The hydrolysis of herbicidal phenoxyalkanoic esters to phenoxyalkanoic acids in Saskatchewan soils

The hydrolysis of the iso-propyl, n-butyl-and iso-octyl esters of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), the n-bytyl ester of 2,4-dichlorophenoxybutyric acid (2,4-DB) and the iso-octyl ester of 2,4-dichlorophenoxypropionic acid (2,4-DP) was studied in four prairie soils of differing textures and pH at 25±1°C. The esters were analysed using gas chromatography. After 24 h in soils at wilting point moisture, and above, less than 20% of the applied iso-propyl and n-butyl esters could be recovered from one soil type and none from the remaining three. Loss of the iso-cotyl esters was slower; however, no trace of the 2,4,5-T and 2,4-DP esters was observed in any of the moist soils after 48 and 72 h respectively. In all cases loss of all esters from air-dried soils minimal. The phenoxyalkanoic acid hydrolysis products were recovered from all soil types, treated with the various esters and identified using thin-layer chromatography.     

The physical and chemical properties of the herbicide cinmethylin (SD 95481)

Cinmethylin (SD 95481), is a novel herbicide developed for the selective pre-emergence control of many annual grass weeds in a wide range of temperate and tropical crops. Representing new herbicide chemistry, cinmethylin is in the cineole family. Cinmethylin is a mobile colourless liquid with a boiling point of 313°C under an inert atmosphere at atmospheric pressure. It has a density of 1015 kg m^?3 and a viscosity of 70–90 mPa s, both at 20°C. It is miscible in all proportions with most organic solvents but has a low solubility, 63 mg litre^?1, in water. It has a vapour pressure of 10.2 mPa (20°C) and the vapour pressure/temperature relationship is given by log_e P(Pa)=28.9–9816/T (K). The n-octanol/water partition coefficient is 6850 and soil organic matter/water sorption coefficient (K_om) ranges between 165 and 235 over the three types of soil used in these studies. Cinmethylin is stable in water over the pH range 3–11. Solutions of cinmethylin in water or solvents are reasonably stable to sunlight, though thin films on a quartz surface photooxidise mainly to an ester within 24 h. This rate can be reduced by the addition of photostabilisers or by sorption onto soil surfaces. In an inert atmosphere cinmethylin is stable to high temperatures, though, in air, oxidation occurs at temperatures above 100°C to give the same product as by photodecay.     

Long- and short-term effects of vapour of the herbicide 2,4-D butyl on the growth of tomato plants

An airflow system has been used to expose tomato plants to a range of concentrations of vapour of the herbicide 2,4-D butyl, from 5 to 50 ng l^?1. Experiments carried out at an air temperature of 20°C indicate that only short periods (less than 2.5 h) are required to produce symptoms of phytotoxicity at concentrations less than 5 ng l^?1, or approximately 2.5% of the saturated vapour pressure of the herbicide. A 5-h period of exposure to approximately 5 ng l^?1 reduced the dry weight and dry matter content of the tomato plants after 7 weeks by 18% and 9%, respectively, compared with the control. Phytotoxicity symptoms were shown by the plants in proportion to the vapour concentration during the period of exposure to herbicide. Rates of photosynthesis of treated plants had begun to decline within an hour of the commencement of exposure, slightly after leaf movement was first observed.     

Some physicochemical factors influencing foliar uptake enhancement of glyphosatemono(isopropylammonium) by polyoxyethylene surfactants

