick glance at the foliage. Thrips also hide during the heat of the day, so check at a consistent time of morning.
Larval thrips are often found in tight places: the base of onion leaves, inside flower buds and in the folds of new growth. Open a few buds each week as part of the routine.
What tools work for thrips?
Blue and yellow sticky cards catch flying adults and show when numbers rise. Tapping a flower or shoot over a white sheet of paper dislodges thrips so you can count them. On onions, inspecting the inner leaf folds near the neck reveals the insects where they hide. In greenhouses and high tunnels, cards placed at plant height near doors and vents give early warning.
Whichever tool you use, use it the same way every week so counts are comparable.
If you sell directly to customers, your tolerance may differ from a wholesale market, because cosmetic scarring on fruit and flowers affects how produce is received.
- Sticky cards at crop height
- Tap sampling over white paper
- Hand lens checks on leaf folds and flowers
- Weekly logs of counts
When in the season should you start?
In greenhouses and tunnels, start at transplanting and continue through the season, since warm protected space lets thrips breed all year. In open fields, thrips often build up as weather warms and as nearby crops such as small grains or hay are cut and the insects move to new hosts. Onion, cabbage, cotton and many flower and fruit crops each have their own pattern, so check your regional extension guidance.
Warm dry weather generally favors thrips, and heavy rain can knock back populations on exposed plants.
Always compare cards from the same position and height from week to week, because cards placed in different spots are not directly comparable.
How do crop stage and damage fit in?
Decide which stage of the crop matters most. For bulb onions, extension programs often tie thresholds to leaf number or growth stage. In flowers and fruit, thrips feed in blooms and can scar fruit surface. Some thrips also spread plant viruses, so a small number can matter in a crop at risk.
Ask your extension office whether tospoviruses are a known concern in your region, and what that means for your tolerance.
How do natural enemies and cultural steps help?
Predatory insects and mites, minute pirate bugs and some parasites feed on thrips. Notice them while you count. Cultural steps include removing weedy hosts near fields, avoiding planting next to freshly cut hay, using reflective mulch where recommended, and keeping plants well watered since stressed plants tolerate feeding poorly.
Clean transplants matter. Inspect incoming plants before they enter a greenhouse or tunnel.
What about treatment decisions?
Compare your counts to the threshold your extension service lists for your crop and stage. Treatment timing and resistance management are complicated for thrips, so rely on current local recommendations, not on general advice.
The label is the law, pre-harvest intervals apply, bees must be protected, and state rules for applicators vary. A certified crop adviser or licensed applicator can help you plan.
What does a worked example look like in a tunnel?
For example, a grower with a tomato tunnel hangs blue sticky cards at canopy height near the door and at the far end. Weekly counts show a slow rise over three weeks. The grower taps flowers over white paper, finds larvae and adults, and checks the neighboring field to see whether hay was cut recently. The grower then compares the counts with the extension threshold and notes the plant stage in the record.
Even without a known threshold, the trend gives information. A card count that doubles every week deserves closer attention than a flat line.
A short monitoring log with the date, card count, tap count and weather lets you compare across seasons.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.