Trap Crops: Plant a Decoy, Save the Harvest

What it is

A sacrificial planting. You grow what the pest prefers, place it where the pest finds it first, and let the main crop sit untouched behind the decoy.

The pest goes where it prefers. That is the whole mechanism. A black aphid will choose a nasturtium over a broad bean every time. A cucumber beetle will pick blue Hubbard squash over your zucchini. Give them what they want, in the wrong place, and they leave the right place alone.

This is not a spray replacement. It is a geometry problem solved with plants.

The decoys that work

Years of field trials and farmer notebooks have narrowed the list. These pairings are reliable.

Nasturtium pulls black aphids off broad beans, fava, and brassicas. Aphids cluster on the stems in colonies you can see from across the bed.
Sunflower pulls stink bugs off tomatoes and beans. The bugs feed on the developing seed heads.
Radish pulls flea beetles off eggplant, kale, and arugula. The beetles shred the radish foliage and ignore everything behind it.
Mustard (especially the giant southern type) pulls harlequin bugs off cabbage and Mexican bean beetles off snap beans.
Blue Hubbard squash is the heavyweight. It pulls cucumber beetles AND squash bugs off summer squash, melons, and cucumbers. One Hubbard at each end of a 10 m row of zucchini can shift 80 to 90 percent of the pest pressure.
Pak choi pulls cabbage white caterpillars off heading brassicas. The whites prefer the open leaf and lay there.

Match the decoy to the pest you actually have. See Garden Pests Overview for identification before you commit to a trap.

Why it works

Insects find host plants by smell, color, and silhouette. The preferred host wins on all three counts, and it usually wins by a wide margin. A flea beetle does not choose between radish and eggplant. It locks onto radish from meters away.

Place the decoy between the pest source and the crop, or upwind of the crop, and the pest never reaches the rows behind. Most of these pests are weak fliers. They land on the first acceptable host they detect.

This is the same logic as a nurse tree or a windbreak. You are using one plant to shape the conditions for another.

Build it

Time it. Plant the trap crop one to three weeks before the main crop. The decoy has to be more attractive when the pest arrives, which means bigger, leafier, and further along. A nasturtium seeded the same day as the beans loses the race.

Place it. At the field edge or upwind, between the pest source and your main crop. If you know your cucumber beetles are coming from a brushy fence line, the blue Hubbard goes between the fence and the zucchini. Not behind. Not in the middle of the row.

Size it. Two plants is not a trap crop. It is a snack. Run the decoy as a continuous border or a substantial block. 10 percent of the planted area is a working rule of thumb. For blue Hubbard around cucurbits, two to four plants per 10 m of main crop. For nasturtiums around brassicas, a 30 cm wide ribbon along the windward edge.

Diversify it. A single decoy row can get overwhelmed. Stack two species if you have multiple pests. Sunflower at the corners, nasturtium along the wind edge, mustard in the gaps.

Tend it

Watch the trap. That is the whole tending strategy.

When the decoy fills with pests, you have a decision window of a few days. Pull the trap plants and bag them. Burn them. Drown them in soapy water. Feed them to chickens. Any of those works. Leaving them in place lets the population complete its cycle and explode outward into your main crop, which is the opposite of what you set out to do.

Some growers spray the trap instead of destroying it. A targeted dose of insecticidal soap or neem on the decoy hits the concentrated pest cluster while sparing the rest of the garden. This works for aphids and flea beetles. It does not work for stink bugs or squash bugs, which are too well-armored.

Re-seed the trap if the season is long enough. A second round of radish three weeks after the first keeps the flea beetle pressure pulled away through summer. See Succession Planting for the rotation logic.

When it goes wrong

The trap got overwhelmed and the pests spread. Too small a planting. Double the decoy area next season. A trap crop is supposed to look ruined. If it does not, it was undersized.

The pests ignored the trap and went straight for the crop. Wrong placement. The decoy was downwind of the main crop, or behind it, or in the same row. Move it upwind, or between the pest source and the crop, and try again.

The trap worked, then the population doubled. You forgot to remove it. Mark a date on the calendar three to four weeks after pest arrival. Pull the decoy whether you remember it or not.

The decoy attracts beneficials too. This is a feature, not a bug. Nasturtium and sunflower both feed hoverflies and native bees while doing trap duty. The trap crop is also habitat. Plan for that.

Trap crops sit inside the broader logic of Integrated Pest Management. They are not a silver bullet, and they will not replace row covers or healthy soil. But for the specific pests on the specific crops above, they are some of the highest-leverage interventions a small grower has. Two dollars of seed, one strip of ground, and a calendar reminder.