Jump to main content or area navigation.

Contact Us

CADDIS Volume 2: Sources, Stressors & Responses


Sources and activities that suggest listing insecticides as a candidate cause

photo of stormwater runoff into a storm sewer
Figure 4. Stormwater runoff entering a storm sewer, which may discharge directly to a stream.
Courtesy of City of Murray, KY

Insecticides should be listed as a candidate cause if insecticide sources are present in a stream or watershed. You should consider both point and nonpoint sources when identifying sources of insecticides.

Point sources of insecticides include wastewater treatment facilities (which receive runoff during non-overflow conditions), combined sewer overflows (CSOs) during wet weather, manufacturing facilities, and spills and leaks of insecticides on farms or other areas where they are stored and handled in bulk quantities. This would include back-siphoning of insecticides into irrigation water wells not equipped with proper safety devices. Also, agricultural ditches that convey runoff or irrigation returns may act as point sources.

Nonpoint sources of insecticides are less spatially localized. Insecticides typically enter waterbodies with surface water runoff. This runoff can be from storms (Figure 4) or return water from irrigated or inundated fields, such as rice and cranberries (Figure 5). Sources may include farms, where insecticides are applied to agricultural crops; urban and suburban areas, where insecticides are applied to gardens and lawns; homes (e.g., deck and other outdoor wood treatment, lawns); commercial buildings (e.g., termite control); forestry land, if insecticides are applied to control forest pests; and locations where insecticides are manufactured, stored, (i.e., leaking storage tanks, washing from storage tanks), or mixed (i.e., washing of application equipment). Insecticides also enter waterbodies as a result of spray drift during application, particularly during aerial applications, forest or orchard spraying, or spraying near roadsides and wetlands to control mosquitoes. Table 3 lists insecticides commonly used with popular agricultural crops.

photo of an agricultural drainage ditch
Figure 5. Agricultural ditches can convey insecticides from fields to receiving streams.
Courtesy of USGS.

Nonpoint sources of insecticides are highly variable over time and space. Causes of this temporal variation include the intermittent timing of application, interaction of applications with weather patterns (e.g., precipitation for runoff, wind speed for drift), and low persistence of most currently used insecticides. Causes of spatial variation include land use patterns, soil properties, slope, and distance to waterbodies (Gilliom et al. 2006).

Insecticide concentrations in base flows increase with urban land use regardless of background land use (Sprague and Nowell 2008). The relative distribution of urbanized areas contributing nonpoint sources of insecticides (and other toxicants) within a watershed can be identified using the U.S. EPA's EnviroMapper for Water (zoom in to state and region of interest). Aqueous concentrations of insecticides may be found in state or tribal databases or federal data repositories: STORET, USGS-NAWQA Exit EPA Disclaimer , and EMAP.

