Most plants can tolerate low levels of pest and disease damage without
yield reduction, unless the damage is to parts of the plant that become
unacceptable (such as fruits for the supermarket trade). The term
‘economic threshold’ is used to summarize this concept. Cucumbers,
for example, require more than 30 per cent leaf area affected by red
spider mite before economic damage occurs in terms of yield loss.
This enables methods of control that depend on some damage being done to ensure continued success, such as the use of predators. Damage
assessments are used in apple orchards to decide whether control
measures are necessary. Thus, at green-cluster stage (before flowers
emerge) chemical sprays are considered only when an average of half
the observed buds has five aphids per bud. Similarly, an average of three
winter moth larvae per bud-cluster merit control at late blossom time.
Pheremone traps enable the precise time of maximum codling moth
emergence to be determined in early June. Catches of less than 10 moths
per trap per week do not warrant control. DEFRA issue spray warning information to growers when serious pests, such as carrot root fly, and
diseases, such as potato blight, are likely to occur. Supervised control
may greatly reduce pesticide costs.