Structure-concentration–foliar uptake enhancement relationships between commercial polyoxyethylene primary aliphatic alcohol (A), nonylphenol (NP), primary aliphatic amine (AM) surfactants and the herbicide glyphosatemono(isopropylammonium) were studied in experiments with wheat (Triticum aestivum L.) and field bean (Vicia faba L.) plants growing under controlled-environment conditions. Candidate surfactants had mean molar ethylene oxide (EO) contents ranging from 5 to 20 and were added at concentrations varying from 0·2 to 10 g litre^?-1 to [¹⁴C]glyphosate formulations in acetone–water. Rates and total amounts of herbicide uptake from c. 0·2–μl droplet applications of formulations to leaves were influenced by surfactant EO content, surfactant hydrophobe composition, surfactant concentration, glyphosate concentration and plant species, in a complex manner. Surfactant effects were most pronounced at 0·5 g acid equivalent (a.e.) glyphosate litre^?-1 where, for both target species, surfactants of high EO content (15–20) were most effective at enhancing herbicide uptake: surfactants of lower EO content (5–10) frequently reduced, or failed to improve, glyphosate absorption. Whereas, at optimal EO content, AM surfactants caused greatest uptake enhancement on wheat, A surfactants gave the best overall performance on field bean; NP surfactants were generally the least efficient class of adjuvants on both species. Threshold concentrations of surfactants needed to increase glyphosate uptake were much higher in field bean than wheat (c. 2 g litre^?-1 and < 1 g litre^?-1, respectively); less herbicide was taken up by both species at high AM surfactant concentrations. At 5 and 10 g a.e. glyphosate litre^?-1, there were substantial increases in herbicide absorption and surfactant addition could cause effects on uptake that were different from those observed at lower herbicide doses. In particular, the influence of EO content on glyphosate uptake was now much less marked in both species, especially with AM surfactants. The fundamental importance of glyphosate concentration for its uptake was further emphasised by experiments using formulations with constant a.i./surfactant weight ratios. Any increased foliar penetration resulting from inclusion of surfactants in 0·5 g litre^?-1 [¹⁴C]glyphosate formulations gave concomitant increases in the amounts of radiolabel that were translocated away from the site of application. At these low herbicide doses, translocation of absorbed [¹⁴C]glyphosate in wheat was c. twice that in field bean; surfactant addition to the formulation did not increase the proportion transported in wheat but substantially enhanced it in field bean.     

Uptake by tomato plants of the herbicide [14C] 2,4-D-butyl in the vapour phase

Uptake of [¹⁴C-phenyl]2,4-D-butyl by 4- to 6-week-old tomato plants was measured for vapour concentrations in the range 0.3–19 ng litre^?1, for an exposure period of 4 h. Calculation of boundary layer thicknesses at the surface of the leaves, and depletion of the vapour concentration due to uptake by the plants, suggested that the plants experienced concentrations very close to the nominal values. Relationships between external vapour concentration and plant uptake, expressed in terms of fresh weight, leaf area, or on a whole plant basis, were linear in all cases. Twenty-four hours after the commencement of exposure, leaves contained 63–93% of the total herbicide in the plant; the proportion retained by the leaves was greater at low vapour concentrations. The largest amount of herbicide was present in leaves from the mid-position on the stem, but in terms of leaf area, the amount was greatest in leaves at the apex and decreased basipetally. It is not known whether this was due to a greater uptake rate per leaf area at the apex, or to translocation. As visible symptoms of phytotoxicity usually develop at the apex, both of these processes, together with the preferential retention of herbicide in leaves at low vapour concentrations, may all contribute to the development of vapour damage.     

Effect of temperature on the toxicity of low concentrations of methyl bromide to diapausing larvae of the warehouse moth Ephestia elutella (Hübner)

The performance of low concentrations of methyl bromide against diapausing larvae of Ephestia elutella at 15 and 25°C was assessed in extended exposure periods. At concentrations of 1.9 mg litre^?1 and below, test batches required higher concentration-time (ct) products for 100% kill at 25°C than at 15°C. The minimum concentration at which the concentration: time relationship still applied was between 1.3 and 1.9 mg litre^?1 at 15°C, whereas at 25°C it was between 2.7 and 4.0 mg litre^?1. For many individuals within each population sample, however, lower concentrations at moderate dosage levels remained lethal. At 25°C, a ct product of about 90 mg litre^?1 h gave between 53 and 77% kill at 6.1, 4.0, 2.7 and 1.9 mg litre^?1. The trends observed suggest that the most tolerant members of the population have an enhanced ability to detoxify methyl bromide at the higher temperature. The implications of the results for the build-up of resistance and for practical control measures are discussed.     

