Many weeds can protect nearby plants from insect problems. At the larger garden plots on our farm, we allow such weeds as ground ivy to propagate. This will mask the scent of the edible companion that grows alongside it to help ward off pests and smaller animal consumers keeping them away from protected edible areas. Ground ivy along with the purslane weed harbors a habitat shelter for the ground beetle which is nocturnal and comes out at night to prey upon soft bellied insects that invade your garden.
The purslane weed is very nutritional and can be used in salads and surprisingly has more omega 3 fatty acids than any edible leaf vegetable you can grow. Acting as a trap crop is another feature which will distract pests away from your edibles. Pests seeking a food plant by smell will land on anything green within the scent and will stay on the weed versus the valued crop that you are protecting.
One of the most interesting examples we have observed on our farm is called flight patter disruption (or host finding disruption). In theory what this suggests is that many insect pests will make an inappropriate landing if your intercropping techniques are surrounded by other plants as a decoy masking both plant scent and insect pheromones. This will cut down on a beetle infestation or caterpillar infestation. For example the cabbage looper moth will always give up if it doesn’t get enough sufficient leaf data to land and lay its eggs on your leaf plants. Therefore the more you intercrop with companions, the harder it is for these pests to hit the desired target so they will simply become frustrated and move elsewhere.
This technique and know how is another natural defence arsenal for protecting your edibles and you can work with what nature has provided by knowing the balance and benefit they provide. This theory also supports the intensive gardening practices using smaller square metre garden vessels created to incorporate intercropping companion planting techniques and to mimic habitat situations.
This is one way human intervention can create a proactive habitat vessel to support a natural formula that already exists within our ecosystem components.
Edit: Here is a very informative link on natural pest control from our friends Speedy Pest Control out of the U.K.