Unit 14

• Penetration of insecticides through the insect cuticle


  • The wax layer of the insect cuticle protects from foreign bodies or harmful substances
  • Most of the chemical pesticides are soluble in wax allowing it to spread in the insect body
  • When the substance is absorbed inside the body, it gets transported through the circulatory system until it reaches it target sites

  
• Carrier efficiency


  • The degree to which a solvent induces rapid penetration of an insecticide
• Penetration resistance


  • Modifications in the cuticle that will eventually slow down the penetration of insecticide molecules within insects' body
  • Mechanisms: cuticle thickening, altering of cuticle composition
• The volatility of the solvent and the solubility of insecticide in solvent are also contributory factors to the rate of penetration
• Carrier efficiency depends on certain physical properties of a solvents
• Certain solvents increase the rate of penetration of contact insecticides through the insect cuticle

  
• Residue


  The spectrum of compounds to be analyzed which may include the parent compound, metabolites, isomers, reaction products and/or degradants
• Pesticide residue


  Any specified substance in food, agricultural commodities, or animal feeding resulting from the use of a pesticide. The term includes any derivatives of a pesticide, such as conversation products, metabolites, reaction products, and impurities considered to be of toxicological significance

  
• Maximum residue limit (MRL)


  Maximum concentration of a pesticide residue (expressed as mg/Kg) by either Codex Alimentarius Commission or national authority to be legally permitted in or on food commodities and animal feeds. MRLs are based on GAP data and foods derived from commodities that comply with the respective MRLs are intended to be toxicologically acceptable

  
• Residue


  Refers to the chemicals found regardless of the place found with the implication by time lapse or alteration of both
• Deposit


  Limited to the insecticidal chemical as initially laid down

  
• Factors affecting chemical insecticide residue decomposition and disappearance


  • Biotic factor: Enzymatic degradation, etc.
  • Abiotic factor: evaporation, weathering etc.

  
• Dissipation


  Initial phase of fast disappearance
• Persistence


  Subsequent phase of slow decrease in the amount of residue
• Factors affecting movement of insecticides in terrestrial plants


  • Method of application
  • Frequency of application
  • Processing (e.g., formulation)
  • Sizes (e.g., chemical/ soil/ plants)

  
• Factors affecting absorption of pesticides by target and nontarget organisms


  • Chemical nature (e.g., synthetic- more persistent)
  • Polar and non-polar characteristics of chemical and the type of cuticle of plants (e.g., chemicals with increased polarity usually penetrate best)

  
• Methods in removing pesticide residue


  • When in the surface: Washing, Brushing, Scraping, Peeling
  • When inside plant tissues: Degradation, Metabolism, Conjugation
• Methods in removing pesticide residue based on pesticide half-life


  • Reaction (e.g., chemical reaction)
  • Dissipation of residue
• Oxygenation is the most frequent first step in the bio-transformation of pesticides and other organic xenobiotics. Many of these reactions are mediated by oxidative enzymes, e.g., cytochrome P450s, peroxidases, and polyphenol oxidases
• Pesticide conjugates


  Products formed by the reaction of pesticides and their metabolites with endogenous chemicals (glucose, glucuronic acid, sulfate, glutathione, amino acid, etc.)
• Factors affecting movement of insecticides in the soil


  • Absorption by the soil particles as well as organic matter in the soil
  • Leaching and washing off by water
  • Evaporation into the air (including mechanical transport by water vapor)- evapotranspiration
  • Degradation/ activation by soil microorganisms
  • Physiological decomposition or activation catalyzed by soil conditions or soil constituents
  • Photodecomposition
  • Translocation through biological systems

  
• Other factors affecting movement of insecticides in the soil


  • Soil type
  • Soil moisture
  • Temperature
  • Mode of cultivation
  • Cover crop practices
  • Application of insecticides
  • Formulation of insecticides
• Presence of organic matter content can absorb residues
• Soil type and soil moisture could determine the depth of penetration of insecticide residue; soil with high silt and clay composition has less or sometimes no migration compared to sand

  
• Insecticide persist in acidic soil than alkaline

  
• Mineral content influences the fate of insecticide (catalyzing decomposition or influencing sorption behaviors)

  
• High disappearance in high temperature
• Shading (ex. cover crops) increase persistence of insecticide by lowering the evaporation rate in the soil

  
• Cultivation may hide or expose insecticide
• Movement of air also affects the disappearance from the soil (cultivation aeration)
• Formulations- granules persist longer than emulsions; emulsions persist longer than miscible liquids and wettable powder
• Movement of residue in the environment


  • Water mediated transport: run-off, percolation, precipitation, eluted (removed) because of the solubility of compounds
  • Air mediated transport: volatilization, evaporated with water vapor, transport with water particles and dusts
• Environmental toxicologist asks whether the disappearance of the insecticides means actual degradation of the hazardous chemical or whether it is actually a signal that the insecticide has been translocated for, say, bioconcentration into some ecosystem or converted into dangerous chemicals

  
• Factors affecting the speed of degradation


  • Chemical nature
  • Physical factors (pH, temperature, humidity etc.)
  • Biological factors (presence or absence of potential microorganisms)
• Microorganisms involved in insecticide metabolism


  • Actinomycetes
  • Fungi
  • Bacteria
• Acute toxicity


  The short term, direct effect of chemicals to animals based on the LD50 of chemicals (e.g., abdominal pain, dizziness, headaches, nausea, vomiting, as well as skin and eye problems)
• Chronic toxicity


  The long term toxicity of chemicals due to the buildup of pesticides by environmental concentrations (e.g., damage to the reproductive system, the liver, the brain)
• Bioaccumulation


  Accumulation of persistent insecticides in various biological systems at levels much higher than their surroundings

  
• Biomagnification


  Increase of the concentration of chemical residues in the body of animals as it moves to the top of the food chain
• Schematic diagram of biological transfer of pesticide residues (adapted from Matsumura, 1975)