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Pest, Disease & Weed Timing CalendarsCaterpillar Monitoring Calendar
Caterpillars do their damage in a short window after eggs hatch, so good monitoring looks for moths and eggs before the feeding starts.
Key points
- Damage comes from larvae, but the earliest signal is the adult moth flight and eggs.
- Combine pheromone traps, plant checks and frass searches.
- Check leaf undersides and plant centers on brassicas, silks on sweet corn.
- Use local thresholds and label directions before any treatment.
Why does caterpillar timing start with the adult moth?
A caterpillar is the larval stage of a moth or butterfly. The adult lays eggs, the eggs hatch, and the larvae feed and grow before they pupate. Damage comes from the larval stage, but the earliest warning comes from the adult flight. By the time you see holes in leaves, many larvae have already had days of feeding.
That is why extension programs for sweet corn and brassicas recommend trapping adults or checking for eggs and small larvae. Penn State and Utah State fact sheets on corn earworm describe moth monitoring as the way to know when risk is rising.
What tools do you use to monitor caterpillars?
There are three basic tools. Pheromone traps attract males of a particular species, and the catch tells you when the flight begins and whether numbers are rising. Plant checks find eggs, tiny larvae and early damage. Frass, the dark droppings left on leaves, shows that larvae are present even when you cannot see them.
Each tool answers a different question. Traps tell you when to start looking hard, plant checks tell you how many are on the crop, and frass helps you find the larvae.
- Pheromone traps for adult moth flights
- Leaf and whorl checks for eggs and small larvae
- Frass and ragged holes as clues
- Weather and degree-day records for timing
How do you scout brassicas for caterpillars?
On cabbage, broccoli and related crops, look at the undersides of leaves and into the center of the plant. Alabama Extension's crucifer scouting guidance describes checking a set number of plants and recording what you find across the field, including in high tunnels. Check from transplanting through harvest, since small plants and heading plants can both be damaged.
Record the species when you can, since different caterpillars on the same crop may behave differently. Photographs help your extension office confirm identification.
How do you scout sweet corn and tomatoes?
In sweet corn, the key stage is silking, when some moths lay eggs on fresh silks. Trap catches in the weeks before and during silking tell you how active the moths are. In tomatoes and peppers, check for chewed leaves and fruit damage, and look for large caterpillars on stems and foliage during warm months.
Fruit damage often shows only after the caterpillar has entered, so earlier checks for eggs and small larvae give you more options.
How does weather change the calendar?
Warm weather speeds development from egg to larva to pupa, so a hot spell can compress the timing between egg laying and feeding damage. Cool, wet weather slows it. Strong storms can also carry moths in from far away, which is why local alerts from your extension service are useful.
For example, if your trap catch jumps sharply after a warm front, increase your plant checks that week even if you scouted recently.
What should you do when you find caterpillars?
Identify the species, note the size and stage and compare your count to the threshold your extension service gives for the crop. Small numbers on a crop that tolerates some feeding may call for rechecking, not action. Natural enemies such as parasitic wasps and predatory insects often help, and some caterpillars are killed by diseases in humid weather.
Any pesticide choice must follow the label, which is the law. Pre-harvest intervals apply, pollinators should be protected, and applicator licensing varies by state. Your county extension office can give current recommendations.
What does a worked example look like?
For example, a grower with fall broccoli sets a pheromone trap for a cabbage-family moth at transplanting and walks the planting each week. The first week shows a few small eggs and no damage. The second week shows a rising trap catch and several tiny larvae on lower leaves. Because the larvae are still small and the heads have not formed, that is the week to confirm the count against the extension threshold and plan a recheck within a few days.
The sequence matters more than the numbers. Eggs, then small larvae, then damage, with the trap catch warning you ahead of all three. A grower who waits for holes in the leaves has skipped the two steps that offered the most choices.
Keep a simple tally sheet with columns for trap catch, eggs, small larvae, large larvae and percent of plants with damage. A glance at the sheet each week shows which stage is climbing.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.