Give an account of the ways in which biological control can be used to control pest numbers.

The effect of predator-prey relationship in regulating populations has been exploited by humans as a method of controlling various pests. Biological pest control is where the natural predators (agent) of the pest ((target) are used to reduce its population to a tolerable level. It is important to make sure that the right predator is chosen, so it must be studied in quarantine to make sure that once the number of prey drop it will not change to attack another species and become a pest itself. It must have a fast enough reproductive rate in order to take control before the crop is ruined and it should be native to the environment to avoid unforeseen effects. In general it is best to use herbivores to get rid of weeds e.g. the use of cactoblastis larvae which makes the prickly pear susceptible to fungi thus killing it is used in Australia. To control herbivores like aphids, carnivores like the hoverfly are used. Parasites can control their hosts e.g. the parasitic wasp is used to reduce the numbers of white fly larvae. In some cases such as the screw worm fly where females can only breed once, by releasing infertile males it makes the next generation smaller. And sex pheromones are used to tempt some species like moths into lethal traps. Biological control is often used in greenhouses for instance by releasing ladybirds into a confined area the number of whitefly can be reduced rapidly.

The ideal situation to reach is where both the target and agent populations are in equilibrium with each other at a level, which is not damaging the crop. Biological control has the advantage that it is specific and as its not using toxic chemicals it will not cause adverse effects to the environment.

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