Pesticides

1. Pesticides • Insecticides: • organochlorines • organophosphates • carbamates • Herbicides: • triazines • chloroacetamides • phosphonates • phenoxy • Fungicides

2. Pesticides • ~109 kg used in North America annually (all uses) • ~108 kg used in California for agriculture • The EPA stopped tracking pesticide use in 2001 • 50% used for agriculture (North America) • 85% used for agriculture worldwide • insecticide – cotton • herbicide – corn and soybeans

3. Pesticides From: NIH

4. Pesticides • Concerns: • exposure of the public to pesticides through food (chronic). • 50% of the food eaten in the US has measurable levels of at least 1 pesticide. • exposure of farm workers to pesticides (acute). • exposure through home use of pesticides. • Some toxicologists believe ‘normal’ exposure to pesticides is not harmful.

5. Pesticides • History • 1000 years ago – Greeks burned sulfur to create sulfur dioxide gas to control insects – used up until the 1800s and sulfur dust is used today as a fungicide. • Romans used arsenic to control insects – used through WWII. • Many inorganic compounds (NaF, B(OH)3) have been used throughout history. • Natural oils from petroleum, fish, and whales, have also been used. • Usually the doses needed were enough to be very toxic to humans. • Organic compounds were synthesized for pesticide use beginning with WWII.

6. Pesticides • Organochlorines • stable – do not require frequent application • highly toxic to insects, low toxicity to humans • persistent in the environment • bioaccumulate – to toxic levels for animals high on the food chain para-dichlorodiphenyltrichloroethane DDT • hexachlorobenzene • HCB • agricultural fungicide • banned in the US, however it is a byproduct of other processes. • very persistent in the environment dichlorodiphenyldichloroethane DDE

7. Pesticides Organochlorines • major replacement for DDT • highly toxic to fish – lakes cannot be restocked for years • relatively volatile – transported through air • currently banned in US – but sparsely used in other countries • LD50 = 50mg/kg toxaphene • flame retardant and insecticide • still used in China and Australia against giant termites • currently banned in US • LD50 = 235mg/kg mirex • similar structure and effectiveness as DDT and DDE • less persistent due to the reactive methoxy groups • metabolization by organisms rather than bioaccumulation • currently used in the US to control flies and mosquitoes • LD50 = 5000mg/kg methoxychlor

8. Pesticides Organochlorines endosulfan From: USGS • Use is permitted since it is not as persistent as other organochlorines (sulfur/oxygen bond). However its degradation product is somewhat persistent. • Close to being banned in the US. • Highly toxic and endosulfan sulfate (degradation product) is also highly toxic. • LD50 is 80-160 mg/kg in rats. • Is blamed for the decline of several fish and amphibian species in areas of high use.

9. Pesticides • Organophosphates • derivatives of phosphoric acid • decompose within days to weeks after application • break down to phosphoric acid, alcohols, and thiols • metabolized quickly by animals – no bioaccumulation • tend to be more acutely toxic than organochlorines • distributed widely both for agriculture and domestic uses • Three types: • A: • B: • C: • tend to be the most volatile • break down the fastest • more stable than type A • break down into type A • some insects absorb type B and C better than type A • generally, but not always less toxic than type A

10. Pesticides Organophosphates • type A • highly toxic – LD50 = 25mg/kg • used in fly strips and flea collars due to its volatility dichlorvos • type B • extremely toxic – LD50 = 3mg/kg • non-specific – can easily kill birds and fish and bees • may be responsible for more human deaths than any other pesticide • still used in developing countries parathion • type C • slightly toxic – LD50 = 985mg/kg • widely used agriculturally and domestically • used to spray communities to reduce mosquitoes and other pests malathion

11. Pesticides • Carbamates • derivatives of carbamic acid • dermal toxicity tends to be lower than for organophosphates • break down within weeks • decompose to CO2, alcohols, and amines • highly toxic towards honeybees carbamic acid carbamate

12. Pesticides Carbamates • extremely toxic – LD50 = 8mg/kg • one crystal, mistaken for a seed by a bird, will kill the bird • absorbed through plant roots and distributed to green vegetation • effectively banned in the US this year carbofuran • somewhat toxic – LD50 = 307mg/kg • widely used in lawn and gardens • extremely toxic toward honeybees • somewhat soluble and tends to contaminate water supplies • production in India caused Union Carbide disaster in Bhopal carbaryl • extremely toxic – LD50 = 1mg/kg • may only be handled by professional applicators • banned for domestic use aldicarb

13. Pesticides Organophosphates and Carbamates as Neurotoxins • Both interfere with cholinesterase activity. • Cholinesterase is an enzyme that breaks down acetylcholine. • Acetylcholine is an important neurotransmitter. • High levels of acetylcholine cause convulsions, irregular heartbeat, and death. • Most poisoning occurs through acute exposure of people applying the insecticides. • A particular problem in developing nations where safety regulations and bans on particularly toxic chemicals are not in place. • Insecticides are also commonly used for committing suicide. • 10,000 – 40,000 die annually, and millions suffer illness from insecticide exposure. (30 die in the US and 20,000 require medical care.)

14. Pesticides Triazines • general formula of a herbicide triazine • substitute a Cl, and 2 amines at the carbons • represents 40% of all weed killer applied in the US • used mostly on corn, since corn is resistant • works by blocking the transformation of CO2 to sugars by photosynthesis • tends to accumulate in water and has been banned in some countries • LD50 = 2000mg/kg atrazine

15. Pesticides From: USGS

16. Pesticides Chloroacetamides • derivatives of chloroacetic acid • replace the OH with a tertiary amine • usually used with atrazine in intensively planted areas • LD50 ~ 1000mg/kg • commonly found in groundwater near applied areas alachlor

17. Pesticides Phosphonates • commonly known as Roundup • extensively used commercially and domestically • inhibits the synthesis of essential amino acids • LD50 = 5000mg/kg • some crops have been genetically engineered to resist glyphosate • absorbs to soil better than other herbicides (doesn’t end up in the water as much) glyphosate

18. Pesticides Phenoxy • most widely used herbicide in the world • kills broadleaf weeds, but not grass • LD50 =640mg/kg • component of Agent Orange 2,4 D • good for clearing brush – used on roadsides • LD50 =500mg/kg • component of Agent Orange • manufacturing process usually introduces unacceptable amounts of the dioxin TCDD • banned in the US 2,4,5-T

19. Pesticides Natural pesticides • found in chrysanthemum flowers • unstable in sunlight • stable derivatives are usually used (pyrethroids) • common in household insecticides • extremely toxic to aquatic life • LD50 = ~1000mg/kg pyrethrin • derived from roots of tropical plants • extremely toxic to fish • LD50 = ~400mg/kg rotenone

20. Pesticides Integrated Pest Management • The use of pesticides has decreased in many developed countries over the past decades due to: • chemical control – the use of both synthetic and natural chemical pesticides. • biological control – reducing pest populations by introducing predators, parasites, or pathogens. • cultural control – using farming practices that minimize pests. • host-plant resistance – using plants that are resistant to pest attack, including genetically engineered plants. • physical control – using non-chemical methods to reduce pests • regulatory control – preventing the invasion of an area by new pests.