THE HYDROLYSIS OF 2,4-DICHLOROPHENOXY- ACETATE ESTERS TO 2,4-DICHLOROPHENOXYACETIC ACID IN SASKATCHEWAN SOILS

Summary. The hydrolysis of the iso-propyl, n-butyl and iso-octyl esters of 2,4-dichloro-phenoxyacetic acid to the free acid was studied in aqueous solutions and on three prairie soils. The esters were analysed using electron-capture gas chromatography following extraction from solution with benzene, and from soils, using either 10% aqueous aceto-nitrile or a 2:1 mixture of benzene and iso-propanol.
Ester hydrolysis in 0·1 n sodium hydroxide solution was very rapid, more than 50% of all the esters being hydrolysed in less than 1 min. In 0·1 n sodium carbonate solution the times for 50% hydrolysis at 25°C were < 5, 5 and 30 min for the n-butyl, iso-propyl and iso-octyl esters respectively. In distilled water negligible hydrolysis occurred over a 5 h period.
The iso-propyl and n-butyl esters underwent a very facile hydrolysis on soils. Shaking the treated soils with 0·1 M calcium chloride solution for 30 min at 25°C resulted in extensive hydrolysis to the acid as determined spectrophotometrically. Under similar conditions negligible hydrolysis of the iso-octyl ester occurred.
After 1·5 h at 25±1°C on soils at their wilting point moisture levels, and greater, less than 15% of the applied iso-propyl or n-butyl esters could be recovered using either extraction procedure, indicating rapid hydrolysis to the acid. After 24 h no iso-propyl or n-butyl ester residues could be detected in any of the soils. Loss of the iso-octyl ester was slower under these conditions with approximately 20–30% of the ester remaining after 24 h, and 10% after 48 h.
L'hydrolyse des esters de 2,4 dichlorophénoxyacétates en acide 2,4 dichlorophénoxyacétique dans les sols du saskatchewan.     

Depression of the minimum concentration of methyl bromide effective against diapausing warehouse moth larvae at 15°c by prior exposure at a moderate concentration

Warehouse moth (Ephestia elutella) larvae in diapause were exposed at 15°C to methyl bromide at 8 mg litre^?1 for 14.5 h and then immediately exposed at a lower concentration. The exposure at 8 mg litre^?1 killed 44–69% of the larvae treated. Subsequent concentrations down to 1.1 mg litre^?1 obeyed Haber's rule (concentration × time= k, a constant for mortality), but a higher concentration-time product (ct) was required for over 90% kill at 0.8 mg litre^?1. Only concentrations down to 1.9 mg litre^?1 obey Haber's rule if there is no prior exposure at a higher concentration. Although minimum effective concentrations are lower at 15°C than at 25°C, exposure at a higher concentration depresses the subsequent level to a similar extent at each temperature. The contribution to the efficacy of a treatment, of low concentrations persisting at the end of fumigation, is thus likely to be even greater at moderate to low temperatures than at 25°C. The implications for the development of resistance to methyl bromide are discussed.     

Effects of low concentrations of vapour of the phenoxyalkanoic herbicide 2,4-D-butyl on growth of tomato plants

Tomato plants (5–7 weeks old)were exposed to a range of concentrations of vapour of 2,4-D-butyl (0.12–2.4 ng litre^?;1), for periods of 6 or 24 h, using an air-flow system. Net carbon dioxide exchange of the whole plant was measured for up to 2 days after exposure and was found to decrease in treated plants approximately in proportion to vapour concentration. Six weeks after exposure, shoot dry-weights of plants exposed to herbicide vapour were less than control plants in all cases, by about 20 % or more. All characteristics measured were affected more by a 24-h exposure period than 6 h, at the same vapour concentration. The amount of herbicide vapour, expressed as the product of exposure period and vapour concentration, gave a similar effect at a low range of vapour concentrations (≤1.0 ng litre^?;1)and air-flow rates of about 200 litre min^?;1, but not at higher concentrations at a flow rate of about 120 litre min ^?;1. Better agreement between the experiments carried out at the two concentration ranges was obtained by expressing the herbicide vapour as the total amount to which the plants were exposed (i.e. the product of air-flow rate, vapour concentration and duration of exposure)instead of concentration alone.     

