Biotic factors and their control

• Agriculture removes most of the previous natural ecosystem and replaces it with a simpler agricultural ecosystem. Some wildlife species may take advantage of the change and become pests, whilst other populations decline, such as crop pollinators and the soil organisms that recycle nutrients.
• Pests are organisms that reduce agricultural productivity or the quality of the product. They may do this by: being predators that eat the crop or livestock, compete for resources, being pathogens, carrying pathogens or reducing marketability.
• Weeds are pests. The problems they cause are competition for nutrients, water and light; they are harvested with crop, reducing quality or spoiling taste; provide food for other pests. An example is wild oats. Certain weeds are parasitism of crop roots, like striga in maize crops.
• Insects can be pests. The problems they cause are eating the crop, destroying the crop, or reducing harvests. They may also spoil the appearance of the harvested crop. Examples of this are aphids which suck the sap and reduce the growth of many crops, locusts which eat almost any plant, and termites which attack tree crops.
• Fungi can be pests. The problems they cause are causing the growing plants or harvested crop to rot. Examples of this are leaf smut or leaf blight.
• Bacteria can be pests. The problems they cause are reducing harvests by causing disease. Examples of this are bacterial wilt and bacterial leaf blight.
• Molluscs can be pests. They problems they cause are eating the crop, reducing harvests and spoiling the appearance of the crop. Examples of this are snails and slugs.
• Nematode worms can be pests. The problems they cause are damaging the roots and reducing water and nutrient uptake. They also increase the risks of fungal and bacterial disease. Examples of this are potato cyst nematodes and soybean cyst nematodes.
• Vertebrates can be pests. They problems they cause are eating the growing or harvested crops. Examples of this are mammals and birds.
• Endemic pests are always present, usually in small or moderate. Epidemic pests are not normally present, but there may be ‘outbreaks’ where they can rapidly become a major problem.
• Indigenous species are native to the area where they are found.
• The majority of pests have been introduced from other areas. These are often more of a problem as they may not have any predators in the new areas.
• Cultural pest control involves non-pesticide methods where crops or livestock are cultivated in a way that reduces the risk of pest damage, often by using natural ecosystem services.
• Crop rotation is an example of cultural pest control. This is done as different crops have different pests. If the same crop is grown consistently the pests from the first year may survive until the second year so pests are already present and don’t need to colonise, which allows pest populations to increase and cause more damage. Crop rotation involves the cultivation of a different crop each year, so the pests remaining at the end of one year will have died off before the crop is grown again.
• Companion crops are an example of cultural pest control. It involves growing crops together to increase productivity. Sometimes both crops produce a harvestable crop or one of the plants is grown to increase the harvest of the other crop.
• The inter-species relationship between companion crops include: nutrient supply - legumes can be intercropped among other crops to increase nitrate availability in the soil; barrier crops - the smell of onion can mask the smell of carrots and reduce damage caused by carrot root flies; pest attraction to protect the other plants - nasturtiums attract blackfly pests that could damage bean crops; support of pollinators - flowering plants that support bess which are important in pollinating fruit crops.
• Predator habitats are an example of cultural pest control. The populations of natural pest predators can be increased by providing suitable habitats, such as beetle banks and hedgerows for black ground beetles and ladybirds which eat aphids.
• Biological control is an example of cultural pest control. Predator or pathogen species may be introduced to control pests. This is important if the pest is a non-indigenous species that has been introduced, so has no native predators. Biological control species should be specialist feeders, as if they have a wider diet they may eat beneficial or non-target species.
• An example of a successful biological control was with the Cactoblastis moth and the prickly pear cactus. The cactus was introduced to Australia from South America where it had no natural predators and rapidly spread over large areas of farmland, so they introduced the Cactoblastis moth from South America to deal with it. Both species are now rare.
• An example of unsuccessful biological control are the introduction of Cane Toads to Australia. They have eaten a wide range of other species other than the beetle pests of sugar can. They are toxic so have few predators and have been able to colonise a huge area.
• Sterile male techniques are an example of cultural pest control. In many insect species, females mate once and then store sperm for all future egg laying. If the females mate with a sterile male, no offspring will be produced. Sterile males are created by exposing them to gamma radiation and then releasing them. Enough sterile males will reduce fertile matings to a level below that needed to produce enough young to compensate for mortality, causing population declines. An example of this is with the Mediterranean Fruit Fly.
• The advantage of sterile male techniques is that it is species-specific so it doesn’t affect non-target species.
