In a text such as this, where insects, diseases, and weeds are treated in terms of their general effect on plants, a discussion of their control must be equally general. Specific products and formulations used in the control of specific pests can fill volumes. Fundamental to the training of professional horticulturists, however, is a solid understanding of why and when certain control measures can be effective against pests. The technology of pest control may change over time, but the principles that direct it will not.
The purpose of pest control is to reduce the damage that can result from an injurious agent. Damage can be either quantitative or qualitative. With quantitative damage, all or part of the host plant is destroyed by the antagonistic agent. With qualitative damage, the host suffers a loss of appearance and sale value. Damage also can be to land and
equipment. Land left unusable by a long-lived, soil-borne pathogen can be considered damaged, as can equipment that depreciates in value without maximum use because of pest presence in a crop.
In seeking and comparing control measures, a grower must decide what degree of effectiveness is being sought. There are three levels of pest control.
1. Partial control is the most common type. When a homeowner sprays shrubs with an all-purpose (broad-spectrum) pesticide, the shrubs may still exhibit some symptoms of insects or disease, but not as severely as if they had not been sprayed.
2. Absolute control is total control. All symptoms of pest injury are absent.
3. Profitable control is the level attained when monetary returns on the crop exceed the cost of the control measures.
Partial control is the most common type but profitable control is the type most sought by growers. Absolute control, even when possible, is usually too costly to be feasible. Absolute control may be practical on a noncommercial scale, with a single foliage plant in a home or a carefully tended flower bed in the backyard, but it seldom exists in commercial practice.
In determining the potential profitability of a control measure, three factors must be considered:
1. The value of a single crop plant. The value of individual plants can vary greatly. The cost of bringing one marigold plant to a saleable state is far less than the cost of one poinsettia or one Japanese maple. More valuable crop plants warrant more costly control measures.
2. The ultimate value of the crop. A grower may justify the expense of spraying a field of young sapling trees on the basis of their sale value five years hence.
3. The average loss over a period of years. Some insects or diseases may be troublesome only irregularly. However, rather than risk a devastating infection, a grower may choose to invest in protective control every year to guard against a potential pest outbreak.