Minimum concentration levels of methyl bromide required for full efficacy against seven species of stored-product beetle at two temperatures

The dosages of methyl bromide required to control insect pests can be expressed as concentration, time (Ct) products for concentrations down to a certain minimal level, here defined as the efficacy threshold, below which Habers Ct rule (C×t=k,a constant for mortality) no longer applies. Twelve strains of seven species of stored-product beetle were tested to identify efficacy threshold concentrations at 15°C and 25°C. For each strain the efficacy threshold was higher at 25°C than at 15°C. Different species differed widely in the threshold levels identified. Strains with a higher than usual tolerance to methyl bromide than their standard stock counterparts had slightly higher efficacy thresholds at 25°C but not at 15°C. The order of tolerance of the standard stocks to methyl bromide at 15°C was Tribolium confusum> T. castaneum> Oryzaephilus surinamensis> Rhyzopertha dominica> Sitophilus granarius> S. zeamais> S. oryzae, and at 25°C Tribolium castaneum > T. confusum> R. dominica> O. surinamensis> S. granarius> S. zeamais> S. oryzae. The order of efficacy thresholds largely reflected the order of susceptibility, S. oryzae and S. zeamais having the lowest efficacy thresholds of about 0.6 mg litre^?1 at 15°C and 1.3-2.0 mg litre^?1 at 25°C, while Tribolium spp. had the highest, 1.3-2.0 mg litre^?1 at 15°C and 2.5-4.0 mg litre^?1 at 25°C.     

Partial purification and properties of an esterase from tomato cell suspension cultures hydrolysing the pyrethroid insecticide cyfluthrin

An esterase which hydrolyses the pyrethroid insecticide cyfluthrin, was isolated from tomato cell suspension cultures and purified 10-fold. The apparent molecular weight of the enzyme was estimated to be 32 000 Dalton, the pH-optimum 8-0, and the temperature optimum 35°C. The esterase showed a low substrate specificity and hydrolysed certain esters, e. g. ethyl 4-nitrobenzoate, p-nitrophenyl acetate and dinoseb acetate, whereas ethyl butyrate and the organophosphates demeton-S-methyl sulfoxide and paraoxon could not be hydrolysed. Because of the additional strong inhibition of the enzyme by these organophosphates the cyfluthrin hydrolase is suggested to belong to the B-esterase type. An apparent K_m-value of 6-25 × 10^?4 mol litre^?1 was obtained for p-nitrophenyl acetate as substrate.     

Effect of low temperatures on 2,4-D behaviour in maize plants

The foliar surface of 4-leaf maize plants was found to be poorly wettable and retained 106 μl g^?1 dry matter when sprayed with a U46D (2,4-D formulation) blank. The third leaf retained 141 μl g^?1. A 7-day cold spell (17/9°C) increased retention per unit dry matter by 53% (135% on the third leaf). Cold stress lowered epicuticular wax quantity by 29% on the third leaf. Contact angles of formulated 2,4-D lay between 115 and 125° and were not significantly affected by cold stress. 2,4-D rapidly entered into maize third leaf (66% in 24 h) but migration from it was less than 1.5%. 2,4-D was readily degraded in maize (80% in 72 h). The most abundant metabolite was probably an ester conjugate; little of the hydroxy derivatives were found. Cold stress reduced 2,4-D degradation, and 72 h after treatment the amount of undegraded 2,4-D was 78% higher in cold-stressed maize plants. It was concluded that 2,4-D selectivity in maize results from low spray retention per unit dry matter and active degradation of penetrated herbicide. Cold stress affects both factors.     

Breakdown rates of dimethoate in fruit and vegetable dips

In order to determine the effect of pH and temperature on post-harvest dip solutions of dimethoate (500 mg litre^?1), the half-lives and pseudo first-order rate constants were calculated from measurements at pH 4, 6, 8, 10, 11.5, and at two temperatures 25 and 52°C. The half-lives ranged from 206 days to 39.3 min at 25°C, and from 5.6 days to 205s at 52°C; the rate constants ranged from 3.9 × 10^?8 s^?1 to 2.9 × 10^?4 s^?1 at 25°C, and from 1.4 × 10^?6 s^?1 to 3.4 × 10^?3 s^?1 at 52°C. The results show that the water used in dips should have a pH≤7. The addition of benomyl to the dip solutions at two concentrations (0.5 and 1.0 g litre^?1) had no effect on the half-lives and rate constants. The use of hard and salted waters in dips also showed no major effect. A formula was developed that gives the half-life of the dimethoate as a function of the pH and temperature.     

