CADDIS Volume 2: Sources, Stressors & Responses
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Authors: G.W. Suter II, S. Cormier, K. Schofield, C. Barbour, J. Diamond
Insecticides are chemicals used to control insects by killing them or preventing them from engaging in behaviors deemed undesirable or destructive. They are classified based on their structure and mode of action. Many insecticides act upon the nervous system of the insect (e.g., Cholinesterase (ChE) inhibition) while others act as growth regulators or endotoxins. Table 1 lists the major classes of insecticides and their modes of action. Understanding these modes of action can aid in the identification of a candidate cause—particularly when enzyme assays or similar tests are used in symptom identification of affected organisms.
Insecticides are commonly used in agricultural, public health, and industrial applications, as well as household and commercial uses (e.g., control of roaches and termites). The most commonly used insecticides are the organophosphates, pyrethroids and carbamates (Figure 1). The USDA (2001) reported that insecticides accounted for 12% of total pesticides applied to the surveyed crops. Corn and cotton account for the largest shares of insecticide use in the United States.
Insecticides are applied in various formulations and delivery systems (e.g., sprays, baits, slow-release diffusion; see Figure 2) that influence their transport and chemical transformation. Mobilization of insecticides can occur via runoff (either dissolved or sorbed to soil particles), atmospheric deposition (primarily spray drift), or sub-surface flow (Goring and Hamaker 1972, Moore and Ramamoorthy 1984). Soil erosion from high intensity agriculture, facilitates the transport of insecticides into waterbodies (Kreuger et al. 1999). Some insecticides are accumulated by aquatic organisms and transferred to their predators. Insecticides are designed to be lethal to insects, so they pose a particular risk to aquatic insects, but they also affect other aquatic invertebrates and fish.
|Insecticide Type||Mode of Action|
|Organochlorine||Most act on neurons by causing a sodium/potassium imbalance preventing normal transmission of nerve impulses, while some act on the GABA (γ-aminobutyric acid) receptor preventing chloride ions from entering the neurons causing a hyperexcitable state characterized by tremors and convulsions; usually broad-spectrum insecticides that have been taken out of use.|
|Organophosphate||Cause acetylcholinesterase (AChE) inhibition and accumulation of acetylcholine at neuromuscular junctions causing rapid twitching of voluntary muscles and eventually paralysis; broad-range insecticide, generally the most toxic of all pesticides to vertebrates.|
|Organosulfur||Exhibit ovicidal activity (i.e., they kill the egg stage); used only against mites with very low toxicity to other organisms.|
|Carbamates||Cause acetylcholinesterase (AChE) inhibition causing central nervous system effects (i.e. rapid twitching of voluntary muscles and eventually paralysis); very broad spectrum toxicity and highly toxic to fish.|
|Formamidines||Inhibit the enzyme monoamine oxidase that degrades neurotransmitters causing an accumulation of these compounds; affected insects become quiescent and die; used in the control of OP and carbamate-resistant pests.|
|Dinitrophenols||Act by uncoupling or inhibiting oxidative phosphorylation preventing the formation of adenosine triphosphate (ATP); All types have been withdrawn from use.|
|Organotins||Inhibit phosphorylation at the site of dinitrophenol uncoupling, preventing the formation of ATP; used extensively against mites on fruit trees, formerly used as an antifouling agent and molluscacide; very toxic to aquatic life.|
|Pyrethroids||Acts by keeping open the sodium channels in neuronal membranes affecting both the peripheral and central nervous systems causing a hyper-excitable state causing such symptoms as tremors, incoordination, hyperactivity and paralysis; effective against most agricultural insect pests; extremely toxic to fish.|
|Nicotinoids||Act on the central nervous system causing irreversible blockage of the postsynaptic nicotinergic acetylcholine receptors; used in the control of sucking insects, soil insects, whiteflies, termites, turf insects, and the Colorado potato beetle; have generally low toxicity to mammals, birds and fish.|
|Spinosyns||Acts by disrupting binding of acetylcholine in nicotinic acetylcholine receptors at the postsynaptic cell; effective against caterpillars, lepidopteran larvae, leaf miners, thrips, and termites; regarded for its high level of specificity.|
|Pyrazoles||Inhibits mitochondrial electron transport at the NADH-CoQ reductase site leading to disruption of ATP formation; effective against psylla, aphids, whitefly and thrips; results of testing on one type (fipronil) indicate no effects on the clams, oysters, or fish, with marginal effects on shrimp.|
|Pyridazinones||Interrupt mitochondrial electron transport at Site 1; mainly used as a miticide; display toxicity to aquatic arthropods and fish.|
|Quinazolines||Acts on the larval stages of most insect by inhibiting or blocking the synthesis of chitin in the exoskeleton; developing larvae exhibit rupture of the malformed cuticle or death by starvation; not registered in U.S.|
|Botanicals||Depending upon the type can have various effects:
Pyrethrum – affects both the central and peripheral nervous systems, stimulating nerve cells to produce repetitive discharges and eventually leading to paralysis; commonly used to control lice.