Table 3. Examples of crops and common insecticides used (sources include UC Davis Extension Center and Iowa State University).
Crop Insecticides
Corn, sweet Permethrin (pyrethroid), Esfenvalerate (pyrethroid), Bacillus thuringiensis (BT—Biologicals), Diazinon (organophosphate), Methomyl (carbamate), Malathion (organophosphate),  pyrethrin (botanical), Carbaryl (N-methyl carbamate), Endosulfan (organochlorine)
Alfalfa Beta-cyfluthrin (pyrethyroid), Carbaryl (carbamate), Chlorpyrifos (organophosphate), Cyfluthrin (pyrethroid), Dimethoate (organophosphate), Gama-cyhalothrin (pyrethroid), Idoxacard (carboxylate), Methomyl (carbamate). Methyl Parathion (organophosphate), Permethrin (pyrethroid), Phosmet (organophosphate), Spinosad (fermentation product), Zeta-cypermethrin (pyrethroid)
Sorghum Beta-cyfluthrin (pyrethyroid), Carbaryl (carbamate), Chlorpyrifos (organophosphate), Deltamethrin (pyrethroid), Dimethoate (organophosphate), Esfenvalerate (pyrethroid), Gama- and Lamda-cyhalothrin (pyrethroid), Malathion (organophosphate), Methidathion (organophosphate), Methomyl (cyclodine), Spinosad (fermentation product), Zeta-cypermethrin (pyrethroid)
Sunflower Bacillus thuringiensis (bacterium), Beta-cyfluthrin (pyrethyroid), Carbaryl (carbamate), Chlorpyrifos (organophosphate), Deltamethrin (pyrethroid), Esfenvalerate (pyrethroid), Gama- and Lamda-cyhalothrin (pyrethroid), Methyl Parathion (organophosphate), Zeta-cypermethrin (pyrethroid)
Wheat Beta-cyfluthrin (pyrethyroid), Carbaryl (carbamate), Chlorpyrifos (organophosphate), Dimethoate (organophosphate), Endosulfan (chlorinated hydrocarbon), Gama- and Lamda-cyhalothrin (pyrethroid), Malathion (organophosphate), Methidathion (organophosphate), Methomyl (cyclodine), Methyl parathion (organophosphate), Spinosad (fermentation product), Zeta-cypermethrin (pyrethroid)
Grapes Sevin (carbaryl), Imidan (phosmet), Kelthane (dicofol), Guthion (azinphos methyl), Vendex (hexakis fenbutatin-oxide), Lanate (methomyl), Methoxychlor (methoxychlor), Provado (imidacloprid), Thiodan (endosulfan), Malathion, Neemix, Pyrethrins
Citrus Cygon 400 (dimethoate), Cythion 57% (malathion), Diazinon AG500 (organophosphate), Dibrom 8E, Dipel 2X, Imidan 50 WP, Lannate L, Lorsban 15 G, Metasystox-R, Parathion 4E, Thiodan 3E, Zolone 3EC
Cotton Acramite (bifenazate); Baythroid (cyfluthrin); Dimilin (diflubenzuron); Fulfill (pymetrozine); MSR (oxydemeton-methyl); Temik (aldicarb); Venom (dinotefuran); Zeal (etoxazole)
Soybeans Asana XL (esfenvalerate); Baythroid 2 (cyfluthrin); Cruiser 5FS (thiamethoxam); Dimethoate 4E (organophosphate); Gaucho 480 (imidacloprid); Lorsban 4E (chlorpyrifos); Mustang Max (pyrethroid); Nufos 4E (chlorpyrifos); Warrior (organophosphate)

Top of page

Site evidence that suggests listing insecticides as a candidate cause

One may list insecticides as a candidate cause when insecticides have been measured in water, sediment, or biota at the site of interest. Any measureable amount of an insecticide in water is suggestive of causation, because concentrations are so variable, but toxic levels are clearly indicative of causation (Table 4). Measurements in sediment are important because many organic insecticides are persistent and hydrophobic (most notably the legacy organochlorine insecticides such as DDT and chlordane).  For example, lindane (an organochlorine insecticide), can be found in some Great Lakes sediments 20 years after application to cherry orchards within the region. While bound to sediments, these insecticides may affect benthic biota that may, themselves, be impaired or may serve as part of the food chain transporting contaminants to fish or amphibians. Accumulation of insecticides in sediment and aquatic biota may occur even though concentrations in the water column may be below detection.

Because most modern insecticides are less persistent, they often require more frequent applications to control pests. As a result, exposures often occur in pulses of varying magnitude depending on the method and rate of application, intensity of runoff, and local land characteristics. Thus, the absence of insecticides in a stream sample may not represent the degree of exposure to insecticides. 

Water column or sediment toxicity testing can be used to identify potential effects of insecticides. If a water or sediment sample is toxic, a toxicity identification evaluation (TIE) can be used to determine the compound or group of compounds causing the problem (Norberg-King et al. 2005). For example, a TIE found that the toxicity of an urban creek in Sacramento, California, was due to diazinon and chlorpyrifos (Miller et al. 2001).