Effect of short exposures to a high concentration on the subsequent toxicity of low concentrations of methyl bromide to diapausing larvae of the warehouse moth,Ephestia elutella (Hübner)

Ephestia elutella larvae in diapause were exposed at 25°C to methyl bromide at 12 mg litre^?1 for 3.5 or 7.5 h and then immediately exposed to a lower concentration. The minimum effective concentration (that at which Haber's rule, concentration × time = k, a constant for mortality, still applied) was about 3 mg litre^?1 in tests with no previous exposure toa high concentration, but it was about 2.5 mg litre^?1 for individuals surviving a 3.5 h exposure to 12 mg litre^?1, and was about 1.6 mg litre^?1 for those surviving a 7.5 h exposure to 12 mg litre^?1. These exposures to 12 mg litre^?1, respectively, killed 2–20% and 50–75% of larvae exposed, and hence the smaller the proportion of survivors of exposure to a high concentration, the lower the minimum effective concentration needed against them. Thus the low concentration persisting at the end of a practical fumigation should contribute significantly to the success of the treatment and be much more effective than any similar low concentration present soon after the introduction of gas.     

Soil persistence experiments with [14C]2,4-D in herbicidal mixtures, and field persistence studies with tri-allate and trifluralin both singly and combined

The persistence of [¹⁴C]2,4-D at a rate equivalent to 1 kg/ha was studied in the laboratory on a heavy clay and a sandy loam at 85%of field capacity and 20°C both alone and in the presence of 1 kg/ha dicamba, dichlorprop, difenzoquat, TCA, and 2,4,5-T. The persistence of 2,4,5-T was also monitored in both soils under the same conditions in the presence and absence of [¹⁴C]2,4-D. All soils were extracted at weekly intervals using aqueous acidic acetonitrile and analysed for [¹⁴C]2,4-D remainining radiochemical techniques. The extracts containing 2,4.5-T were additionally analysed gas chromatographically for that herbicide. In each soil type the half-life of the 2,4-D was similar regardless of whether applied singly or in combination with the five herbicides tested. Similarly, [¹⁴C]2,4-D did not affect the breakdown of 2,4,5-T in either soil type. The persistence of tri-allate (1·5 kg/ha) and trifluralin (0·75 kg/ha) both singly and in combination were compared using small field plots at two locations in Saskatchewan. Applications were made during May of 1977 and 1978 and the plots were sampled and analysed for herbicide(s) remaining after 10 and 20 weeks, respectively. The results indicate that within experimental error the loss of both tri-allate and trifluralin from the plots treated with the mixture was the same as from plots treated with the individual compounds.     

Determination of residual 2,4,5-T in vegetables by high performance liquid chromatography with ultraviolet and fluorometric detection after derivatization with 2-(2,3-naphthalimino)ethyl trifluoromethanesulfonate (NE-OTf)

2,4,5-T was extracted with acetone at below pH 1·0 and the extract was concentrated. After adding 100 g litre^-1 sodium chloride solution to the residual solution, 2,4,5-T was extracted with ethyl acetate+hexane (20+80 by volume). The extract was evaporated to dryness and the residue was dissolved in acetonitrile. 18-crown-6, potassium fluoride and NE-OTf were added to the acetonitrile solution and then allowed to react at 50°C for 20 min. The product was injected to a HPLC with ultraviolet detection operated at 259 nm and fluorometric detection at 394 nm emission and 259 nm excitation. The determination limits of the 2,4,5-T derivative in the sample were 20 μg litre^-1 with UV detection and 10 μg litre^-1 with fluorometric detection. © 1997 SCI.     

