Secondary plants used in biological control: A review

In crop systems, different types of plant or secondary crop may be grown together with the primary crop for pest management purposes. These additional plants – henceforth called secondary plants – may increase the efficiency and sustainability of biological control of pests by natural enemies. Such plants fall into several categories: companion, repellent, barrier, indicator, trap, insectary, and banker. Despite their effectiveness and accepted function in biological control, to date the full potential of secondary plants in integrated pest management has not been put to good use. This may be partly attributed to a lack of detailed knowledge of the way the secondary plant–crop systems operate, including the effects of the secondary plants on tritrophic interactions. The biggest constraint upon progress, however, has been confusion over definitions and terminology. In this paper, we review the knowledge of the currently employed plant categories and provide clear definitions.