Aldicarb rates

Since 1945 the use of synthetic pesticides in the United States has grown 33-fold. The amounts of herbicides, insecticides, and fungicides used have changed with time due, in large part, to changes in agricultural practices and cosmetic standards (14, 15). At the same time, the toxicity and biological effectiveness of these pesticides have increased at least 10-fold (15). For example, in 1945 DDT was applied at a rate of about 2 kg/ha. With the more potent insecticides available now, similar effective insect control is achieved with pyrethroids and aldicarb applied at 0.1 kg/ha and 0.05 kg/ha, respectively. 

In aerobic soils, aldicarb degraded rapidly (half-life = 7 d) releasing carbon dioxide. Mineralization half-lives for the incubation of aldicarb in aerobic and anaerobic soils were 20-361 and 223-1,130 d, respectively. At an application rate of 20 ppm, the half-lives for aldicarb in clay, silty clay loam, and fine sandy loam were 9, 7, and 12 d, respectively (Coppedge et al, 1967). Other soil metabolites may include acids, amides, and alcohols (Hartley and Kidd, 1987). 

Given, C.J. andDierberg, F.E. Effect of pH on the rate of aldicarb hydrolysis. Bull Environ. Contam. Toxicol, 34(5) 627-633, 1985. 

Smelt, J.H., Leistra, M., Houx, N.W.H., and Dekker, A. Conversion rates of aldicarb and its oxidation prodncts in soils. III. 

Base hydrolysis kinetic data are reported for ppb solutions of carbofuran,3-OH carbofuran, methomyl and oxamyl. The results are compared with those reported previously for aldicarb, aldlcarb sulfoxide, and aldicarb sulfone. Second order reaction rate constants, k, have been calculated and range from 169 liter mln mole for oxamyl to 1.15 liter mln mole for aldicarb. The order for rate of base hydrolysis is as follows oxamyl >3-hydroxycarbofuran >aldicarb sulfone v- carbofuran >aldicarb sulfoxide > methomyl -v aldicarb. The activation energy for the base hydrolysis of carbofuran was measured to be 15.1 +0.1 kcal mole , and is similar to the value previously reported for aldicarb sulfone. Rapid detoxification of aldicarb, a representative oxime carbamate pesticide, by in situ hydrolysis on reactive ion exchange beds is reported. 

Materials. Aldicarb standards were obtained from the United States Environmental Protection Agency (USEPA), Quality Assurance Section and from Union Carbide Corporation. Crystalline samples of carbofuran and 3-hydroxycarbofuran were supplied by the Agricultural Chemical Group of FMC Corporation. Reference standards of methomyl (99% pure) and oxamyl (99% pure) were obtained from USEPA. HPLC grade methanol was purchased from Burdick and Jackson, Inc. Methylene chloride used for bulk extractions of the carbamate pesticides in solution was recovered, distilled and reused. Analytical reagent grade chemicals and solvents were used in all experiments. Doubly distilled deionized water was used for solution rate studies. Deionized distilled water (DDW) was used for dilutions in reactive ion exchange experim ts., , ... 

Figure 10. Reaction chromatograms for A, Amstel river water and B, Amstel river water fortified with 3 ng of aldicarb (peak 1) 3 ng of methomyl (peak 2) 5 ng of propoxur (peak 3) 5 ng of carbaryl (peak 4) and 10 ng of methiocarb (peak 5). Conditions 150-mm X 4.6-mm i.d. column packed with Spherisorb ODS mobile phase of 50% water and 50% methanol (v/v) at a flow rate of 1.0 mL/min 60-mm X 4.6-mm i.d. reactor column packed with Aminex A-28 reaction temperature of 100 °C OF A reagent flow rate of 30 pL/min detection with Perkin-Elmer Model 204A fluorescence spectrometer excitation wavelength of 340 nm emission wavelength of 455 nm. (Reproduced with permission from reference 46. Copyright 1983 Elsevier Scientific Publishers.)...

Aldicarb is highly water soluble and soil application of this insecticide has the potential to result in runoff or leaching of the insecticide or active metabolites and contamination of surface or groundwater. Aldicarb is acutely toxic to bees, birds, and fish. Species-specific rates of bioactivation may influence the sensitivity of a particular organism to this insecticide. 

According to recent estimates, some 35 chemical pesticides are currently used to control various pests on cotton (1). There is considerable variation across the USA cotton belt in seasonal pesticide application rates, ranging from a high average of 7.4 pounds per harvested acre in Florida to a low of 0.3 in Oklahoma. Currently, methyl parathion (21.0%), azinphosmethyl (13.0%), various pyrethroids (8.0%), chlordimeform (7.4%), propargite (6.8%), and aldicarb (6.8%) account for 63% of all insecticides applied to cotton (1). 

The next three compounds are carbamates. Eserine is important because it is a very strong inhibitor of all cholinesterases and is used to verify that an esterase is a cholinesterase. Carbaryl also has a very high affinity to some cholinesterases, but a very low affinity to others. It is an important insecticide. Aldicarb was made to resemble acetylcholine. In spite of this, its affinity is very low (high Kd), but due to its high reaction rate (k+2 value), aldicarb is very toxic. 

These equations predict that the concentration of aldicarb is monotonically decreasing, while the sulfone and sulfoxide forms change in concentration depending on the value of the rate coefficients. 

Table V. Degradation Rates for Aldicarb and its Metabolites Measured...

In contrast to measuring 0C or values, the measurement of pesticide degradation half-lives (t. / ) in soils is a much more difficult and time-consuming task. Hence, the spatial variations in t. trs could not be assessed in as much detail as we did with OC and K. . Metolachlor and aldicarb TTR half-lives were measured in a selected number of soil samples collected from the Georgia field site. Soil samples were selected to represent the three major soil series present at the site (Clarendon, Ardilla, and Tifton) and the four major morphologic soil horizons (0-20, 25-46, 48-63, and 94-107 cm) within the crop root zone. Soil samples collected from 4 depths at 10 sites were used to characterize aldicarb TTR degradation rates. Metolachlor half-lives were measured in soils taken from four depths at one site for each soil series and also in samples collected at two depths (0-20 cm and 94-107 cm) at 6 sites. 

If aldicarb residues leach into the saturated zone, or groundwater, degradation of the residues continues, mainly by chemical hydrolysis in cold areas and by both chemical hydrolysis and microbial degradation in warm areas. Factors which tend to increase the degradation rate are high temperature and high pH. Microbial populations may provide a significant contribution to... 

The driving force for movement of aldicarb residues is the movement of water. Therefore, the movement of water must be defined before the movement of aldicarb residues can be quantified. Factors influencing the movement of water include rainfall, irrigation, evapotranspiration and soil field capacity in the unsaturated zone. The amount of organic matter determines the rate of aldicarb movement relative to water movement in both the unsaturated and saturated zones. 

TABLE II. Results of Laboratory Studies Measuring Degradation Rates of Aldicarb Residues in Saturated Zone Samples... 

Figure 2(C). Simulated effect of dehydration rate on the leaching of aldicarb residues under Florida Ridge conditions.

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