Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 2: Separation of effects on driving force and mobility and reversibility of surfactant action

Polydisperse ethoxylated fatty alcohol (EFA) surfactants can improve the performance of crop protection agents. At the cuticular level they act as accelerators of penetration by increasing the mobility of active ingredients in the cuticle, the barrier properties of which are mainly caused by cuticular waxes. Polydisperse Genapol C-050 (GP C-050, average formula C12.5E5.8) was also found to increase mobility in wax-extracted polymer matrix membranes (MX) of bitter orange and pear, indicating that sorption of surfactants increased segmental mobility of polymethylene chains in cutin and wax. Sorption into MX of the active fraction of GP C-050 from 5g litre⁻¹ micellar solutions was in equilibrium in less than 1 h after establishing contact. This is almost 100-fold faster than with cuticular membranes (CM). Temperature dependence of solute mobilities in CM was studied in order to measure activation energies (E_D) of diffusion in the presence and absence of aqueous surfactant solutions. Monodisperse fatty alcohol ethoxylates C8E3, C8E4 and C12E6, and (non-surface-active) tributylphosphate decreased E_D of the model compounds WL 110547 and bifenox in Citrus, Pyrus and Stephanotis CM by more than 100 kJ mol⁻¹. This corresponds to 50 to 275-fold increases of mobilities at 15 °C. Our data suggest that the decrease in activation energies with the concomitant accelerating effect on mobility contributes considerably to the effects of so-called activator surfactants. High temperature and accelerators act similarly on barrier properties of CM. It is shown that effects of both monodisperse and polydisperse EFA surfactants on solute mobility are reversible and that radiolabelled C12E8 penetrated pear CM rapidly. However, rates of penetration were lowered by excess amounts of WL 110547 and especially phenylurea. Partition coefficients of seven organic solutes between Capsicum fruit cuticles and GP C-050 were very low and, with the exception of methylglucose, smaller than 1. They decreased with lipophilicity and differed about 100-fold. Especially for the lipophilic compounds they were orders of magnitude lower than octanol/water or cuticle/water partition coefficients, which indicates the limited usefulness of these values for an understanding of penetration of active ingredients from formulation residues. © 1999 Society of Chemical Industry     

Concentration-Dependent Mobility of Chlorfenvinphos in Isolated Plant Cuticles

The mobility of chlorfenvinphos in isolated pear (Pyrus communis cv. Bartlett) leaf cuticular membranes (CM) was studied as a function of concentration of chlorfenvinphos sorbed in the cuticle. Mobilities of chlorfenvinphos increased approximately 9-fold when the amount sorbed increased from 1 to 100 μg cm⁻² pear leaf cuticle. From the amounts per area, average volume fractions of chlorfenvinphos in the cuticle were calculated ranging from 2×10⁻³ to 5·1×10⁻². The increase in mobilities was steepest at the lower and levelled off at higher volume fractions. This correlation could be described for the whole range of volume fractions investigated by an equation which assumes homogeneously dispersed chlorfenvinphos. Temperature dependence of mobilities was studied at 17, 25 and 35°C and chlorfenvinphos volume fractions of 5·5×10⁻³ and 0·12, respectively. Arrhenius graphs were linear for both volume fractions, showing that cuticles did not undergo a phase transition due to the high amount of sorbed chlorfenvinphos. However, at a volume fraction of 0·12, the activation energy of diffusion, E_D, was significantly lower (83·6 kJ mol⁻¹) than at 5·5×10⁻³ (135 kJ mol⁻¹). We interpret these findings as evidence for a plasticising effect on cuticular waxes by chlorfenvinphos. So far, such an effect had been demonstrated only for certain adjuvants (ethoxylated alcohols) but not for active ingredients. Chlorfenvinphos not only increased its own mobility in pear leaf cuticles, but also that of 2,4-D in Citrus leaf cuticles. This would be expected if plasticising of waxes was the sole mechanism responsible for increased mobilities. From these data we predict that permeabilities of cuticles to chlorfenvinphos are not constant. Depending on temperature as well as types and amounts of adjuvants, rates of foliar penetration of chlorfenvinphos can be higher if its concentration in the spray liquid is increased.     

