Pesticides Uses, Classes and Environmental Toxicology

 Background  Mechanisms of Action  Agricultural Practices  Pesticide Environmental Risk Assessment

Background  Pesticides controlled by EPA through FIFRA (Federal Insecticide, Fungicide and Rodenticide Act  The perfect pesticide kills it target organism, but harms nothing else, then disappears quickly  A non-perfect example  DDT  PCBs—similar toxicological characteristics as pesticides

A. Herbicides  Weeds causes an $11 billion dollar loss annually  Herbicides highly toxic but decompose quickly, must be reapplied often  Use has increased in recent years  Pesticide have received most benefit from advances in biotechnology

B. Insecticides  Definition – agents of chemical or biological origin that control insects  Includes Acaricides (kill spiders and mites)  Insects et al., cause $4 billion loss annually  10, 000 species are crop-eating (field, storage)  700 major species do most of the damage  Use has decreased over time, because insecticides have become more toxic and more specific  Have benefited from some biotechnological advances  Bacterial genes coding for insecticidal proteins into crop plants  insects bite and die!

C. Rodenticides  Kill primarily rats, mice, moles  Secondarily prairie dogs, squirrels (esp. flying squirrels)  Cause $4 billion in losses annually

D. Fungicides  $4 billion per year in damages on potatoes alone  Kills rust, smut, blight  An important after-the- fact treatment  Historically important problem (mid 1800’s - potato blight  famine  1 million deaths) Soybean rust Looking for non-blighted potatoes

Mechanisms of Action  Important when dealing with the issue of target vs. non-target organisms  Most pesticides have become more specific towards target species

A. Herbicides  Hormonal  Phenoxy acids, e.g. 2,4-D  mimic plant growth hormone, causes weeds to grow too fast and “burn out”  Photosynthetic inhibitors  diquat, atrazine  cause plants to starve

B. Insecticides  Growth Regulators  Dimlin  break down the enzyme that breaks down chitin, prevents insect from molting  Acetylcholinesterase inhibitors (e.g., organophosphates (malathion, diazinon, and chlorpyrifos), carbamates, Raid)—inhibit enzyme which enables nerves to stop firing  Nerve Toxins (e.g., pyrethroids, a chrysanthemum derivative, organochlorines, e.g., DDT)—very acutely toxic to arthropods and fish, but not mammals (but may have long-term effects)

C. Rodenticides  Ulceratives (e.g., warfarin)—cause internal bleeding  Supposedly safe for children and pets, because bait contains an emetic (causes vomiting); rodents cannot vomit  die

D. Fungicides  Includes chlorothalonil (Bravo®, Echo®), pyraclostrobin (Headline®), azoxystrobin (Quadrise®)  Many previous compounds have been banned (e.g., creosote, pentachlorophenol) but are still around on railroad tracks, utility poles

 Net Benefits from Fungicides = $ 73.4 million  For every $1.00 Arkansas growers spend on fungicides and their application, they reap $3.28 in benefits.  Arkansas growers apply 402,000 pounds of fungicides on 1,361,000 acres Arkansas - The Value of Fungicides

E. Resistance  A resistant few organisms, or species, will live and reproduce; these can be resistant to the pesticide  Combated using binary pesticides (a mixture of two pesticides having different mechanisms of toxicity e.g an acetylcholinesterase and

Agricultural Practices

A. Hydrological alterations  Tile drain—buried in ground, with perforations to draw water down  Since pesticides are water soluble, this also causes increased movement of pesticides into ground water  Some pesticides will also be introduced to streams, rivers, lakes via runoff from agricultural areas

B. Tillage Practices  By repeated plowing (tillage), soil is broken down into smaller particles  Easier to farm (buries weeds seed bank) but increases soil transport by wind erosion and runoff  “No till”—a practice in which cover crop residue is left in place  This reduces soil loss, but increased use of herbicide is necessary

C. Aerial Application  Very difficult to avoid drift or overspray  Even under correct conditions (low wind, accurate application) trailing vortices generated by wings can disperse pesticides to untargeted areas

Pesticide Risk Assessment  Usually takes 6-10 years to go through the process of getting a pesticide registered  Cost approximately $1,000,000 /year

Properties Assessed  Bioaccumulation—particularly important for fat soluble compounds  Degradation—what amount is left over time  Toxicity—acute, chronic, aquatic, terrestrial  Carcinogenicity—to fish, mice, rats (surrogates for human toxicity)

B. Tiered testing approach  Tier 1—1-2 years of acute tests (e.g., using oral doses to rats) to determine potential human toxicity  Tier 2—2-4 years of chronic tests using mammals, birds, fish, invertebrates  Tier 3—about 2 years of simulated field tests using micro- or mesocosms

What do you do with the empty containers?

Human Exposure Which is worse? Human duster, circa 1890