Some insects seek nectar to get through a period when their preferred food source is scarce. For example, clover blossoms produce nectar that attracts hungry lacewings, honey bees, lady bugs, and wasps. Often, beneficial insects will be lured in to prey on the pest of a soil crop but then remain to control other pests on your vegetable crop. Bell beans attract large numbers of bean aphids which in turn attract lady beetles. After the lady beetles have finished off the bean aphids, they scout for other prey and may clean them out as well.
Different plants harbor different populations of insect pests.
If populations build to harmful levels in one crop, a different crop that does not harbor those pests or actually diminishes their population may be planted to interrupt the pest's life cycle. After the break crop has done its work, the rotation may include the vulnerable crop once again. Reducing pest populations is one of the best reasons to rotate crops in the garden and on the farm. Plants with a particularly strong ability to capture, store, and slowly release nutrients are called catch crops. They are traditionally planted in late summer or early fall to trap nutrients from freshly spread manures, previously turned under legumes or decomposing vegetation.