Mobilities of Organic Compounds in Reconstituted Cuticular Wax of Barley Leaves: Effects of Monodisperse Alcohol Ethoxylates on Diffusion of Pentachlorophenol and Tetracosanoic Acid

Effects of monodisperse alcohol ethoxylates on mobilities of ¹⁴C-labelled pentachlorophenol (PCP) and tetracosanoic acid (C₂₄AC) in reconstituted cuticular wax of barley leaves were measured. Depending on the respective alcohol ethoxylate investigated, the diffusion coefficient (D) of PCP in barley wax was increased by factors ranging from 3·3 to 19·6, whereas D of C₂₄AC, was increased by factors varying between 22 and 315. In order to analyse the relationship between the concentration of surfactants in the wax and their effects on D, the amounts of alcohol ethoxylates dissolved in the wax at equilibrium with external concentrations well above the critical micelle concentration (CMC) were determined. Wax/water partition coefficients (K_ww) of the alcohol ethoxylates were about one order of magnitude lower than cuticle/water partition coefficients (K_cw), which is a consequence of the semi-crystalline structure of the wax compared with amorphous cutin. Correlations between effects on D and maximum amounts of alcohol ethoxylates dissolved in the wax were obtained indicating an unspecific wax/surfactant interaction. This was solely dependent on the amount of surfactant sorbed to the wax, leading to increased mobilities of pesticides in the wax. Applying ESR-spectroscopy, which gave an insight into the molecular structure of the wax, supported this interpretation of an unspecific plasticising effect of the alcohol ethoxylates on the molecular structure of the wax. The results obtained in this study are in good accordance with the results obtained in a recent study investigating the effects of the same group of alcohol ethoxylates on mobilities of pesticides in isolated, but intact, cuticular membranes of Citrus. This demonstrates that the investigation of isolated and subsequently reconstituted cuticular wax is a useful model system analysing the mechanisms of the surfactant interaction with the transport-limiting barrier of plant cuticles.     

Mobilities of organic compounds in reconstituted cuticular wax of barley leaves: Determination of diffusion coefficients

Isolated cuticular wax, obtained from barley leaves, was mixed with ¹⁴Clabelled organic chemicals including aromatic pesticides and long-chain linear alkanes, alcohols and acids. These mixtures were reconstituted from the melt and labelled chemicals were desorbed from the wax by immersing the wax samples in aqueous phospholipid suspensions. Diffusion coefficients (D) of radiolabelled test compounds in the wax were derived from desorption kinetics. Diffusion coefficients ranged from 10^?17 to 10^?22 m² s^?1 and decreased rapidly with increasing molar volumes of solutes. However, size selectivity of D was much more pronounced with the linear, long-chain molecules than with the aromatic compounds. It is argued that the two different groups of chemicals (compounds occurring naturally in cuticular waxes vs pesticide molecules) were trapped in different fractions of wax during crystallization from the melt. The normal long-chain aliphatic compounds appear to be incorporated into the crystalline fraction of the wax, while the cyclic pesticide molecules are confined to the solid amorphous regions. Our data indicate that constituents of cuticular waxes are not immobile. In fact, relatively small linear aliphatic molecules have mobilities that do not differ too much from those of cyclic pesticides. However, the pronounced size selectivity of diffusivities of long-chain aliphatic compounds causes a rapid decrease in D with increasing chain length. The value of D of hexadecanoic acid was 3.81 × 10^?18 m² s^?1 while that of dotriacontane was only 4.07 × 10^?22 m² s^?1. Thus, increasing the carbon number by a factor of two resulted in a decrease in mobility by almost four orders of magnitude. Diffusivities of selected pesticide molecules in reconstituted wax were compared with permeances measured using intact barley leaves and were found to be in satisfactory agreement.     

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.     

Physicochemical factors affecting the uptake by roots and translocation to shoots of amine bases in barley

