Pests and Pesticides

Pesticides Pesticides are chemicals that are designed to kill pests. A pest is an organism that humans consider harmful or inconvenient. E.g.: weeds, insects, fungi, rodents

Why are pesticides used? Pests can destroy crops. This costs farmers money. (e.g. estimated that 30% of crops in Canada is lost to pests) Pests can carry diseases (e.g. mosquitoes carry Malaria)

4 Types of Pesticides: Herbicides : kills plants Insecticides : kills insects Fungicides : kills fungi Bactericides : kills bacteria

History of Pesticide Use Pesticides started being used around 500 B.C. when sulfur was used to repel insects. 15th century: arsenic, lead and mercury were applied to crops as insecticides 1700’s: farmers started extracting chemicals from plants that had developed defenses against insects. 1939: Paul Mueller discovered that DDT, a laboratory made chemical, was an effective insecticide

2 categories of Pesticides: Water soluble: will dissolve and wash away (not “persistent”) can be harmful and affect nervous system of organisms Fat Soluble: will dissolve in fatty tissue stay in the body will be passed on in the food chain E.g. DDT

Dichloro, Diphenyl Trichloroethane (DDT) Pesticide that is suitable for bioamplification/biomagnification Has a half-life of 15 years That is, every 15 years the amount left in the system will be reduced by one half. If you use 100 kg of DDT, after 15 years it will be reduced to 50 kg, and after another 15 years it will be 25 kg, and so on… NOTE: Modern pesticides are designed to last one growing season and then break down into less harmful substances.

Bioaccumulation & Bioamplification – an increase in the concentration levels of a pesticide within the body of an organism over time Bioamplification/Biomagnification – a species at a higher trophic level feeds on more than one organism below it therefore pesticide concentrations tend to increase rapidly the higher up the food chain you look

Effects of DDT in Birds Shell thinning Carnivorous birds, such as peregrine falcons and bald eagles, eat other birds, dead animals and fish which contain a build-up of DDT DDT causes the shells to become too thin to allow the large females to sit on the eggs without them breaking Since eggs are being broken, the overall population of these birds is declining

The peregrine falcon almost became extinct in Canada because of DDT use.

After DDT was banned in the US and Canada in the early 1970’s the bird populations began to recover. DDT bioaccumulates in humans the same as it would other animals Male birds have also become more feminine as the DDT mimics female sex hormones

Resistance When pests are no longer affected by the chemical - particularly for bactericides and insecticides because of these pests’ high reproduction rate Some insects have genes that help them survive the pesticide application. These pests pass on this gene, eventually making the pesticide ineffective.

Questions 1. Not all countries, like Mexico, for example, have banned the use of DDT. Since birds migrate from winter to summer from one country to another, do you think the birds are 100% safe from the presence and the effects of DDT? Why? 2. DDT is used responsibly in mosquito-infested parts of Africa. This saves millions of lives by combatting an often lethal disease called malaria, which is carried by mosquitoes. Do you think there should be a world-wide ban on the use of DDT? Explain why or why not. 3. How do you think a pesticide like DDT would affect the overall biodiversity (the number of different types of organisms in an area) in an ecosystem?