Pesticides Flashcards
Herbicides
Kill weeds
Pesticides
Chemical means to control weeds, pests and diseases.
Fungicides
Control fungal diseases
Insecticides
Kill insect pests
Molluscicides
Kill mollusc pests
Nematicides
Kill nematode pests
Herbicide categories
Selective, systematic and contact
Selective herbicides
Stimulate growth until the broad-leaved weed exhausts its food reserves and dies. Similar properties to plant hormones. Example - PREEN
Contact herbicides
Non-selective and short-lived. Perennials with
storage organs not affected. Example - RESOLVA
Systematic herbicides
Absorbed by the weed then spreads through the vascular system of the plant. Very effective, but may take time to act. Will destroy the whole plant and regrowth/regeneration prevented.
Example - ROUNDUP
Systematic insecticides
Systemic insecticides, molluscicides and nematicides are absorbed by the plant and transported in the phloem to all of its parts. This kills pests feeding on the plant
Diseases forecasts
Where invasion of a plant by a pathogen requires a narrow range of environmental conditions, preventative action can be taken.
Conditions can be monitored and growers alerted to periods when chemicals, such as fungicides, should be applied.
Protective applications of fungicides
Protective applications of fungicide based on disease forecasts are often more effective than treating a
diseased crop.
When to apply the fungicides is determined by the disease forecasts
Problems with plant protection chemicals
- Toxicity to non-target species;
- Persistence in the environment;
- Accumulation or magnification in food chains;
- Production of resistant pest populations.
Toxicity to non target species
Some pesticides are poisonous not only to the intended species but to a broader range of animals, including humans
Example dieldrin used to eradicate beetles, however humans ingested it and linked to human health problems
Persistence in the environment
Some protective chemicals may be persistent – this means they do not biodegrade and will remain in the environment for a long period of time.
DDT killed top predators in food chains
Bioaccumulation
Accumulation of chemicals can occur
(particularly when they are persistent) if an organism absorbs the chemical at a faster rate than it is lost. This can mean the chemical becomes toxic in the organism due to the build up.
Biomagnification
Some chemicals can also be magnified along the food chain. This results from the chemical concentration increasing from one trophic level to the next.
Production of resistant population
When pesticide is applied to a crop a few members among the pest population many already be resistant to the chemical due to random mutation. This feature is now of selective advantage to them given the selection pressure of the pesticide. This advantage could be the possession of a thick coat or the ability to produce an enzyme that breaks down the toxin.
