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9.4 Caring for the country: 4. Problems arising from pesticide use
| Syllabus reference (October 2002 version) | ||
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4. The effect of excessive use and long-term consequences of using some
pesticides |
Students learn to: | Students:
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Extract from Earth and Environmental Science Stage 6 Syllabus (Amended
October 2002).© Board of Studies, NSW
[Edit: 22 Jun 06]
Prior Learning: Preliminary module 8.3 (subsection 3 and 5)
Background: Australian soils are naturally low in phosphates
and native flora have adapted to these low levels. Most crops need higher levels of
nutrients, including nitrogen and phosphorus, so fertilisers have been used in some parts
of the country for over 100 years. Monocultures, such as crops, are at risk of attack by
pest insects, decimating the crop, so pesticides have been used heavily in some areas.
gather
information from secondary sources to identify a pesticide whose use is now banned and
summarise its use, the reasons for stopping its use and use available evidence to assess the impact of its
residual chemicals in the environment
- The best way to gather information is to go to relevant
and reputable web sites. Some previous uses of organochlorine pesticides in Australia
has some relevant information. Department of Environment, Water, Heritage
and the Arts. Choose one banned chemical such as DDT or dieldrin and do a
search.
- An example of information about one banned pesticide, DDT, has been provided
here.
Pesticide DDT shows up in Antarctic penguins ABC News, Sunday May 11 2008.
Other chemicals you could study are dioxin or endosulfin.
DDT
DDT is an insecticide that was widely used during and after the Second World War to kill disease vectors (carriers), such as mosquitoes that carried malaria, and as an agricultural spray to kill pests that ate crops. Although DDT is still used in some countries for the prevention of malaria, typhus and other vector borne diseases, many countries stopped it use. Some reasons for stopping its use were:
- Growing concerns about the adverse effects of DDT, especially on the fertility of birds. The characteristics of DDT and its metabolites mean that they are ubiquitous (found everywhere). DDT, as it accumulates up the food chain, becomes a major problem in birds of prey. As the DDT concentration in birds increases, the egg shell thickness decreases
- DDT and its metabolites are very persistent in the environment and as much as 50% can remain in the soil for 10 to15 years after application. This persistence, together with it being lipophilic (attaches to fat), provides the necessary conditions for DDT to bioconcentrate in organisms.
DDT
a banned insecticide DDT@3Dchem.com, UK
When using available evidence to assess the impact of residual chemicals in the environment, identify cause and effect relationships and use these to build an argument for cessation of use due to effects which are unacceptable and detrimental.
discuss the effect of continually introducing new pesticides into the environment, including:
- effect on non-target species
- accumulation in individuals (bio-accumulation) and magnification in animals higher up the food chain (biomagnification)
- human health impacts
Non-target species
- Pesticides can often kill beneficial organisms as well as pests, e.g. insecticides can kill predatory insects that would eat the insect pests, as well as economically important insects, such as bees, that are commercially important to apiarists. As well, the animals that eat the poisoned insects and other animals that are further up the food chain, can be poisoned.
Bio-accumulation and bio-magnification
- Some pesticides, especially organochlorides and some
organophosphates, are readily taken up into and remain in the fatty tissue of animals.
Single doses may not reach lethal levels, however, because these pesticides are very
persistent, the poison can accumulate if the animal is exposed to the poison over a longer
period of time. Each time an organism is exposed to the pesticide, the concentration
increases.
- In other situations, organisms low in the food chain may not have accumulated lethal levels of the pesticides, however, due to the persistence of the pesticides, animals higher in the food chain feeding on these organisms may accumulate lethal pesticide levels. An example is the accumulation of DDT, which causes bird’s egg shells to be thin due to the bird having eaten small mammals exposed to DDT. The mammals had not been sprayed but the insects they ate had been sprayed.
Human health impacts
- A large number of pesticides are found to have adverse effects on
human health, including effects on the immune system and hormones. Also, a large number
have been identified as carcinogens (cancer causing).
