Trap Cropping to Control Pests

4 June 2010, written by Jeremy Dore

Most gardeners have experienced the disappointment of carefully raising a vegetable crop only to have it damaged or destroyed by an invasion of pests such as slugs, aphids or other bugs. The traditional advice in such situations is to heavily spray crops with pesticides but many of us prefer to use nature's own organic controls for the food we are going to eat. Trap cropping is one of those valuable organic techniques and is regularly used by organic farmers yet few gardeners are aware of the benefits it offers.

A trap crop, also known as a sacrificial crop, is a plant that you add to your garden to attract pests away from the main crops you are growing. The reasoning is this: just as many children will choose ice-cream over a plate of vegetables, likewise most garden pests have preferences for what they like to live on. By planting rows of a trap crop (the 'ice-cream') near to your vegetables the pests will be attracted to the trap crop and will usually leave your main crops alone. You don't harvest anything from your trap crop ñ it is just there to keep the pests off the plants you want to do well.

There are several factors which need to be taken into consideration when planting a trap crop:

Different insects prefer different trap crops: Which trap crop you choose depends on the pests you are trying to trap. If the plant isn't sufficiently attractive to the pest then it won't be any use so it is vital to pick the right ones. Often this is a matter for experimentation coupled with observation of what the pests go for in your garden.
Layout of the trap crop: For some insects it is sufficient to plant the trap crop around the border of your growing area. Others are harder to stop and it may be necessary to interplant them to draw them off the main crops. Quantities will depend on the insect you are trying to deter but farmers usually set aside something in the region of 20% of the main crop area for the trap crop. Smaller gardens growing a variety of vegetables will often need less than this.
Timing: Most insect invasions happen at a specific time of year. For example, I almost always get an aphid invasion in late May or early June. It is important to have the trap crop already well established by the time pests arrive.
Beneficial Insects: Trap crops are just one part of good organic pest control and need to be balanced with adequate companion planting of flowers to attract beneficial insects such as lacewings and ladybirds which feed on the pests.

A common concern with trap crops is that they could just act as breeding grounds for the pest which will then move on to the main crops. This can happen but several factors usually prevent things getting out of hand:

When pests increase so do their predators. The pests may be building up on the trap crop but as long as you have built in some companion plants there will usually be a hungry population of beneficial insects to start eating them. Often this controls the pests sufficiently and no further action is necessary.
When a trap crop becomes overrun with a pest you can remove it or thin it out. The pests are then disposed of with the plant on the compost heap or somewhere further away from the garden.
Alternatively choosing a trap crop with a long growing season or making more than one sowing of the trap crop can keep the pests at bay until the main harvest is complete.

Examples of Trap Crops

So, what plants are available to act as trap crops for the various pests that affect vegetable gardens?

Nasturtium makes an excellent trap-crop to keep blackfly off other crops

Nasturtiums are very attractive to aphids (blackfly, greenfly, whitefly) which will often completely cover the stems. Expect to see ants 'farming' the aphids for the honeydew they release.
Nettles also attract aphids and because they do this early on in the season they are often followed by beneficial insects such as ladybirds.
Chervil is said to be very attractive to slugs.
French Marigold is variously reported to attract slugs, thrips and nematodes.
Radish is said to attract flea beetle and root fly away from cabbages although various other brassicas can be used as trap crops too so it is best to experiment with Chinese cabbage and collards as well.