|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|28. Plant protection|
Plant protection measures are carried out to limit performance and yield losses in crop production during the growing season and afterwards (storage protection) as well as for quarantine purposes. They serve primarily to safeguard yields, although in combination with other cultivation measures they can also help to raise yields.
A wide variety of individual measures - with varying ecological, economic and socio-economic impacts - are available for keeping harmful organisms (diseases, pests, weeds) below the economic threshold. To reduce the probability of damage, preventive measures are taken in the areas listed below. Some of these can be regarded as belonging to the field of plant production (cf. environmental brief Plant Production), which reflects the close links between the two sectors:
- site design (hedges, border strips etc.)
- site and variety selection
- sowing, planting
- healthy seed and planting stock
- crop rotation, intercropping
- tillage, land improvement
- crop tending
Measures in these areas are backed up by the following direct forms of plant protection:
- physical methods
- chemical methods
- biotechnical methods
- biological methods
- integrated methods
Physical methods directly destroy harmful organisms, aim to retard their development or prevent them from spreading. They can be divided into mechanical and thermal measures. The former include tillage to control weeds and pests (hoeing, removal of affected parts of plants and intermediate hosts), flooding of fields to combat soil-borne harmful organisms (e.g. Fusarium oxysporum, which causes banana wilt), laying of sticky belts to trap flightless insect pests and other measures for catching pests or keeping them away from crops, such as fences, trenches (locust control), traps and picking-off of pests. Thermal methods utilise the harmful organisms' sensitivity to high or low temperatures. They include hot-water treatment of seed and planting stock (e.g. to combat viruses and bacteria in sugar cane cuttings), solarisation (covering the surface of the ground with plastic sheeting produces phytosanitary effects by virtue of the greenhouse effect resulting from insolation, e.g. for controlling parasitic seed plants, soil-borne harmful organisms etc.), burning-over to control weeds and burning of crop residues. Low temperatures inhibit the spread of certain storage pests.
Eradicative, protective and curative methods are used in chemical plant protection to destroy harmful organisms or keep them away from plants, to protect plants against attack and penetration by harmful organisms and to cure plants (or parts of plants) that have already become infested or diseased. Although chemical methods can be subdivided in this way on the basis of their effects, the boundaries between the individual categories are somewhat fluid, as many pesticides have more than one type of effect. Pesticides generally kill the harmful organism by influencing vital metabolic processes or disrupting the conduction system. Selectivity can be varied through appropriate selection of the active ingredient, formulation, application technique and time of application.
Biotechnical and biological methods of plant protection have gained in significance, among other things because the risks and limits of chemical measures are today assessed more realistically. Biotechnical methods utilise the natural reactions of the (almost exclusively motile) harmful organisms to physical and chemical stimuli in order to bring about changes in their behaviour for the purpose of plant protection (e.g. light and colour traps, chemical attractants, antibodies, pheromones, hormones, growth regulators). The emphasis is on measures which aim not to directly kill the harmful organisms, but rather to permit population monitoring for the purpose of forecasting, defensive action and deterrence. The harmful organisms can be killed by combining biotechnical methods with chemical measures.
Biological plant protection involves using organisms and their activity to protect plants and enhance their resistance to biotic (harmful organisms) and abiotic limiting factors. For the purpose of pest and disease control, beneficial organisms are specifically preserved and fostered, released in large numbers or introduced into habitats where they have not been found hitherto. Biological control of weeds has to date primarily involved introducing beneficial organisms into new habitats.
Another biological method is that of inducing resistance to disease. This can be done, for example, by infecting plants with pathogens having low virulence.
There are close links between biological and integrated plant protection in that both methods attach major importance to regulation by means of biotic limiting factors. If such methods are to prove effective, moreover, there must be little or no use of preventive and broad-spectrum pesticides. Biological methods can be applied on only a limited scale in intensively used agrobiocoenoses which are poor in species, but can play a more important role in areas where extensive farming is practised and in coenoses comprising a greater variety of species. Their limits are determined above all by the efficiency of the beneficial organisms and the latter's dependence on environmental conditions.
Integrated plant protection is a concept which involves coordinated use of all ecologically and economically justifiable methods in order to keep harmful organisms below the economic threshold. The emphasis is on utilising natural limiting factors. The main aim is to preserve the balance of nature as far as possible; this is to be achieved by reducing use of chemical plant protection methods and simultaneously employing a variety of measures from the other categories. It is here that the links with the plant production sector are particularly close. Use of pesticides is to be reduced to the essential minimum by abandoning the practice of routine or calendar-based spraying, gearing pesticide dosage to actual conditions, refraining from the use of broad-spectrum persistent agents (liable to harm beneficial organisms) and selecting the time of application such that beneficial organisms suffer no adverse effects.
Integrated plant protection methods generally prove more successful in permanent crops than in short-lived crops, since the biocoenoses of the former are more stable and can be more permanently influenced whereas those of the latter are inevitably subject to constant change. The limits and risks attaching to these methods become clear if the work is performed by untrained personnel. Use of integrated plant protection methods generally calls for detailed knowledge of biological, ecological and economic factors.