- The use of DDT provides a good example of how pesticides can impact
on humans. Humans can still be exposed to DDT by eating food that contains small amounts
of these compounds. Although DDT has been banned since the early 1970s small amounts of
DDT can be found in soil and can still be transferred to crops grown in this soil.
- Imported foods may have also been directly exposed to DDT. However, the
amount of DDT in crops has been decreasing and is expected to continue decreasing with
time.
- Short-term exposure to high doses of DDT affects the nervous
system. People who have taken in high amounts of DDT experience excitability, tremors and
seizures. These effects on the nervous system appear to be reversible once exposure to DDT
stopped.
- Some people who have come in contact with DDT have complained of rashes or irritation of the eyes, nose and throat. People exposed for a long-term at low doses such as people who were involved in the production of DDT has some changes in the levels of liver enzymes but there was no indication that DDT caused irreversible harmful effects.
gather and analyse information from secondary sources about alternatives to pesticides as a management practice
Gather as many alternative measures as you can that are used by farmers and horticulturists to control or prevent pests from eating crops. Try to find information that states how effective these measures are. You might like to compare methods used in different countries. You could investigate the following methods:
- Physical barriers, e.g. sawdust around plants to prevent slugs and
snails getting onto and eating the plants.
- Biological control, e.g. releasing an insect,
such as a particular type of wasp, that lays its eggs in another insect
that is a pest and eats the crops. When the wasp larvae hatch out, they
will eat and kill the pest insect.
- Genetic modification, e.g. when geneticists
insert genes into a plant to code it to produce a protein that will make
the plant less susceptible to pests such as cotton with resistance to weevils.
- Use of herbs and companion plants, where
plant combinations work together in fending off insect pest problems. Actual
research data about this is not well established. Much of the information
relates to the experience and suggestions from gardeners. An example is
garlic as a companion plant for fruit trees. When planted close to the trunk,
the garlic repels aphids and blight.
- Use of decoy crops, that is, crops that will
attract the pests so they will leave the main crop untouched or only slightly
affected.
- Integrated pest management, which may incorporate
physical and biological controls and genetic modification.
Integrated pest management - the good, the bad and the genetically modified Nova, CSIRO.
Organic farming -huge potential for producers Click on Organic Farming- huge potential
for producers, NSW Department of Primary Industries, Agriculture.
Analyse the methods listed above or any other methods you have identified by finding out which methods are the most successful. Record your results in a table and come to conclusions about the most effective methods. Cite evidence that supports your generalisations.
assess alternative management practices that do not require the use of pesticides
Management practices that do not require pesticides are often more labour intensive, but have many long-term benefits that are worth the effort.
A greater level of vigilance is needed to identify and manage pests before their numbers become a significant problem. Therefore there is a need for farmers to develop knowledge of the habits and life cycles of pests and to have the skill to apply this knowledge to control the pests.
Often the use of chemicals is replaced with labour and time-consuming activity. Weeds need to be controlled by labour intensive techniques, such as hand weeding and cultivation. The use of some specialised tools, such as flame weeders can reduce this at times.
Parasites of animals must be managed by selecting resistant animals, varying the stocking density, adopting suitable grazing strategies, and appropriately timing operations that reduce the risk of disease, such as shearing and crutching.
There needs to be a strong emphasis on the use of preventive measures and long-term strategies. Although this seems difficult to justify, the long-term productivity of a farm may depend on the reduction of chemicals that build up in soils to levels that destroy their ability to support crops.
Some alternative management practices can achieve multiple benefits. For example, crop rotations help break disease cycles and can be used to manage weeds. Some tall leafy crops can shade out weeds so that fewer weeds survive to cause problems for a following wheat crop. Healthy cover-crops protect young plants, reduce weeds and encourage predators of some pests.
Some strategies make common sense and are very cost affective. These include the establishment of tree groves, which provide shelter for birds and other animals that help in pest management, The trees can also act as windbreaks. The use of natural chemicals and pheromones can confuse and intercept pests.
Management practices that do not require pesticides have the potential to improve the productivity of a farm over a long time frame.
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