Through the international exchange of plants and plant products, many harmful insects and destructive plant diseases have travelled with their hosts to distant lands. Inadequate plant quarantine services have failed to exclude such pests and diseases, many of which have become established with even greater vigour in their new environments.
Plant pests and diseases destroy over 20% of the potential world harvest annually. Among the hundreds of noxious insects and plant diseases that countries seek to exclude through quarantine regulations and controls are the golden nematode of potatoes and tomatoes, pink bollworm of cotton and cotton boll weevil, harmful species of fruit flies, such as the Mediterranean oriental and Mexican fruit flies and the melon fly, Colorado potato beetle, citrus canker and various virus diseases. Because there is greater impetus behind species introduction, commercial and otherwise, and behind international transport, than there is behind quarantine controls, the execution of controls remains inadequate, and plant pests and diseases are carried to new regions on plant products, seed and nursery stock. The worldwide distribution of many major crop and forest pests - the Hessian fly, Japanese beetle, Colorado potato beetle, grape fungus [Phylloxera], spruce sawfly and gypsy moth; tree diseases - Dutch elm disease and chestnut blight; and crop diseases - golden nematode of potatoes, potato blight and vine powdery mildew - testify to the ability of man to spread plant pests and disease.
Quarantines are effective, preventing the introduction of many pests and diseases and retarding the movement of others, giving scientists time to combat such pests and diseases before they become well established; and annually save agriculture large amounts of money, more than sufficient to offset the business and trade losses due to, and the costs of, quarantine control measures.
As man is unable to effectively prevent the movement of microscopic pathogens, many quarantines are scientifically unsound and ineffectual. Occasionally, quarantines have been used as economic sanctions in restraint of free trade and have caused unnecessary economic losses.