The uptake by barley roots from nutrient solution and subsequent transport to shoots of two series of amine bases were measured over 6 to 72 h. The compounds were chosen to span systematically ranges of lipophilicity (assessed using 1-octanol/water partition coefficients, K_ow) and pKa that would include commercial pesticide amines. In a series of six substituted phenethyl amines, strong bases with pKa∽9·5, all the compounds were strongly taken up by roots from solutions of pH 8·0; uptake declined substantially as the pH was lowered to 5·0, especially for the compounds of intermediate lipophilicity (log K_ow 2 to 3). This uptake could be ascribed to three processes: (i) accumulation of the cation inside the root cells due to the negative charge on the plasmalemma, as given by the Nernst equation and important only for the polar compounds which have low permeation rates through membranes; (ii) accumulation into the vacuole by ion-trapping, which was the dominant process at high pH for all compounds and at all pH values for the compounds of intermediate lipophilicity; (iii) partitioning on to the root solids, substantial only for the most lipophilic compounds. Translocation to shoots was proportional to uptake by roots, this ratio being independent of external pH for each compound and being optimal for the compounds of intermediate lipophilicity. Such proportionality was also observed in a series of three weaker bases of intermediate lipophilicity, in which compounds of pKa 7·4 to 8·0 were also well taken up and translocated whereas the very weak base 4-ethylaniline (pKa 5·03) was much less so. Tests with quaternised pyridines confirmed that organic cations move only slowly through membranes. The observed behaviour of the amines could be modelled reasonably well assuming that transport within the plant was dominated by movement across membranes of the non-ionised species, and this appeared to be true even for the most lipophilic phenethylamine (log K_ow 4·67) studied, though its long-distance movement would be as the protonated species. © 1998 Society of Chemical Industry     

Effects of some nonionic polyoxyethylene surfactants on uptake of ethirimol and diclobutrazol from suspension formulations applied to wheat leaves

The influence of a number of commercial nonionic polyoxyethylene surfactants on the foliar penetration and movement of two systemic fungicides, ethirimol and diclobutrazol, was studied in outdoor-grown wheat plants at different growth stages and post-treatment temperatures in two consecutive growing seasons. Both fungicides were applied as ca 0·2 μl droplets of aqueous suspension formulations containing 0·5 g litre^?1 of ¹⁴C-labelled active ingredient; surfactants were added to these suspensions at concentrations ranging from 0·2-10 g litre^?1. To achieve optimum uptake of each fungicide the use of surfactants with different physicochemical properties was required. For diclobutrazol, a lipophilic compound, uptake of radiolabel was best with surfactants of low mean molar ethylene oxide (E) content (5-6) but it was necessary to use concentrations of ca 5 g litre^?1 to attain this. The surfactant threshold concentration for uptake enhancement of radiolabel from ethirimol formulations (< 2 g litre^?1) was much lower than that for diclobutrazol but surfactants with E contents > 10 induced the greatest amount of uptake. For both fungicides, surfactants with an aliphatic alcohol hydrophobe were generally more efficient in promoting their uptake than those with a nonylphenol moiety. The sorbitan-based surfactant ‘Tween 20’ proved to be an effective adjuvant only for the ethirimol formulation; the uptake enhancing properties of the block copolymer ‘Synperonic PE/F68’ were weak. Uptake performance could not be related to the spreading properties of the respective formulations on the wheat leaf surface or to differences in solubilisation of the two fungicides by the surfactants. Although surfactants could substantially increase the amount of acropetal transport of radiolabel from both fungicides, none of those tested specifically promoted it; a constant proportion of the radioactive dose absorbed by a treated leaf was usually exported away from the site of application. The results are discussed in the light of current theories about the mode of action of surfactants as spray adjuvants.     

Some physicochemical and environmental factors affecting transformation rates and sorption of the herbicide metamitron in soil

In addition to the molecular structure of a pesticide, environmental conditions may influence its persistence through their effect on the growth and activity of pesticide-degrading micro-organisms. As a result, transformation rates may decrease rapidly when a compound is leached into subsoil. Metamitron sorption isotherms were determined and incubation series were set up for a sandy loam soil, simulating single and combination effects that occur during transport of metamitron into subsoils. K_OC values increased with increasing depth from 185 to 700 litre kg⁻¹. A combination of conditions that are unfavourable for microbial activity, such as low temperature (5°C), low concentrations (0·5 mg kg⁻¹) and a large sorbed fraction (K_OC = 700) resulted in half-lives of over one year. Oxygen inhibition decreased the transformation rate of metamitron from 0·058 to 0·019 day⁻¹. In order of significance, the transformation of metamitron appears to be a function of temperature, oxygen availability and sorption to organic carbon. Increasing doses did not change transformation rates significantly, although different transformation pathways were observed.     

