|Figure 16.1 Biological
control in a greenhouse
A good general principle for the gardener or grower is that they use
as many different kinds of control as possible within a plant or crop
cycle to bring about precise and efficient control for a pest or disease.
And so a parasitic wasp may be encouraged or applied against an aphid
attack, whilst also considering a cultural control such as the removal of
alternate host weeds or a carefully considered pesticide if this is needed.
A second principle highlights a distinction between pests and diseases.
For pests, biological control
is a major form of control in the natural
environmental habitat (and to a similar extent in the garden/commercial
holding when predators and parasites are encouraged). For diseases,
however, in the natural environment, plant resistance
biological control) is the important control method. In gardens and
horticultural units, plants which have not been highly bred often exhibit
a high level of resistance similar to their wild relatives. But intensively
bred cultivars of annual flowers, annual vegetables and fruits may
largely lack this important form of control.
A third principle suggests that a programme of control against a pest or
disease should consider non-pesticide controls first before relying on the
alternative route. It has to be admitted that few people would disagree
with the principle, but many growers will be able to cite instances when
this idealistic attitude has left them with serious pest or disease problems
on their hands (such as slugs in wet summers). In the ‘supervised
control’ section of this chapter, there is a brief discussion of ‘economic
damage’. At relevant points in this chapter, distinction is drawn between
those measures available to the amateur gardener and those used by the