PESTICIDES BISC 312 Simon Fraser University Onkar Bains

What are Pesticides? n An agent (chemical, physical, or biological) that kills organisms classified as pests n insects, birds, rodents or fungi, bacteria, viruses n a. k. a “_______” n Common groups (divided by targets): n n n _____________ _______

Pesticide use: an old practice n Invention of agriculture about 12, 000 years ago; production and storage of food (grain) immediately involved problems with pests n Earliest pest control techniques n n n 4500 years ago—use of sulfur with stored grain (Sumaria) 2, 000 years ago—ants used to control leaf beetles on citrus (China) 1100 A. D. —insecticidal soap (China)

Pesticide use: an old practice n n n 16 th century—Japanese mixed poor quality whale oil with vinegar to spray on rice paddies to prevent development of insect larvae by weakening cuticle 1600’s—tobacco infusions, arsenic, some herbs used against insects 1800’s—Europeans used sulfur as fungicide to control powdery mildew on fruit Late 1930’s—discovery of insecticidal properties of DDT by Swiss chemist Paul Muller 1930’s—Gerhard Schrader discovered insecticidal properties of organophosphates

The ideal pesticide n Requirements for an 'ideal' pesticide: ________________ n High activity results from minimal usage n ________________ n Low development of resistance by pests n ________________ n Easy to make !! n

Pesticides—Why are they needed? n Each year, pests consume or destroy 3048% of world's food production n Increase crop yields n Improved food quality

A price worth paying? n Hazards of pesticide use are now widely recognized n World Health Organization estimates that at least 3 million people are poisoned by pesticides every year and more than 200, 000 die n In Malaysia and Sri Lanka, 7 to 15% of farmers experience poisoning at least once in their lives n In Thailand, 5, 500 people were admitted for pesticide poisoning in 1985 alone, of whom 384 died n In Latin America, 10 to 30% of agricultural workers show inhibition of the blood enzyme, cholinesterase, which is a sign of organophosphate poisoning n In Venezuela, 10, 300 cases of poisoning with 576 deaths occurred between 1980 and 1990 n In Egypt, more than 50% of cotton workers in the 1990 s suffered symptoms of chronic pesticide poisoning, including neurological and vision disorders From New Internationalist, Issue 323, May 2000

ORGANOCHLORINE INSECTICIDES

Organochlorine insecticides n Composed of carbon, chlorine, hydrogen and sometimes oxygen and sulfur n Cyclic carbon chains (benzene rings) n OC’s banned in _______ and ____, but still used in developing countries because: n n n _______________ highly effective Risk/benefit weighted in favor of control of insects for better food production and disease control in developing countries

Structural classifications From http: //aquaticpath. umd. edu/appliedtox/pesticides. pdf

Examples From http: //www. nau. edu/~chem/courses/chm 440/lectures/lec 19. html

DDT n 1, 1, 1 -trichloro-2, 2 -bis(p chlorophenyl)ethane n _____ water solubility n n 0. 025 mg/L at 21 o. C Log Kow = 4. 89 -6. 91 n Log Koc = 5. 15 -6. 26 VP = 2. 5 x 10 -5 Pa at 25 o. C n __________ n First synthesized in _____ n

DDT n Used to kill moths and carpet beetles in 1939 n Used on humans to control typhus epidemic by killing lice in Italy (1943 -1944) n Used extensively from 1940 s to 1960 s in: n n n From http: //www. udel. edu/inst/jun 2001/files/1 agriculture and forestry building and structural protection from termites ______ in humans— control spread of mosquitoes (malaria threat)

Synthesis of DDT Cl Cl Cl 3 CCHO H 2 SO 4 H Cl CH Cl 3 OH chlorobenzene Cl Cl Cl trichloroacetaldehyde p, p-DDT (70%) The other main product of this reaction was o, p-DDT (~25%) !! From http: //www. whoi. edu/science/B/people/mhahn/20

Mode of Action n Opens ______ gate in nerve membrane n Nerve keeps firing and the insect is stimulated to death (______)

Environmental Fate n Soils and Groundwater: n Binds _______ to organic matter and becomes ____ n In cases where soils have low organic matter, it may with time leach into groundwater n Residues found on surface of soil may be subject to runoff, photolysis and biodegradation

Environmental Fate n Surface Waters: n May reach fresh water by direct application (for the control of malaria mosquitoes), runoff from agricultural fields, atmospheric transport, etc. n Half-lives: n n n Lakes = ~56 days River water = ~28 days Can be removed from aquatic environment by sorption to sediments and organic particles, photolysis and via uptake by aquatic organisms

