Environmental Fate and Toxicology of Dimethoate

Abstract

The insecticide dimethoate, an organophosphate, was first introduced in 1962 for broad spectrum control of a wide range of insects including mites, flies, aphids, and plant hoppers. It inhibits AChE activity, resulting in nerve damage, which may lead to death. It is considered highly toxic to insects although dimethoate resistance has been observed. Dimethoate has both a low vapor pressure (0.247 mPa) and Henry's law constant (l.42x10(-6) Pa m3/mol), thus volatilization is not a major route of dissipation from either water or moist soils. Photolysis is considered a minor dissipation pathway. However, studies have shown that in the presence of a catalyst, the rate of photolysis does increase. The insecticide has high water solubility (39,800 mg/L) and under alkaline conditions, hydrolysis predominates representing a major degradation pathway. It has a low soil sorption capacity (Koc=20) which varies by soil type and organic matter content. Dimethoate is degraded by microbes under anaerobic conditions and bacterial species have been identified that are capable of using dimethoate as a carbon source. Although many intermediate by-products have been identified by abiotic and biotic processes, the major degradation product is omethoate. Dimethoate has been found to adversely impact many organisms. In plants, photosynthesis and growth are highly impacted, whereas birds exhibit inhibition in brain enzyme activity, thus sublethal effects are apparent. Furthermore, aquatic organisms are expected to be highly impacted via direct exposure, often displaying changes in swimming behavior. Toxicity results include inhibition in growth and more importantly, inhibition of acetylcholinesterase activity.

Isolation and Characterization of Microorganisms that Degrade Dimethoate

“…The half-life of dimethoate in clay loam soil was determined to be 10 and 5 days at 10°C and 20°C, respectively. This confirmed a faster decrease in the pesticide due to the increased temperature [9]. HPLC analysis was carried out to check degradation.…”

“…Organism SS3 c showed the best degradation potential and HPLC (High Performance Liquid Chromatography) analysis was carried out for the sample degraded by the organism. Dimethoate adsorption was measured on eight soil types (pH from 8.0-8.45; OM from 0.73-2.95%; clay content from 5.9-14.9%) in which resulting isotherms followed an L-shape [9]. Some pesticides hydrolyze very rapidly.…”

Reduction of Pesticide Toxicity Under Field‐Relevant Conditions? The Interaction of Titanium Dioxide Nanoparticles, Ultraviolet, and Natural Organic Matter

“…The 96‐h EC50s of dimethoate that indicate higher toxicity at test series PEST + nTiO 2 than at test series PEST despite more efficient degradation (Table 4) could, in addition, be induced by the formation of metabolites that are more toxic than the parent substance (Evgenidou et al 2006; Farner Budarz et al 2017). It was shown that during photocatalytic degradation with nTiO 2 , dimethoate is degraded into 9 by‐products (Evgenidou et al 2006), whereas the major by‐product, omethoate (Van Scoy et al 2016), is 90‐fold more toxic than the mother compound (Pesticide Properties Database; Lewis et al 2016). Likewise, the photocatalysis (UV = 5.8 W/m 2 ; TiO 2 = 120 mg/L) of malathion sharply increases its toxicity toward Vibrio fischeri , whereas no increase in toxicity was observed when treating malathion with UV alone (Li et al 2019).…”

Decomposition of Dimethoate and Omethoate in Aqueous Solutions – Half-Life, Neurotoxicity and Mechanism of Hydrolysis

“…Due to dimethoate's high water solubility and low soil persistence, its potential to runoff into surface waters and leaching into groundwater is high [15]. The most important degradation pathways of dimethoate in the environment are hydrolysis, photolysis and microbiological degradation [15].…”

“…Due to dimethoate's high water solubility and low soil persistence, its potential to runoff into surface waters and leaching into groundwater is high [15]. The most important degradation pathways of dimethoate in the environment are hydrolysis, photolysis and microbiological degradation [15]. The photocatalytic oxidation and microbial metabolism of dimethoate often have omethoate as the final product, which is not desirable due to its extreme toxicity.…”