• The disadvantages of sterile male techniques is that it is only successful if the sterile males behave normally and are successful in finding a mate. For example, with mosquito populations, sterile males behave differently which causes them to be rejected by females, which continue to mate with wild fertile males. There also may be re-colonisation from nearby areas that have not been cleared, so regular re-treatment may be needed.
• Pheromone traps are an example of cultural pest control. In many insect species, such as moths, mates are attracted by a scent called a pheromone. Pheromone traps release an artificial scent to attract pests. This can be used in two ways: to show that the pest is present so that pesticides can be used to protect the crop or to kill all the pest individuals or enough of one gender so there are not enough fertile matings to maintain the population.
• Genetic resistance to disease is an example of cultural pest control. Selective breeding may enhance the resistance of a variety to pests or disease. Pathogens evolve to overcome crop disease resistance so it’s necessary to regularly introduce new characteristics to maintain resistance. Commercially cultivated crops have little genetic diversity, so search for new characteristics focus on wild or CWR species.
• The issues with relying on genetic resistance for pest control is that the CWR species which are relied on to provide new characteristics for commercially cultivated crops are habitat loss, spread of commercial crop varieties with small gene pools and climate change.
• GM crops are an example of cultural pest control. They are genetically modified crops to control pests more effectively by reducing susceptibility to pests.
• The toxicity of a pesticide can influence their effectiveness and environmental impacts. Pesticides which have high toxicity require the use of smaller volumes. Most pesticides act by inhibiting enzyme action.
• The specificity of pesticide can influence their effectiveness and environmental impacts. A measure of the range of taxa can be affected by the pesticide. More specific pesticides are less likely to harm non-target species.
• The persistence of a pesticide can influence their effectiveness and environmental impacts. Persistent pesticides are chemically more stable and degrade slowly. This reduces the frequency of re-application but can increase the likelihood of a pesticide dispersing more widely in the environment and may extend the period of time that it may harm non-target species.
• The solubility of a pesticide can influence their effectiveness and environmental impacts. Pesticides that are water soluble are more likely to be washed off a crop, requiring re-application. Liposoluble pesticides may be absorbed and stored within the crop, possibly entering the human food chain.
• There are two main modes of action by which chemical pesticides kill pests: contact action and systemic action.
• Contact action is: contact herbicides which kill plants by damaging the tissues they are sprayed onto; contact insecticides which kill insects that are sprayed directly or come in contact with the pesticide that was sprayed onto the crop that is being protected. The pesticide only protects the crop surfaces that are sprayed, and can still be washed off by the rain.
• Systemic action is: systemic pesticides are absorbed by the crop and translocated throughout the plant. This protects all of the plant and will also protect new growth. The pesticide cannot be washed off by rain but can be retained in the harvested crop and eaten by humans.
• Washing food may remove contact pesticides but not systemic ones. Public safety is increased if a sufficient time period is left between application and harvest for the pesticides.
• Antibiotics are an example of chemical pest control. They are used for several reasons: to treat infections and kill pathogenic bacteria; to prevent infections through regular large doses of antibiotics, especially where the livestock stocking density is high; to promote growth and increase Gross Growth Efficiency of livestock through small doses of antibiotics reduce the population of non-pathogenic gut bacteria so more food is used towards growth, therefore increasing productivity and farm income.
• The disadvantages of antibiotic use in agriculture are: smaller doses may only kill the most sensitive individuals so the surviving population will be less easily controlled by the antibiotic; large scale use of antibiotics as growth promoters increases the risks of producing antibiotic resistant bacteria. Some of these may be zoonoses which cause disease if they are transferred to humans e.g salmonella, E.coli and Campylobacter.
• Hormone pesticides are an example of chemical pest control. These kill pests through their biochemical action. They increase or start natural processes in a way that is harmful to the pests, such as insect hormone pest control chemicals control development in a way that causes death. Some cause insects to metamorphose into adults before they are large enough to function properly so they die, and other prevent the formation of chitin skeleton when they moult. Hormone pesticides have low persistence and are more specific than most pesticides.
• Integrated control is the use of a combination of techniques to maximise effective pest control whilst minimising environmental impacts. Integrated control often has an order in which techniques are used based on cost and ease of use, effectiveness and environmental impacts.
• The principles of integrated control are: use of cultural techniques which make growth environment less suitable for the pests; use of cultural techniques that prevent the build-up of a pest population; cultivating species and varieties that are less likely to suffer pest attacks; use of other appropriate non-pesticide techniques; use of pesticides when essential with carefully timed applications of specific, non-persistent pesticides.