Nicotine – mimics acetylcholine (Ach) in the central nervous system ganglia, causing twitching, convulsions and death; used most to control aphids and caterpillars.
Rotenone – acts as a respiratory enzyme inhibitor; used as a piscicide that kills all fish at doses non-toxic to fish food organisms.
Limonene – affects the sensory nerves of the peripheral nervous system; used to control fleas, lice, mites, and ticks, while remaining virtually non-toxic to warm-blooded animals and only slightly toxic to fish.
Neem – reduces feeding and disrupts molting by inhibiting biosynthesis or metabolism of ecdysone, the juvenile molting hormone; commonly used against moth and butterfly larvae.
|Synergists/Activators||Inhibit cytochrome P-450 dependent polysubstrate monooxygenases (PSMOs) preventing the degradation of toxicants, enhancing the activity of insecticides when used in concert; synergists and activators are not in themselves considered toxic or insecticidal.|
|Antibiotics||Act by blocking the neurotransmitter GABA at the neuromuscular junction; feeding and egg laying stop shortly after exposure while death may take several days; most promising use of these materials is the control of spider mites, leafminers and other difficult to control greenhouse pests.|
|Fumigants||Act as narcotics that lodge in lipid-containing tissues inducing narcosis, sleep or unconsciousness; pest affected depends on particular compound.|
|Inorganics||Mode of action is dependent upon type of inorganic: may uncouple oxidative phosphorylation (arsenicals), inhibit enzymes involved in energy production, or act as desiccants; pest group depends on compound (e.g., sulfur for mites, boric acid for cockroaches).|
|Biorational||Grouped as biochemicals (hormones, enzymes, pheromones natural agents such as growth regulators) or microbials (viruses, bacteria, fungi, protozoa and nematodes), acting as either attractants, growth regulators or endotoxins; known for very low toxicity to non-target species.|
|Benzoylureas||Act as insect growth regulators by interfering with chitin synthesis; greatest value is in the control of caterpillars and beetle larvae but is also registered for gypsy moth and mushroom fly; some types are known for their impacts on invertebrates (reduced emergent species) and early life stages of sunfish (reduced weight) (Boyle et al. 1996).|
Insecticides should be listed as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for insecticides (Figure 3). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case.
The checklist below will help you identify key data and information useful for determining whether to include insecticides among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate insecticides as a candidate cause. For more information on specific entries, click on checklist headings or go to the When to List tab of this module.
Consider listing insecticides as a candidate cause based on the presence of the following sources and activities, site evidence, or biological effects:
- Agricultural runoff
- Irrigation return water
- Tree farms and orchards
- Forestry application
- Mosquito control
- Insecticide manufacturing and storage
- Insecticide mixing and transfer to application equipment
- Urban and suburban runoff
- Combined sewer overflows (CSOs)
- Wastewater treatment plant discharges
Site data for insecticides in water or sediment
Bioaccumulation of insecticides (e.g., in aquatic insects or fish tissue)
- Mortality or developmental effects, especially in aquatic insects (Kreutzweiser 1997)
- Catastrophic or mass drift of aquatic insects (Kreutzweiser and Sibley 1991; Beketov and Liess 2008)
- Reduced biological diversity (Relyea 2005), especially among aquatic insects
- Sudden, massive kills of aquatic life (e.g., fish kill in Mills River, NC)
- Fish exhibiting cough, yawn, fin flickering, S-and partial jerk, nudge and nip; difficulty in ventilation and aberrant behavior (Alkahem 1996)
- Elevated muscle and liver pyruvate levels in fish (Alkahem 1996)
- Decreased acetyl cholinesterase (AChE) activity in fish (Alkahem 1996)
Contributing, modifying, and related factors that are important contributors to the aquatic toxicity of insecticides are not identified. However, other stressors, such as low dissolved oxygen or high temperatures, may exacerbate the effects of insecticides.
Consider other causes with similar evidence, since other stressors may cause effcts that are the same as or similar to those caused by insecticides (Table 2).
|Aquatic insect or fish kills||Other toxics, low dissolved oxygen, low or high pH, high ammonia|
|Fish cough, yawn, jerk||Metal contamination|
|Difficulty in respiration||Low dissolved oxygen, high temperature|