Home / Guides / Pest, Disease & Weed Timing Calendars
Pest, Disease & Weed Timing CalendarsAphid Monitoring Calendar
Aphid populations can climb quickly, so a steady monitoring routine matters more than any single date. Here is how to set one up.
Key points
- Monitor from planting, and check more often when weather is mild.
- Check growing tips and leaf undersides, and record trend, not only count.
- Note natural enemies and mummies before deciding to act.
- Use thresholds from extension and follow the label for any treatment.
Why do aphids need regular monitoring?
Aphids reproduce rapidly in warm weather, and many species produce winged forms that move to new plants when colonies get crowded. Cornell and University of New Hampshire fact sheets describe how a few insects can become a large colony in a short time. Because of this, checking at long intervals can miss the window when numbers are still easy to manage.
Monitor from planting onward, and increase frequency when weather is mild and plants are growing fast.
Where and when do you find aphids?
Look at growing tips, new leaves, flower buds and leaf undersides. Curled or sticky leaves, shiny honeydew and ants moving up plants are clues. In high tunnels and greenhouses, aphids can appear any time of year, as Utah State Extension notes in its high tunnel pest page. In open fields, early season arrivals often land on the edges first.
Different species prefer different crops and times, so check your extension pest list for the aphids that matter in your area.
- Growing tips and new leaves
- Leaf undersides
- Honeydew, sooty mold, curled leaves
- Ants tending colonies
How do you count aphids?
Choose a set number of plants spread across the field or bed and check the same plant part each time. Record whether aphids are present and roughly how many. For some crops, extension thresholds use the percentage of infested plants and whether the population is rising, as University of Nebraska-Lincoln soybean aphid guidance does.
Pay attention to trend as much as the count. A small colony that doubles in a few days needs a recheck sooner than a larger colony that is stable.
What role do natural enemies play?
Lady beetles, lacewings, hoverfly larvae and parasitic wasps all feed on aphids. Mummified aphids, which look bloated and tan or black, show that parasites are active. Record these alongside aphid counts, since a field with many natural enemies may hold aphids in check without further action.
Protecting these insects is part of the plan. Avoid broad spectrum treatments when possible, follow label instructions and protect pollinators during bloom.
What do you do after you find aphids?
Check the extension threshold for your crop and stage. If you are below it, plan a recheck in a few days. If you are above it, confirm your options with your county extension office before acting. Non-chemical choices such as pruning infested tips, a strong water spray on small plants and reducing excess nitrogen can help in gardens.
Pesticide use must follow the label, observe pre-harvest intervals and meet state licensing rules.
What does a season of aphid monitoring look like?
For example, a market gardener might check brassicas and lettuce weekly from transplanting, looking at the underside of the oldest and youngest leaves and the center of each plant. In early season, a few winged aphids landing on plants is normal and not a reason for action. The signal to watch is wingless colonies that grow from one visit to the next.
In midsummer heat, some aphid colonies decline on their own as natural enemies build up and plants mature, while others keep growing in cool, moist spells. Local records over several seasons help you see which pattern your site follows.
In autumn, many species move to overwintering hosts or produce overwintering eggs. Late season checks on fall crops such as kale and spinach matter because those plants hold aphids that can spoil harvest quality even when the plants themselves are healthy. Over time, you will learn which crops on your farm attract aphids first, and you can scout those beds more closely and sooner.
What mistakes make aphid monitoring less useful?
The first is counting only the top of the plant. The second is ignoring the undersides of leaves. The third is treating every aphid sighting as an emergency, which can wipe out the natural enemies that would have done the work for you.
Another mistake is using a threshold designed for a different crop or region. Thresholds depend on crop, stage and local conditions, so always use the numbers from your state extension service rather than a number you saw online or heard from a neighbor.
Keep your monitoring notes with your planting records so you can compare aphid pressure across crops, varieties and planting dates from one year to the next.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.