THE ADSORPTIVE BEHAVIOUR OF ACID AND ESTER FORMS OF 2,4-D ON SOILS

Summary. The adsorption of acid, n-butyl, and iso-octyl ester forms of 2,4-D (2,4- dichloiophenoxyacetic acid) was studied for several Canadian prairie soils. The k values for the acid ranged from 0·09 to 1·30 and were correlated to the soil organic matter and not to the clay content. The Q , values (μg adsorbed/g of organic matter) for the acid were 7–2±l. The n-butyl and the iso-octyl esters hydrolysed to the acid form in the aqueous medium and this process was accelerated in the presence of the moist soil. Consequently, the k values for the esters, using slurry type adsorption experiments, or their distribution coefficients, using leaching columns, could not be determined. It was concluded that the adsorptive behavior of the two esters in moist soil was similar to that of the acid form.
Comportement des formes actde, et ester du 2,4-D quant à leur adsorption dans tes sols     

N-Nitrosodimethylamine content of aqueous dimet hylamine and phenoxy herbicide/dimethylamine formulations during storage

N-Nitrosodimethylamine (NDMA) levels in samples of 600 g litre^?1 aqueous dimethylamine (DMA) solution stored at 4, 25 and 40°C, in formulations of the DMA salt of 2, 4-dichlorophenoxyacetic acid (2, 4-D) and of the DMA salt of 4-chloro-2-methylphenoxyacetic acid (MCPA) at 40°C were monitored using a gas-liquid chromatograph coupled with a thermal energy analyser (g.l.c./t.e.a.). The NDMA content of aqueous DMA at 4°C increased from 0.32μgg^?1 to 8.6μgg^?1 in 156 days. At 25°C it increased from 0.20μgg^?1 to 10.5 μgg^?1 in 156 days and at 40°C from 0.76μgg^?1 to 7.12μgg^?1 in 54 h. At 40°C, the NDMA in the 2, 4-D/DMAsalt increased from 0–53μgg^?1 to 2.79μgg^?1 in 96 days and in the MCPA/DMA salt from 0.48μgg^?1 to 5.51 μgg^?1 in 21 days.     

The algaecidal activity of some substituted 1,3-diphenylureas, 1-phenyl-3-(2-pyridyl)ureas and the corresponding thioureas

Twelve ureas and thioureas with 1,3-diphenyl- and 1-phenyl-3-(2-pyridyl) were tested as potential herbicides in a simple screen against two species of algae Chlorella fusca and Anabaena variabilis. Several were shown to inhibit growth at 100 mg litre^?1 but only 1-[2,4-bis(azidosulphonyl)phenyl]-3-(2-pyridyl)urea and 1,3-bis(4-isopropyl- idenehydrazinosulphonylphenyl)thiourea showed any activity at 1 mg litre^?1. This compares with the well-established urea herbicide diuron which, in identical tests, gives similar inhibition of growth at concentrations as low as 0.01 mg litre^?1.     

The toxicity of cyromazine to Chironomus zealandicus (chironomidae) and Deleatidium sp. (leptophlebiidae)

The toxicity of cyromazine and a commercial formulation, ‘Vetrazin’®, to Chironomus zealandicus (thummi) Hudson and Deleatidium sp. was investigated. Under acute test conditions, the LC₅₀ values for each species were quite comparable. For C. zealandicus, the value varied according to instar, 100–400 mg litre^?1 for second- and third-instar to 1000–10000 mg litre^?1 for older fourth-instars. For the one size class of Deleatidium tested (c.10 mm long), the value was 300–400 mg litre^?1. High control mortalities of C. zealandicus limit that species' usefulness as an acute bioassay candidate. Under chronic test conditions, cyromazine showed a high toxicity to eggs or early-instar larvae of C. zealandicus. The maximum acceptable toxicant concentration for cyromazine against C. zealandicus was approximately 17.5 μg litre^?1. The possibility of water contamination at this level is discussed. Whole-of-life chronic tests with C. zealandicus indicated that the most susceptible stage was in the egg or soon after larval emergence. These results highlight the dangers of using short-term acute toxicity results to formulate environmental exposure limits for modern pesticides that do not have dysfunction of the nervous system as their mode of action.