The main problems with biological control are:
- Unsuccessful application of biological control organisms that lead to a severe pest problem.
- Introduction of a biological control organism that subsequently kills desirable or beneficial organisms in the environment.
These can be minimized by:
- understanding both the pest's and predator/parasites' life cycles in order to achieve reliable control;
- carefully choosing the best predator or parasite for the problem pest or disease concerned;
- taking care that environmentally useful species are not subject to the attacks of the predators and parasites.
In most horticultural situations, there are important examples of natural balance between species.
- With pests, their naturally occurring predators and parasites are an important form of crop protection (see page 271).
- With diseases, naturally occurring predators and parasites are less common, but the nutritional condition of the plant and the resulting naturally occurring bacterial and fungal populations on leaf, stem and root surfaces (see phyllosphere p235and rhizosphere p322) often help slow a disease's progress.
- The garden represents a complex situation. There may be plant species present from every continent (see p73), and any of these plant species may be accompanied by a specific pest from its country of origin. Plant species that have been present in the UK for many years (such as apple) often have beneficial predators and parasites introduced accidentally or deliberately, from their country of origin, that limit pest numbers. It is quite likely, however, that for more recently imported plant species, there may not be appropriate predators or parasites to control an introduced pest occurring on the plant species in the British Isles.
Some horticultural practices can disturb natural balances.
- In a natural habitat such as woodland, a climax population of plants and animals develops (see p52). Here, a complex balance exists between indigenous pests and their predators/parasites. The food webs (see p53) include several types of predator/parasite found on each plant species that limit (but do not eliminate) the pests. This development of food webs is not achieved to such an extent in most gardens since the natural succession of wild plant species mentioned above is not desirable to gardeners as they aim for optimum production of edible crops or for an aesthetic layout of decorative plants free from weeds (see also Chapter 3).
- Regular movement or removal of cultivated plants and weeds without particular thought to the natural balance between predator/parasites and pests will make pest attacks more likely in the garden/nursery situation.
- The removal of the rotting hollow stems of herbaceous perennials and branches of decaying wood which are common sheltering sites for predatory beetles and centipedes reduces their control potential.
- In a similar way, removal of old plants such as brassicas or bedding plants in autumn may take away the parasitized aphids or caterpillars that would normally serve as the next year's control measures.
- The absence in gardens of plant species acting as a pollen food source to adults such as hoverflies may delay the emergence of their predatory larvae amongst aphid populations.
- The lack of good soil structure (see p310) resulting from poor cultivation or inadequate incorporation of organic matter in a garden may hinder the movement of predatory animals in their search for soil pests.
- A poor physical preparation of soil, and lack of attention to pH and nutrient levels in soil may result in poor soil microbial action (see p321).
- The repeated planting of crops or annual bedding plants into the same area of soil often leads to serious attacks of persistent soil-borne pests or diseases. Notable examples are club root disease on brassicas (see p228) and potato cyst nematode pest on potatoes (see p244). A comparable situation is found when young trees and shrubs (such as roses) are planted into a soil previously occupied by an old specimen of the same plant species, with the resulting problem called 'replant disease' caused by high level of Pythium fungus (see p246).
- The unconsidered use of pesticides may result in a rapid decrease in predators and parasites and may considerably delay their appearance and build-up the following growing season.
The natural balances of organisms can be maintained and restored in order to reduce pesticide use. At the private garden level, there are an increasing number of practices being used that encourage natural balances in order to reduce pesticide use. These physical and cultural methods have been described earlier.
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