Table 4. Acute and chronic toxicity values for different pesticides, expressed in μg/L, for fish and invertebrates (from Office of Pesticide Programs’ Aquatic Life Benchmarks, updated April 2009).
Pesticide Fish Invertebrates
  Acute Chronic Acute Chronic
Acephate 416,000 5,760 550 150
Acrolein 7 11.4 <15.5 7.1
Aldicarb 26 0.46 10 1
Azinphos methyl 0.18 0.055 0.08 0.036
Carbaryl 110 6.8 0.85 0.5
Carbofuran 44 5.7 1.115 0.75
Chlorpyrifos 0.9 0.57 0.05 0.04
Diazinon 45 <0.55 0.105 0.17
Dicrotophos 3,150 - 6.35 0.99
Dimethoate 3,100 430 21.5 0.5
Disulfoton 19.5 4 1.95 0.01
Endosulfan 0.42 0.11 2.9 0.07
Ethoprop 150 24 22 800
Fenitrothion 860 46 1.15 0.087
Pesticide Fish Invertebrates
  Acute Chronic Acute Chronic
Fenthion 415.0 7.5 2.60 0.013
Imidacloprid >41,500 1,200 35 1.05
Malathion 0.295 0.014 0.005 0.000026
Methidathion 1.1 6.1 1.5 0.66
Methomyl 160 12 2.5 0.7
Naled 46 2.9 - 0.045
Oxydemeton methyl 365 5 95 46
Permethrin 0.395 0.0515 0.0106 0.0014
Phorate 1.175 0.34 0.3 0.21
Phosmet 35 3.2 1.0000 0.8
Profenofos 7.05 2 0.465 0.2
Resmethrin 0.14 0.32 1.550 -
Terbufos 0.385 0.64 0.1 0.03
Tribufos 122.5 3.5 13.5 1.56

The toxicity of insecticides can be influenced by various factors in an aquatic environment. These include local water quality characteristics that affect bioavailability (e.g., dissolved organic carbon, suspended sediment and temperature) and interactions between insecticides and other pollutants. These mechanisms are affected in various ways by temperature. Adsorption of hydrophobic insecticides to particulate organic carbon may decrease with increasing temperature (Lyman 1990). The breakdown and transformation of many insecticides slows at lower temperatures, while the toxicity of some insecticides increases with increased temperature (Osterauer and Kohler 2008).

photo of farm machine applying insecticide to a crop field
Figure 6. Insecticide application on a large farm.
Courtesy of U.S. EPA, Region 9.

Mixtures of insecticides may have additive, synergistic, or antagonistic interactions. Mixtures are concentration additive if the constituent chemicals have the same mode of action, so that their toxicity-normalized concentrations can be added to estimate the effective concentration. For example, mixtures of organophosphate insecticides, such as diazinon and chlorpyrifos, have concentration-additive effects. Synergistic effects occur when mixing two insecticides provides a greater response than the equivalent total concentration of either insecticide alone. The presence of an insecticide and another stressor with a different mode of action may result in synergistic effects. Synergistic effects are particularly challenging to identify in environmental monitoring. Antagonistic effects are those exhibited by a mixture of insecticides that is less toxic than the insecticides individually.

Mixtures of insecticides with common contaminants such as metals or ammonia also may produce joint toxic effects. Forget et al. (1999) reported studies of the joint toxicity of nine mixtures of a metal (arsenic, copper, or cadmium) and an insecticide (carbofuran, dichlorvos, or malathion) that resulted in synergistic lethal effects in all cases. Because of the persistence of heavy metals and metalloids and their high toxicity, these compounds and their associations should be considered when evaluating potential insecticide effects.

