A trap crop is a plant the pest prefers to the main crop, grown beside or around it to concentrate the pest where it can be removed, and it only helps if the pest actually favours it, if it is placed where the pest arrives, and if the infested trap plants are destroyed before the pest breeds on them. A trap left standing and ignored becomes a pest nursery that sends more pests to the main crop than before.
Trap cropping is the use of a sacrificial plant to attract a pest away from a valuable crop so that the pest can be removed or killed.
Three Conditions For A Trap To Work
- The pest prefers it. The trap plant must be a better host than the crop at the time the pest arrives. This can be a different species, a different variety or the same crop planted earlier.
- It sits where the pest arrives. A border strip works when pests come in from outside; an interior row works for pests already in the field.
- You deal with the pest on it. Pull, bag and destroy the infested plants, or hand-kill the pests, before they lay eggs or mature. If the carrier’s warning is ignored, the trap multiplies the pest.
A review by Shelton and Badenes-Perez (Annual Review of Entomology 51, 2006) describes the main designs: perimeter traps, interior traps, and “dead-end” traps on which the pest lays eggs but its young cannot survive. Collards as a trap for diamondback moth are the standard example of a dead-end trap (recalled; check before quoting).
The Carrier’s Three Traps
| Trap plant | Protects | Pest | Note |
|---|---|---|---|
| Blue Hubbard squash, planted around the edge | Squash and cucumber | Squash bugs, cucumber beetles and borers | Northeastern US extension trials used it as a perimeter trap |
| Radish among brassicas | Cabbage, broccoli | Flea beetles | Test it; radish is often as attractive to flea beetles as the crop |
| Mustard near brassicas | Brassicas | Harlequin bugs | Mustard is itself a brassica, so destroy it before it flowers and seeds |
The squash trap needs an early start: sow it a little ahead of the main crop so it is the first and largest host when beetles emerge.
Method
Step 1: Name the pest and when it arrives. Use your monitoring record (Monitoring and Identification).
Step 2: Choose a trap plant that the pest prefers and sow it ahead of the main crop, around the edge for pests that come in from outside.
Step 3: Watch the trap at least as often as the crop. Count pests on it weekly.
Step 4: Act on it. Hand-pick pests into soapy water, or pull infested plants into a sack, tie it and burn it or bury it deep. Do not compost.
Step 5: Compare the main crop with an untrapped control row to learn whether the trap did anything (Pest-Repelling Plant Combinations gives the paired-row method).
Where Trap Crops Fail
- The trap is only slightly more attractive. Pests spread across both.
- No follow-up. A trap is a labour commitment; a trap you will not tend is better not planted.
- The trap plant is also a host for the disease of the main crop. Radish and mustard share brassica diseases such as clubroot.
- It competes with the crop. Squash grown beside squash is a rich target, and the edge plants shade the rest.
Common Mistakes
- Planting the trap and forgetting it. The pest builds up.
- Planting the trap late. The pest finds the main crop first.
- Using a trap crop to avoid rotation. It does not break a soil-borne cycle (Crop Rotation for Disease Break).
What’s Next
- Learn next: Beneficial Insect Attractors — the other plant-based pest control
- Combine with: Physical Controls — dealing with what the trap catches