A trap crop is a plant that attracts agricultural pests, usually insects, away from nearby crops. This form of companion planting can save the main crop from decimation by pests without the use of pesticides.  Trap crops can be planted around the circumference of the field to be protected, or interspersed among them, for example being planted every ninth row.
Trap crops, when used on an industrial scale, are generally planted at a key time in the pest's lifecycle, and then destroyed before that lifecycle finishes and the pest might have transferred from the trap plants to the main crop.
Examples of trap crops include:
- Alfalfa planted in strips among cotton, to draw away lygus bugs, while castor beans surround the field, or tobacco is planted in strips among it, to protect from the budworm Heliothis.
- Rose enthusiasts often plant Pelargonium geraniums among their rosebushes because Japanese beetles are drawn to the geraniums, which are toxic to them.
- Chervil is used by gardeners to protect vegetable plants from slugs.
- Rye, sesbania, and sicklepod are used to protect soybeans from corn seeding maggots, stink bugs, and velvet green caterpillars, respectively.
- Mustard and Alfalfa planted near strawberries to attract lygus bugs, a method pioneered by Jim Cochran.
Recent studies on host-plant finding have shown that flying pests are far less successful if their host-plants are surrounded by any other plant, or even "decoy-plants" made of green plastic, cardboard or any other green material. The host-plant finding process occurs in three phases.
The first phase is stimulation by odours characteristic to the host-plant. This induces the insect to try to land on the plant it seeks. But insects avoid landing on brown (bare) soil. So if only the host-plant is present, the insects will quasi-systematically find it by landing on the only green thing around. This is called an "appropriate landing". When it does an "inappropriate landing", it flies off to any other nearby patch of green. It eventually leaves the area if there are too many "inappropriate" landings.
The second phase of host-plant finding is for the insect to make short flights from leaf to leaf to assess the plant's overall suitability. The number of leaf-to-leaf flights varies according to the insect species and to the host-plant stimulus received from each leaf. But the insect must accumulate sufficient stimuli from the host-plant to lay eggs; so it must make a certain number of consecutive "appropriate" landings. Hence if it makes an "inappropriate landing", the assessment of that plant is negative and the insect must start the process anew.
Thus it was shown that clover used as a ground cover had the same disruptive effect on eight pest species from four insect orders. An experiment showed that 36% of cabbage root flies laid eggs beside cabbages growing in bare soil (which resulted in no crop), compared to only 7% beside cabbages growing in clover (which allowed a good crop). Also that simple decoys made of green card disrupted appropriate landings just as well as did the live ground cover.
- Dr. Stefan Brueckmann. "Trap cropping". Oisat.org. Retrieved 2010-02-25.
- Horticulture Research International, Wellesbourne : “Insects can see clearly now the weeds have gone.” Finch, S. & Collier, R. H. (2003). Biologist, 50 (3), 132-135.
- "Trap Crop". Archived from the original on March 22, 2007. Retrieved 24 May 2011.
- Alfredo Flores (March 8, 2010). "Geraniums Could Help Control Devastating Japanese Beetle". United States Department of Agriculture Agricultural Research Service. Retrieved 2010-04-04.
- Samuel Fromartz (2005). Organic Inc.: Natural Foods and How They Grew. Harcourt. ISBN 978-0-15-603242-1.