An example of apparent antagonistic effects is from a study by Bailey et al. (2001), that discussed the relationship between diazinon, an organophosphorus insecticide, and ammonia. It was found that the toxicity of the mixture was 30% less than the toxicity of each compound individually. Because these two contaminants may co-occur, their relationship with respect to joint toxicity may be of interest particularly when interpreting the results of effluent tests and subsequent toxicity identification evaluations.

Many pesticides are specific to certain types of pests (e.g., mites, snails, or insects) that occur in different types of environments (Table 5). However, pesticides that are intended to control one type of organism may have toxic effects on other types of aquatic biota.  For example, an insecticide would be expected to have its greatest effect on insects, but smaller effects might occur in mollusks. Hence, information concerning the relative sensitivity of taxa to an insecticide and the relative magnitude of effects at the impaired site can help determine the cause.

Top of page

Biological effects that suggest listing insecticides as a candidate cause

Insecticides should be considered as a candidate cause when the impairment involves gross pathologies or community changes that are indicative of adverse insecticide effects, such as:

  • Sudden, massive kills of aquatic life (e.g., fish kill in Mills River, NC Exit EPA Disclaimer)
  • Catastrophic drift of insects (Kreutzweiser and Sibley 1991, Beketov and Liess 2008)
  • Reduced biological diversity (Relyea 2005)
  • Fish exhibiting cough, yawn, fin flickering, S-and partial jerk, nudge and nip, difficulty in respiration, and aberrant behavior (Alkahem 1996)

Site evidence that supports excluding insecticides as a candidate cause

There are no site observations that specifically provide evidence of the absence of insecticides. Step 2 of the Step-by-Step guide and the Tips for Listing Candidate Causes provide general advice for excluding candidate causes from your initial list. Exclusion of insecticides as a candidate cause should be based upon high quality in-stream measurements and the absence of evidence of sources or activities that may result in the input of insecticides to the stream.

Table 5. Summary of the active ingredient in common insecticides and where they are applied (from Lawn & Garden Insecticides Exit EPA Disclaimer).
Insecticide Active Ingredient Common Product Names General Application Specific Places of Application Target Species

Sevin garden insecticides. Slug, Snail and Insect Killer Bait. Urban/Suburban Ornamental ground cover, roses, shrubs, dogs and cats Mexican bean beetles, armyworms, leafhoppers, grass hoppers, tomato fruitworms, flea beetles, corn earworms, fire ants, ticks and fleas
Malathion 50% Malathion Spray. Home Orchard Spray Urban/Suburban Home outdoor, lawn, ornamental outdoor residential aphids, scales, beetles, and lepidopteran larvae
Malathion 50% E.C. Agriculture

many vegetables and fruits, ornamentals, citrus and certain animals aphids, scale, lepidopteran larvae, Japanese beetle, horn flies, lice, ticks, fleas, bed bugs, thrips, leafminers, spider mites, and mosquitoes
Boric Acid Boric Acid Roach Powder Urban/Suburban Most indoor areas including homes, restaurants, schools, offices, warehouses, vehicles, boats, etc. cockroaches, ants, silverfish
Bacillus thuringiensis in liquid form. Thuricide HPC Agriculture Many vegetables, citrus, ornamentals and tobacco Controls only lepidopteran larvae including. cabbage looper, orange dog, tobacco hornworm, imported cabbageworm, rindworm
Permethrin Pet & Livestock Pest Control Spray; Home Pest Control Spray Agriculture, Urban, Suburban Cattle, goats, sheep, hogs, horses, dogs, Indoor surfaces, outdoors, and ornamental flower gardens fleas, flies, lice, bed bugs, ticks, whitefly, aphids, lacebugs, leafminers, japanese beetles, ants, thrips, armyworms, palmetto bugs, scorpions, millipedes, carpet beetles, centipedes, pillbugs, silverfish, spiders, crickets, weevils, rust red flour beetles, meal worms, mites
Azadirachtin Neemix 4.5 Forestry Forests balsam fir sawfly

Top of page

Jump to main content.