Effects of temperature and concentration of the accelerators ethoxylated alcohols,diethyl suberate and tributyl phosphate on the mobility of [14C]2,4‐dichlorophenoxy butyric acid in plant cuticles

Intrinsic activities of monodisperse ethoxylated dodecanols (MEDs), diethyl suberate (DESU) and tributyl phosphate (TBP) were investigated using Stephanotis floribunda leaf cuticular membranes (CMs) and [¹⁴C]2,4‐dichlorophenoxy butyric acid (2,4‐DB) as a model solute. When sorbed in cuticular membranes, MEDs, DESU and TBP increase solute mobility and are called accelerators for this reason. With MEDs, dose‐effect curves (log mobility vs accelerator concentration) were linear but, with DESU and TBP, curves convex to the x axes were obtained that approached a maximum at 90 and 150 g kg⁻¹, respectively. Accelerators increased the mobility of 2,4‐DB in the CMs by 9‐ to 48‐fold, and effects were larger at lower temperatures (range 15–30 °C). Activation energy for diffusion of 2,4‐DB was 105 kJ mol⁻¹, decreasing with increasing accelerator concentrations to 26 kJ mol⁻¹ with DESU at 90 g kg⁻¹ and 64 kJ mol⁻¹ with TBP at 150 g kg⁻¹. Thus, the intrinsic activity of DESU was much higher than that of TBP, which implies that, for a given effect, less DESU than TBP would be needed. MEDs were also very effective accelerators, lowering activation energies to 36 kJ mol⁻¹. Data are discussed in relation to increasing rates of foliar penetration of active ingredients at low temperatures. © 2001 Society of Chemical Industry     

Diffusive movement of simazine and lindane in river-bed sediments

River-bed sediments are active zones for pesticide deposition and subsequent movement by diffusion, mass transport and sorption to solids. The aim of this work was to investigate the importance of diffusion as a means of pesticide movement. In laboratory experiments, simazine and lindane were introduced to well-mixed aqueous solutions overlying two different river sediments. Sediment cores were sectioned horizontally and analyzed for pesticide content by supercritical fluid extraction. Experiments were used to determine sorption isotherms of the compounds to suspended sediments at 10°C. Vertical profiles of the pesticides in the sediments showed that the compounds reached a maximum depth of 89 mm over a period of 37 days. A mathematical model was developed to describe pesticide transport by diffusion within the sediment porewaters and sediment sorbed phases, taking into account sorption of the compounds to sediment particles. Effective diffusion coefficients ((0.5–1.6)×10^-10 m² s^-1) were obtained for simazine and lindane in the characterized sediments. These were used to calculate values for diffusion in the dissolved phase (0.38×10^-10 and 6·16×10^-10 m² s^-1 for simazine and lindane respectively) and diffusion in the sorbed phase (0.39×10^-10 m² s^-1 for simazine and negligible for lindane). Sorption onto the sediment significantly influenced the rate of penetration of the compounds into the sediment; thus although lindane had a larger effective diffusion coefficient than simazine, its larger sorption affinity and negligible diffusion in the sorbed phase led to less penetration into the sediment. © 1998 Society of Chemical Industry     

Recent developments in our understanding of the plant cuticle as a barrier to the foliar uptake of pesticides†

The plant cuticle is a highly complex membrane which forms the outer surface of the aerial portion of plants. The nature of the plant cuticle is reviewed with particular regard to its action as a potential barrier to the penetration of pesticide molecules; the role of the cuticular waxes is highlighted. The physicochemical properties of the cuticle influence the behaviour of spray droplets and, in turn, may affect the rate and efficiency of cuticle penetration. The permeation of active ingredients is influenced by their solubility characteristics as indicated by octanol/water (log K_ow) and cuticle/water (K_cw) partition coefficients. Penetration of hydrophilic compounds (low log K_ow) may be enhanced by hydration of the cuticle, while transcuticular transport of non-polar solutes (high log K_ow) is increased by factors which reduce wax viscosity. The use of in-vitro models involving isolated cuticle membranes, isolated cuticle waxes, or isolated leaves has helped to focus on the activities of the cuticle in the absence of other physiological factors. Using these systems, the role of the waxes as a transport-limiting barrier has been identified and the factors influencing sorption, permeance and desorption examined. The action of surfactants, in vitro and in vivo, has been briefly addressed in regard to their role in facilitating cuticle penetration; other steps involving surfactant/solute/cuticle are complex, and synergy appears to depend on a number of factors including test species, concentration of active ingredient, surfactant type and concentration. Adjuvants may greatly influence the surface properties of the droplet, predispose the cuticle to solute transport, and enhance pesticide activity. The nature of these complex inter-relationships is discussed. © 1999 Society of Chemical Industry     