Metabolism n Very slowly metabolized to produce: n n 1, 1 -dichloro-2, 2 -bis(p-dichlorodiphenyl)ethylene (_______) 1, 1 -dichloro-2, 2 -bis(p-chlorophenyl)ethane (_______) n Metabolites readily stored in _______ tissues n Ultimately metabolized to produce bis(dichlorodiphenyl) acetic acid (______) excreted from the body via urine

Toxic Effects n n n n ______ of the tongue, lips and face Apprehension Irritability _________ and _______ convulsions Increased salivation Loss of weight (due to anorexia) Decline in male reproductive health n n Reduced sperm count; increased incidence of testicular cancer p, p’-DDE binds to androgen receptors and acts as antagonist (blocks effects of testosterone)

Problems with DDT n _____ development with pests n ________ n half life of approx 100 years ________ n n n accumulation of a compound through the food chain Non-target effects n ________ n Induce ________ n n DDT was found to induce feminization of embryos in contaminated gull eggs Explains skewed sex ratios and reduced numbers of breeding males found in a California gull population (Fry and Toone 1981) Photograph of a crushed Caspian tern egg, next to a normal egg. Source: Marco, G. J. , Hollingworth, R. M. , and Durham, W. , Silent Spring Revisited, American Chemical Society, Washington, DC, p 100, 1987.

From http: //jan. ucc. nau. edu/~doetqp-p/courses/env 440_2/lectures/lec 25. html

From http: //www 2. bren. ucsb. edu/~keller/courses/esm 202/ESM 202 Lecture 11_2004. pdf

Other problems—DDT toxicity n Highly toxic to many aquatic invertebrate species: n n n Highly toxic to fish species: n n 96 -hour LC 50 s in various aquatic invertebrates (e. g. , stoneflies, midges, crayfish, sow bugs) range from 0. 18 ug/L to 7. 0 ug/L 48 -hour LC 50 s are 4. 7 ug/L for daphnids and 15 ug/L for sea shrimp (Johnson and Finley, 1980) 96 -hour LC 50 s are less than 10 ug/L in coho salmon (4. 0 ug/L), rainbow trout (8. 7 ug/L), bluegill sunfish (8. 6 ug/L), largemouth bass (1. 5 ug/L), and fathead minnow and channel catfish are 21. 5 ug/L and 12. 2 ug/L respectively (Johnson and Finley, 1980) Moderately toxic to some amphibian species—larval stages appear to be more susceptible than adults (Hudson et al. 1984; WHO, 1989).

ORGANOPHOSPHATE INSECTICIDES

Organophosphorous insecticides Parathion n First recognized in 1854, but their general toxicity was not established until the 1930 s n First insecticides used on a large scale to replace the chlorinated hydrocarbons (OC’s) n Unlike most of OC’s, OP’s are not stored in the body for long periods of time Malathion

Organophosphorous insecticides Parathion n This property, combined with a much shorter residual life also reduces the chances of long-term environmental contamination n All share the same general formula with a central ______ atom surrounded by four functional groups Malathion

Water Solubility (mg/L) Log Kow Log Koc V. P. (Pa) 40 at 25 o. C 3. 81 2. 28 0. 011 Fenamiphos 700 at 20 o. C 3. 25 2. 29 0. 00012 Chlorpyrifo s Parathion 2 at 25 o. C 5. 11 3. 73 0. 0025 12. 4 at 25 o. C 130 -145 at 25 o. C 3. 15 1. 05 0. 00089 2. 36 2. 46 0. 0053 at 30 o. C Diaxinon Malathion

Metabolism n Readily undergo conversion from ______ (P=S) to ______ (P=O) when metabolised by CYP 450 n n Parathion to paraxon Malathion to malaoxon n Conversion occurs in environment (under influence of oxygen and light) or in body (chiefly by liver microsomes) n ______ more toxic than ______ but ______ breaks down more readily n Ultimately, both ______ and _______ are hydrolyzed at the ______ linkage n n Can yield, for example, alkyl phosphates and leaving groups, both of which are of relatively low toxicity They are either excreted or further transformed in the body before excretion

OR Esterases From http: //www. cpes. sussex. ac. uk/undergrad/coursenotes/ehh/lec 13/13. pdf

Mechanism of Action n Bind irreversibly to _____________ n Acetylcholine (Ach) is a neurotransmitter; it chemically conveys a nerve signal across a synapse, by binding with a receptor site n Acetylcholinesterase is an enzyme that breaksdown acetylcholine (so the nerve stops firing) n OP’s phosphorylate a _____ residue in the active site of acetylcholinesterase, and remains bound

Mechanism of Action n Inhibitors of acetyl cholinesterase prevent the normal cessation of nerve activity, the nerve keeps firing through prolonged Ach action n Inhibition of enzyme lasts _____ to ______ n Inhibition is overcome physiologically by the synthesis of new acetylcholinesterase

