Read through sections 1-6, then click here to read about a case study where biological control might be used. Answer the questions that go with it.
To find out more about the types of organisms that can be used for biological control visit the Cornell University site.
Extension
An alternative approach to biological control. Rather than introducing novel organisms an alternative approach would be to encourage natural predators. If you are happy with what you have read so far take a look at this article.
Find out more about biological control visit the Biological Control Virtual Information Center.
Using a mixture of approaches to pest control is called integrated pest management. Click here to find out more about it.
Make a table in your notes comparing the advantages and disadvantages of biological control.
Biological control of pests
Biological Control
1. What are the characteristics of pest species?
They cause economic damage OR have health implications (e.g. they could be vectors of disease). Many are capable of very rapid population growth. Often the normal factors which would regulate their numbers (i.e. natural enemies) are not present. This may be because they have been imported into a part of the world where their natural enemies do not exist, or because these enemies have been suppressed in some way.
2. What is the aim of biological control?
This is not always easy to determine. For example if you live in a house infested with cockroaches your pest control aim might be to eliminate all of them. However, in general it is accepted that the aim is to depress the pest population below the Economic Injury Level (EIL):- That is where the costs of the control measures start to exceed those of the extra revenue.
The graph shows an idealised situation. It is assumed that the pests normal population equilibrium size is greater than the EIL. The introduction of a control measure depresses the mean population size below the EIL. Biological control will not eliminate the pest completely.
3. How in theory could a biological control programme be mounted?
For introduced control organisms it should preferably
* reproduce rapidly
* be specific to the pest
* good searching capacity, so that it can keep the pest at low numbers
If the pest species is an introduction then the following sequence of events could be followed if biological control is felt to be desirable.
i. Search in the pest species native country for a suitable organism
ii. Identify if the geographic, climatic and political conditions are right for release
iii. Quarantine the organism, rear the control organisms for release to ensure any unwanted parasite/parasitoids are not introduced with it.
iv. Select a suitable release area.
4. What groups of organisms can be used in a biological control programme?
A. Insect parasitoids These have the great advantage in their specificity to a particular host, which they lay their eggs in. The larvae eat the host from the inside once the eggs hatch. The main taxonomic groups are:- Hymenoptera; superfamilies Ichneumonidae and Chalcidoidea. Diptera; family Tachinidae. B. Predators. Coleoptera; e.g. Carabidae (ground beetles) and Coccinellidae (ladybirds) Hemiptera; e.g. Reduviidae (assassin bugs) (also included here could be herbivores to attack weed species, such as weevils) C. Pathogens The best known example is the bacterium Bacillus thuringiensis. There are several 'pathotypes', one of which has been used to control winter moth in Canada.
6. What examples of biological control are there?
The control of Icerya purchasi by Rodolia cardinalis. The control of the white fly Trialeurodes vaporarium by the parasitic wasp Encarsia formosa in greenhouses. This is a form of 'inoculation'; the wasp is sold as pupae stuck to a bit of cardboard. This is now widely used as an alternative to pesticide control.
The control of water hyacinth (Eichhornia crassipes), which was introduced to the USA from S. America, by a weevil.
The use of grass carp (Ctenopharyngodon idella) in the UK to control weed growth in ponds and waterways. It is claimed that water temperatures in the UK are too cold for them to breath. However, the use of an exotic species such as this is still controversial.
Can things go wrong? Yes, especially if a non-native species is used. On Granada mongoose were released to control the rat population, however, they have had a devastating effect on native species- especially some birds.
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 (N.B. not in the 'inoculation' examples).
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.
The last two could be true of chemical control as well as biological control.