Analysis of foliar uptake of pesticides in barley leaves: Role of epicuticular waxes and compartmentation

Uptake of pesticides into barley leaves was measured under controlled conditions. Leaves detached from plants were submerged in aqueous solutions of ¹⁴C-labelled (2,4-dichlorophenoxy)acetic acid, triadimenol, bitertanol and pentachlorophenol. Uptake was biphasic. A short (30-min) period with high rates of uptake was followed by uptake that proceeded more slowly and was steady over hours. Compartmentation of pesticides was studied by desorbing pentachlorophenol from leaves previously loaded with [¹⁴C]pentachlorophenol. From the uptake and desorption kinetics it was concluded that penetration of pesticides proceeds as follows: the compounds are first sorbed at the surface of epicuticular wax aggregates where they are in contact with the donor solutions. Solutes then diffuse through the surface wax aggregates into the cuticle. Equilibrium between donor solutions, surface wax and cuticle is established in about 30 min. After this time the amounts of solutes in these compartments no longer increase. Uptake after this time represents penetration into the leaf cells. This fraction of the pentachlorophenol is retained irreversibly, while that sorbed in wax and cutin can be desorbed again. All compounds were sorbed in cuticular waxes and partition coefficients wax/water were determined. On a mass basis only 5 to 10% of the amounts sorbed in cutin are sorbed in wax. This comparatively low solubility in wax contributes to the barrier properties of cuticular waxes. The other determinant of permeability is the very low mobility of solutes in cuticular waxes.     

In vitro and in planta nematicidal activity of Fumaria parviflora (Fumariaceae) against the southern root‐knot nematode Meloidogyne incognita

Root and stem extracts of Fumaria parviflora showed strong nematicidal activity against Meloidogyne incognita in in vitro and in planta experiments. Phytochemical screening of F. parviflora revealed the presence of seven classes of bioactive compounds (alkaloids, flavonoids, glycosides, tannins, saponins, steroids and phenols). Quantitative determination of the plant extracts showed the highest percentages of alkaloids (0·9 ± 0·04) and saponins (1·3 ± 0·07) in the roots and total phenolic contents in the stem (16·75 ± 0·07 μg dry g^?1). The n‐hexane, chloroform, ethyl acetate and methanol extracts of roots and stems at concentrations of 3·12, 6·24, 12·5, 25·0 and 50·0 mg mL^?1, significantly inhibited hatching and increased mortality of second‐stage juveniles (J2s) compared with water controls. Percentage J2 mortality and hatch inhibition were directly related to exposure time. In pot trials with tomato cv. Rio Grande, root and stem extracts at concentrations of 1000, 2000 and 3000 ppm, applied as soil drenches, significantly reduced the number of galls, galling index, eggs masses, eggs and reproduction factor compared with the water control. Regardless of concentration, all the extracts significantly increased the host plant growth parameters studied. The n‐hexane extracts from the roots and stem were the most active, followed by the methanol ones, at all concentrations. The in vitro and in planta results suggest that extracts from the roots and stem of F. parviflora may be potential novel nematicides.     

Effects of alcohols,glycols and monodisperse ethoxylated alcohols on mobility of 2,4-D in isolated plant cuticles

Effects of n-alcohols, ethoxylated alcohols and glycols on mobility of 2,4-dichlorophenoxy acetic acid (2,4-D) in cuticular membranes (CM) isolated from bitter orange (Citrus aurantium L.) leaves were studied. 1-Heptanol, 1-octanol and 1-nonanol had the highest effects, as they increased solute (2,4-D) mobility by 25- to 30-fold. Increasing the number of carbon atoms in the alcohols decreased their effectiveness. Ethoxylation of alcohols did not increase 2,4-D mobility and effectiveness decreased with increasing ethoxylation. Free glycols had no effect on solute mobility in isolated cuticles. The results show that ethoxylation is not required for increasing solute mobility in cuticles. It is suggested that alcohols and ethoxylated alcohols are sorbed in cuticular waxes and plasticize them. The data show that alcohols and ethoxylated alcohols having between seven and ten carbon atoms are powerful accelerator adjuvants, as long as the degree of ethoxylation is not too high. Free short-chain alcohols synthesized by leaves may act as endogenous plasticizers and modulate permeabilities of cuticles, depending on environmental and growing conditions.     