Treatment n ______________ (2 -PAM) n n Binds to glutamic acid residue in active site of acetylcholinesterase, next to the phosphorylated serine residue Causes dephosphorylation, thus reactivating acetylcholinesterase Relieves muscarinic and nicotinic effects of poisoning ______________ n n Administer—IV or intramuscularly Muscarinic antagonist to block effects of excess Ach (especially in the CNS where the danger lies in respiratory depression)

Toxic Effects n All due to prolongation of the action of Ach SLUD (Salivation, Lacrimation, Urination, Defecation) n Bradycardia n Confusion n Convulsions n Depression of respiratory center n CNS effects n

Environmental Fate n n n OP’s rapidly degraded by most animals not undergo biomagnification in terrestrial or aquatic food chains Not persistent in soils = hydrolysis, volatilization and metabolism by soil microorganisms/animals ensure rapid removal Limited persistence in surface waters and sediments due to rapid degradation and metabolism Susceptible to photodecomposition and hydrolysis in atmosphere HOWEVER…OP’s sometimes formulated to increase persistence—formulated as _______

HERBICIDES

Herbicides n Constitutes 51% of pesticide market n Herbicides can be classified into n n n Soil application—attacks via roots, often at seed stage Leaf-application—onto the growing plant Latter classification divided further n n Systemic—chemical enters plant via roots or leaves and translocates within (most modern herbicides) Contact—attacks/kills only at point of contact

Paraquat n Non-selective contact herbicide, used extensively in Asian countries n Poorly absorbed from the gut due to 2 positive charges

Toxicity—in plants Monocation Superoxide Radical Hydrogen Peroxide

Toxicity—in mammals n Lung is major target organ for toxicity n WHY?

ANSWER… n Shape of the paraquat molecule is perfect substrate for specific uptake systems present in Type II alveolar cells n n n ______/_____ transport system Distance between 2 charged nitrogen ends is critical for its selectivity There is _____ ___ in the lungs n Mechanism of toxicity of paraquat is via an oxidative process. generating oxygen radicals

Toxic Effects n Haemorrhage n _____ n Damage to type II alveolar cells n Fibrosis n Toxic effects are stow to develop (3 to 4 weeks) since paraquat is poorly absorbed from gut

Treatment n If ingested: _________ n Since it is poorly absorbed from gut, it gives us some time advantage n __________ n If in an emergency (say for example out in the fields), it is possible to eat dirt n n n Clay would best because it contains bentonile which is a negatively charged compound Binds to paraquat (which is positively charged) to form an insoluble complex which is excreted in the feces Reduce oxygen to level sufficient to maintain acceptable arterial tension of 40 – 50 mm. Hg n Reduces the formation of oxygen radicals

Metabolism n Toxic effects are uncommon in mammals because: n n They are rapidly hydrolysed by esterases in man and in the environment, hence there is no accumulation (unlike DDT) HOWEVER…insects do not have esterases, therefore chemicals are toxic to them

Agent Orange n When _______ and _______ are mixed together n Used extensively in Vietnam to defoliate jungle from hidden enemies n However…contamination by ____ n Resulted in much medical controversy on the exposure of “agent orange” to Vietnam veterans

Agent Orange 2, 4 -D (50%) 2, 4, 5 -T (50%) trace impurity 2, 3, 7, 8 -tetrachlorodibenzodioxin (2, 3, 7, 8 -TCDD) From http: //www. whoi. edu/science/B/people/mhahn/20

Pesticide resistance n Evolution of resistance is increasingly problematic for pest control in agriculture n For example, in 1970, there were no known weeds with herbicide resistance, while now there are over 273 herbicideresistant weeds known n Estimated that, across all kinds of agricultural pests and diseases, approximately 1000 species are resistant to at least one pesticide n Some pests are now resistant to multiple pesticides, with some being resistant to all pesticides that are legally approved for use on them

Pesticide resistance n Resistance increases because of: n High rates of pesticide use (intense selective pressure) n Changing nature of pesticides themselves n Early generation pesticides attacked multiple sites in their targets (more difficult for pests to develop resistance)…BUT… n Newer pesticides are often specific for one biochemical pathway (faster development of resistance)

Mechanisms of resistance n Mechanisms by which resistance develops varies according to pest species and/or to the class of chemical to which the pest is exposed. Resistance mechanisms include: n Better/more enzyme systems with an increased capacity to detoxify and excrete the pesticide once it has entered the pest's body (reducing exposure); n More effective retention/immobilization of toxins in fatty deposits (i. e. in insects)—DDT stored in inert fat (reduces exposure to sensitive tissues); n Decreased sensitivity of the target site that the pesticide acts upon, or; n Decreased penetration of the pesticide through the cuticle or epidermis of the pest species