Sustainability Interdependance Key Area 3 b c d

Sustainability & Interdependance Key Area 3 b, c & d Cultural Control Chemical Control

Learning Intentions By the end of this topic you should be able to: State there advantages of plant protection chemicals which are selective or systemic. State protective applications of fungicide based on disease forecasts are more effective than treating a diseased crop. State that many plant protection chemicals create a number of problems including toxicity to animal species, persistence in the environment, can accumulate or be magnified in food chains, produce resistant populations. State that biological control is the reduction of a pest population through the introduction of a natural predator or parasite of the pest. Give examples of biological control mechanisms. i. e. ladybird (control) and aphid (pest) State that the most effective control of pests will involve a combination of chemical and biological controls. State that this is known as IPM. (Integrated Pest Management ) State that IPM is a control measure rather than a complete eradication of the pest. State that IPM will cause the least impact on surrounding wildlife.

Control of weeds, pests and diseases Weeds, pests and diseases can be controlled in two ways: 1. By cultural means 2. By chemical means

Cultural methods of crop protection These are techniques for controlling weeds, pests and diseases which are non-chemical. They tend to be preventative rather than curative and involve long-term planning They have been developed over a long period of time, usually by trial and error: • Ploughing • Removing any weeds early • Crop rotation

Ploughing …by turning over the top 20 cm of soil every time a field is ploughed many weeds are buried deep enough for them to die and decompose. Crops can then be planted before the weeds have a chance to re-establish

Weeding …removal of weeds should be done early in the life of the crop to reduce competition. The crop is then given time to grow more sturdy and can tolerate the weeds when they return Weeds should also be removed from the edges of fields as they provide an alternative host - a breeding ground for pathogens.

Crop Rotation … a series of different crops are grown one after the other on the same piece of ground – over 4 growing seasons. Field 1 Potatoes Field 4 Onions Pathogens in the soil from the first crop cannot then grow on the second crop. Growing leguminous (nitrogen fixing) plants can add nutrients to the soil. Field 2 Cabbage Field 3 Peas (nitrogen fixing)

Other cultural means. . … include growing a cover crop on fallow areas as part of crop rotation. The cover crop outcompete weeds so that the field is weed-less when it is ready to be used again. Complete removal of any non-harvested parts of the crop prevents pathogen spores surviving and being present to infect the next crop

Chemical means of crop protection Pesticides include: • • • herbicides (to kill weeds) fungicides (to control fungal disease) insecticides (to kill insect pests) molluscicides (to kill mollusc pests) nematicides (to kill nematode pests)

1. Herbicides These are chemicals used to kill weeds and can be either: (a) Selective (b) Systemic

(a) Selective herbicides mimic the action of plant growth hormones and tend to be biodegradable. This speeds up the metabolism of broad leafed plants (weeds) to the extent that they use up their food reserves and die. Narrow leafed plants e. g. cereal crops, are not affected.

(b) Systemic herbicides are absorbed by the plant and quickly transported to all areas of the plant through their vascular system. This prevents regrowth of the weed. This has a lethal effect on the leaves and roots of the plant and are effective as they can reach underground storage organs and intricate root systems and kill them.

2. Systemic Insecticides, Molluscides & Nematicides These pesticides are essential to protect crops from invertebrates. Without them about a third of crops would be lost due to invertebrate “attack”. These systemic pesticides also spread throughout he vascular system of the plants and kill pests that feed on them.

3. Fungicides These chemicals kill fungal parasites which cause diseases in crop plants. Again, these can be contact or systemic. However, application of fungicides are more effective when used with disease forecasting. Disease forecasts can warn growers of conditions that may lead to fungal infections – spraying crops at this point will be more effective than treating a damaged crop.

Problems with plant protection chemicals The ideal pesticide would be: Specific Short-lived Safe Non-persistent Biodegradable However, this is not always the case

Problems with plant protection chemicals Not specific – the chemical could be toxic to animals or other plants as well as the targeted plants/weeds/pests If the chemical kills all pests but some survive, resistant populations will appear and as there is now no competition they could quickly reproduce

The use of pesticides can result in a population selection pressure (directional) producing a resistant population (eg: the weed/pest/fungi has an enzyme that can break down the toxin or possession of a thicker coat).

Problems with plant protection chemicals Some of the chemicals could remain persistent in the environment (not easily broken down) which would lead to… Bioaccumulation or Biomagnification

Bioaccumulation Plant protection chemicals can accumulate within the body of an organism. Even if levels of the chemical in the environment are relatively low, accumulation can occur if the organism absorbs the chemical at a faster rate than it can excrete (lose) it. This accumulation can mean that chemicals build up to toxic levels in an organism and cause poisoning.

Biomagnification Many plant protection chemicals have been found to persist in the environment. Whilst they are found at low concentrations in the environment and are harmless to all but the pests, they can accumulate between trophic levels – this is called biomagnification.

Biomagnification Although at low concentration in the producers, at each level in the food chain it becomes more concentrated until in the tertiary consumers it becomes toxic and they suffer from poisoning

Biological control describes the control of a pest population through the introduction of one of its natural “enemies”: (a) predator (b) parasite (c) pathogen

Predator Parasite Pathogen Using ladybirds to control aphids Using Encarsia – a parasitic wasp which lays its eggs inside whitefly and destroy it. Using Bacillus thuringiensis – a bacterium which infects caterpillars with a toxin called Bt toxin.

Biological control It is important to time when a biological control is introduced: • An outbreak must already have occurred so the predator can find its prey • Environmental conditions are present that favour the control (predator/pathogen/parasite)

Risks associated with biological control Biological control works best in “closed systems” such as greenhouses so that the control agent cannot “escape” into the wider environment and temperature can be controlled to favour the predator If escape occurs into an environment which is free from predators, parasites or disease then their numbers could increase rapidly and infect the local populations. (e. g. Cane toads in Australia)

Problems with Biological Control Predators can become pests if their population rises by becoming an invasive species, parasite, prey on or be a pathogen of other species

Integrated pest management IPM uses a combination of chemical, biological, cultural methods and plant resistance to improve yield. The main aim of IPM is to: • control rather than eradicate the pest • to reduce chemical use • only use chemicals which do not persist • reduce pests to levels which allow biological control methods to take over.

Integrated pest management Chemicals used as part of IPM are chosen so that: • They are effective even when used sporadically at specific points in the host’s or pest’s life cycle • They show low persistence • They reduce pest numbers to a level where biological control can take over • They are selective and so leave useful predators unharmed