Pests are species that lower the yield of crop and that need to be controlled.  They might be competitor plants (weeds) that grow in the field or greenhouse, taking vital minerals from the soil and using sunlight that might be available to the crop plant.  Pests may be insects that feed on the crop plant such as aphids or caterpillars.

Farmers can control pests in two ways;  chemical pesticides are toxic chemicals that kill the pest species and there is also biological control.

Chemical pesticides have two significant disadvantages.  Firstly if the pesticide is persistent (DDT is a good example of this) it will not be broken down in the ecosystem and so will pass up the food chain.  This leads to the pesticide occurring in a much higher concentration in the cells of animals at the top of the food chain and it can cause unexpected and catastrophic effects. DDT made the egg shells of the birds of prey much more fragile so chicks were hatched very prematurely and populations of eagles and falcons were decimated.  This process is called Bioaccumulation.

Can you think why the concentration of the pesticide in the cells of the animals should increase as it moves up the food chain?

The second problem of chemical pesticides is that the pest species quickly develops a resistance to the chemical through natural selection.  The pest can pass its resistance onto its offspring, and so the pest species gradually evolves to become more and more resistant to the pesticide.  This means the farmer has to use more and more of the pesticide for less and less effect.  In every generation the most resistant organisms survive to breed.

Biological Pest Control introduces a different species into the ecosystem that either predates or causes disease in the pest.

The control organism reduces but does not completely eliminate the pest species.  It does take the pest population lower than the EIL (economic injury level) and so at these small populations, the presence of the pest does not damage the farmer’s yield.

Advantages and disadvantages of biological control.

Claimed advantages are

1. Selectivity, it does not intensify or create new pest problems. 

2. No manufacturing of new chemicals, the organisms are already available. 

3. Control organisms will increase in number and spread. 

4. The pest is unable (or very slow) to develop a resistance. 

5. Control is self perpetuating as the control organism will itself breed.

Disadvantages

1. Control is slow. 

2. It will not exterminate the pest. 

3. It is often unpredictable. 

4. It is difficult and expensive to develop and supply. 

5. It requires expert supervision. 

A good example of biological pest control might be the use of ladybirds in a greenhouse to reduce the numbers of whitefly, an aphid that feeds on the crop plant.  Ladybirds eat the aphids and so reduce their population below the EIL. In a greenhouse, the control agents (ladybirds) cannot really escape into the wild and cause environmental damage and so the risk of the procedure is low.  Sadly there are many examples from all over the world of a non-native control species being introduced to act against a specific pest and the biological control agent causing more harm than good through unpredictable behaviour in the new ecosystem.  Biological control needs careful monitoring and detailed research in advance of the introduction of the new species.