Resistance to pesticides
The development of resistant individuals from the millions of susceptible weeds, pests and diseases occurs most rapidly when exposure to a particular chemical is continuous or when a pesticide acts against only one body process of the organism. Resistance, e.g. in powdery mildews, to one member, e.g. carbendazin, of a chemical group confers resistance to other chemicals in the same benzimidazole group. Growers should therefore follow the strategy of alternating between different groups and not simply changing active ingredients. Particular care should be taken with systemic chemicals that present to the organism inside the plant a relatively weak concentration against which the organism can develop resistance. Increase in dosage of the chemical will not, in general, provide a better control against resistant strains. Biological control, unlike chemical control, does not create resistant pests.
Active ingredients are mixed with other ingredients to increase the efficiency and ease of application, prolong the period of effectiveness or reduce the damaging effects on plants and man. The whole product (formulation) in its bottle or packet is given a trade name, which often differs from the name of the active ingredient. The main formulations are as follows:
- Liquids (emulsifiable concentrates) contain a light oil or paraffin base in which the active ingredient is dissolved. Detergent-like substances (emulsifiers) present in the concentrate enable a stable emulsion to be produced when the formulation is diluted with water. In this way, the correct concentration is achieved throughout the spraying operation. Long chain molecular compounds (wetter/spreaders) in the formulation help to stick the active ingredients onto the leaf after spraying, particularly on smooth, waxy leaves such as cabbage.
- Wettable powders containing extremely small particles of active ingredient and wetting agents form a stable suspension for only a short period of time when diluted in the spray tank. Continuous stirring or shaking of the diluted formulation is thus required. An inert filler of clay-like material is usually present in the formulation to ease the original grinding of particles, and also to help increase the shelf life of the product. It is suggested that this formulation is mixed to a thin paste before pouring through the filter of the sprayer. This prevents the formation of lumps that may block the nozzles.
- Dusts are applied dry to leaves or soil, and thus require less precision in grinding of the constituent particles and less wetting agent.
- Seed dressings protect the seed and seedling against pests and diseases. A low percentage of active ingredient, such as iprodione applied in an inert clay-like filler or liquid reduces the risk of chemical damage to the delicate germinating seed.
- Baits contain attractant ingredients, e.g. bran and sugar, mixed with the active ingredient, e.g. methiocarb, both of which are eaten by the pest, such as slugs.
- Granules formulated to a size of about 1.0 mm contain inert filler, such as pumice or charcoal, onto which the active ingredient is coated. Granules may act as soil sterilants (e.g. dazomet), residual soil herbicide (e.g. dichlobenil), residual insecticide (e.g. chlorpyrifos), or broad-spectrum soil nematicide and insecticide (e.g. aldicarb). Granular formulations normally present fewer hazards to the operator and fewer spray-drift problems.
Labels on commercial formulations give details of the active ingredient contained in the product. Application rates for different crops are included. DEFRA approves pesticide products for effectiveness.
Phytotoxicity (or plant damage) may occur when pesticides are unthinkingly applied to plants. Soil applied insecticides, such as aldicarb, can cause pot plants, such as begonias, to go yellow if used at more than the recommended rate. Plants growing in greenhouses are more susceptible because their leaf cuticle is thinner than plants growing at cooler temperatures. Careful examination of the pesticide (particularly herbicide) packet labels often prevents this form of damage.
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