Control of Avena fatua L. and Cirsium arvense (L.) Scop. with mixtures of 3,6-dichloropicolinic acid and four herbicides for control of A. fatua

Control of Avena fatua (L.) (wild oat) with diclofop methyl applied at 0·7 kg ha^?1 at the two-leaf stage and difenzoquat at 0·84 kg ha^?1 at the four-leaf stage in wheat (Triticum aestivum L.) under field conditions was good and not affected when either of these herbicides was mixed with 3,6-dichloropicolinic acid as the monoethanolamine salt at 0·14, 0·20 or 0·30 kg ha^?1. In the glasshouse, mixtures containing 3,6-dichloropicolinic acid at rates as high as 0·6 kg ha^?1 also did not affect control of A. fatua. When barban at 0·35 kg ha^?1, or flamprop methyl at 0·56 kg ha^?1 was mixed with similar rates of 3,6-dichloropicolinic acid and applied at the two-leaf and four-leaf stage of A. fatua respectively, a reduction in control of A. fatua (antagonism) occurred under both field and glasshouse conditions. The herbicides for control of A. fatua did not influence the fresh weight suppression of C. arvense shoots obtained in the glasshouse with 3,6-dichloropico-colinic acid at 0·3 kg ha^?1. Early tolerance of wheat (cv. Neepawa) was acceptable with all mixtures. Wheat yields with diclofop methyl or difenzoquat alone or in mixture with 3,6-dichloropicolinic acid were increased over the yields from the A. fatua-infested control.     

Effect of the ecdysone agonists,RH-2485 and tebufenozide,on the southwestern corn borer,Diatraea grandiosella

The effect of the ecdysone agonists RH-2485 (proposed name methoxyfenozide) and tebufenozide (RH-5992), was examined on eggs and larvae of the southwestern corn borer, Diatraea grandiosella Dyar. Both compounds exhibited a concentration-dependent ovicidal activity. More than 95% of eggs died when egg masses were dipped in solutions of 100 or 200 mg liter^-1 of either compound in acetone+distilled water (1+1 by volume). Although some eggs treated with 1 or 10 mg liter^-1 of the compounds hatched, the survival rate was low. Newly hatched larvae were fed for seven days on an artificial diet containing RH-2485 or tebufenozide. The LC₅₀ values were 0·049 mg kg^-1 for RH-2485 and 0·185 mg kg^-1 for tebufenozide, showing that RH-2485 was about four times more active than was tebufenozide. Although increasing the time of exposure to either compound decreased the LC₅₀ value significantly, the relative potency of RH-2485 versus tebufenozide was not changed. Newly ecdysed 4th-instar larvae fed with diets containing 0·125, 0·25 or 0·5 mg kg^-1 RH-2485 or tebufenozide ceased feeding approximately 8 h after exposure, indicating that larvae had prematurely entered a molting cycle. Larvae treated with RH-2485 ecdysed earlier and died more quickly than those treated with tebufenozide. Ingestion of sublethal concentrations of RH-2485 (0·005 and 0·01 mg kg^-1) or tebufenozide (0·03 and 0·06 mg kg^-1) retarded larval growth, and decreased pupal weight and adult emergence. Increasing exposure time to tebufenozide tended to increase the larval mortality, significantly retarded larval growth, and decreased the mean weights of male and female pupae and adult emergence. RH-2485 (0·125 and 0·25 mg kg^-1) and tebufenozide (0·25 and 0·5 mg kg^-1) were lethal to newly hatched larvae, even after diets containing these compounds were held for 20 days at 30°C under long days (16 h light: 8 h dark). Our results suggest that field trials to assess the potential of RH-2485 and tebufenozide to control D. grandiosella are warranted. © 1998 SCI     

Presence of Muscarinic Acetylcholine Receptors in the Cattle Tick Boophilus microplus and in Epithelial Tissue Culture Cells of Chironomus tentans

