Plant protection can encompass a number of activities. Weed control in cereals, competitor suppression for pine seedlings, weevil control in lucerne, control of fruit worm in tomatoes, black spot on apples, powdery mildew on peas, clematis vitalba and possums on native trees, rabbits on pasture, ultra violet light damage, salt toxicity and many others. The common element in all of these situations is a yield reduction effect, whether due to a predator, a competitor, or other other cause. When faced with these situations, there are several possible responses which 'managers' can take. Choice amongst these options is likely to be based upon several criteria including: profitability, efficiency, environmental acceptability, potential and actual effects on humans and other non-target species.