A muscarinic acetylcholine receptor (mAChR) has been demonstrated and partially characterized in larvae of the cattle tick Boophilus microplus. Its properties are compared with mAChR from an epithelial cell line from the dipteran insect Chironomus tentans. Competition studies with cholinergic ligands of different specificity revealed the muscarinic nature of the cholinergic receptors investigated in both species. In homogenates from tick larvae, specific binding sites for [³H]quinuclidinyl benzilate (QNB) with high affinity (1·2±(0·13) nM ; B_max 22·5 pmol mg protein⁻¹) were detected that do not bind nicotinic compounds specifically. The estimated IC₅₀ values for nicotine, imidacloprid and α-bungarotoxin were all in the mM range. Additionally, with tick larvae, high-affinity nicotinic binding sites were detected with [³H]nicotine which could be displaced by high concentrations of imidacloprid or QNB. The estimated IC₅₀ values for nicotine, α-bungarotoxin, imidacloprid and QNB were 43(±8) nM , 0·8(±0·2) μM , 2·8(±0·6) μM and 78(±1·9) μM , respectively. With homogenates of the non-neuronal insect cell line from C. tentans, only high-affinity binding sites for [³H]QNB were found. Muscarinic antagonists selectively displaced [³H]quinuclidinyl benzilate (QNB) binding to tick larvae homogenates. The mAChR of B. microplus preferred pirenzepine (IC₅₀ 2·13(±1·02) μM ) among different subtype-specific mAChR antagonists (4-DAMP had IC₅₀ 49·9(±9·13) μM and methoctramine had IC₅₀ 121(±14·2) μM ) indicating a type of binding site similar to the vertebrate M1 mAChR subtype. The tick muscarinic receptor seems to be a G-protein-coupled receptor, as concluded from the 4·8-fold reduction in receptor affinity for binding of the muscarinic agonist oxotremorine M upon treatment with the non-hydrolysable GTP-analogue γ-S-GTP. Binding data for the agonists oxotremorine M (IC₅₀ 71·3(±19·6) μM ) and carbachol (IC₅₀ 253(±87·1) μM ) parallel the biological efficacy of these compounds, in that, while oxotremorine M showed some activity against ticks, carbachol was ineffective.     

Comparison of screening methods to evaluate the response of St. Augustine grass to Magnaporthe oryzae

The objective of this study was to develop a reliable and high throughput screening method to evaluate the response of St. Augustine grass (Stenotaphrum secundatum) genotypes to the grey leaf spot (GLS) caused by Magnaporthe oryzae infection. Whole plant, detached stolon and detached leaf assays under growth chamber conditions were compared to field conditions on eight commercial and nine advanced breeding lines of St. Augustine grass. Disease was assessed using two variables, lesion size (LS) and overall plant disease severity (SEV). LS and SEV were highly correlated for field and growth chamber screening methods using the whole plant assay (LS r² = 0·79; SEV r² = 0·83; P ≤ 0·001), the detached stolon assay (LS r² = 0·75; SEV r² = 0·72; P ≤ 0·001), and the detached leaf assay (LS r² = 0·46; SEV r² = 0·60; P ≤ 0·001). Genotypic variation for resistance in 17 St. Augustine grass genotypes was identified using all screening methods for LS (P < 0·05) and SEV (P < 0·05). The rank‐sum method was used to classify St. Augustine grass genotypes into highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) classes based on the rank‐sum values of LS and SEV. Two introduced African polyploids used as parents, and two F₁ interploid progeny obtained using an in vitro embryo rescue technique, were classified as highly resistant (HR), or resistant (R), across all screening methods.     

Sorption of low levels of nonionic and anionic surfactants on soil: effects on sorption of triticonazole fungicide

The sorption of two anionic surfactants and a series of seven nonionic alkylphenolethoxylate surfactants of increasing hydrophilic/lipophilic balance (HLB) in a loamy clay soil was evaluated. The effect of low doses of these surfactants on the sorption characteristics of the fungicide triticonazole was investigated. The critical micellar concentration (CMC) of the surfactants in pure water and soil–water systems, and surfactant sorption were estimated by surface tension measurements using a batch equilibration technique. Triticonazole sorption, alone and in the presence of low doses of surfactants, was also measured by batch equilibration. CMC of the alkylphenol surfactants increased with their HLB. The sorption of surfactants increased with their lipophilicity. CMC in the soil–water systems were considerably higher than in pure water. Sorption of the most lipophilic alkylphenol surfactants at the higher doses significantly increased triticonazole sorption. Proposed mechanisms are modifications of soil surface properties, and increase of soil organic carbon content. Sorption of the other nonionic and anionic surfactants only resulted in monomeric surfactant concentrations in pore water, and did not affect triticonazole sorption. © 1998 Society of Chemical Industry