Adjuvants and herbicidal efficacy – present status and future prospects

The influence of adjuvants on spray liquid behaviour and herbicide performance is reviewed. Total formulation efficacy can be expressed as a function of [deposition:retention:uptake: translocation:a.i. toxicity]. Adjuvants influence the physico-chemical and plant interactions involved for each factor. Deposition efficiency of spray droplets on to a target is dependent largely on the droplet spectrum, whereas retention performance is dependent on plant leaf surface character, orientation and canopy architecture, as well as droplet volume, velocity and dynamic surface tension effects. Uptake into plant foliage is affected by the leaf surface wax, cuticle age and composition and species variability. Uptake can be improved through appropriate formulation to provide either stomatal infiltration or much greater and faster cuticular absorption of the active ingredient. The inherent translocation capability of the a.i. is not affected directly by adjuvants, which are relatively immobile, but they can increase the mass of absorbed a.i. translocated, as a consequence of improved uptake or may reduce it as a result of localized contact phytotoxicity. Considerable progress has been made in developing models of spray droplet deposition, adhesion and retention, as well as uptake. In future, individual models may be combined to provide an integrated formulation efficacy decision support system.     

Spray application factors and plant growth regulator performance: II. Foliar uptake of gibberellic acid and 2,4-D

Effects of droplet size and carrier volume on foliar uptake and translocation of gibberellic acid (GA₃) and 2,4-D were investigated. Simulated spray droplets were applied to primary leaves of 10-day-old Phaseolus vulgaris (cv Nerina) in droplet sizes and carrier volumes ranging from 0.5 to 10 μl and 10 to 200 μl per leaf, respectively. Doses of GA₃ (2 μg per leaf) and 2,4-D (100 μg per leaf) were held constant. Total uptake of GA₃ approached a penetration equilibrium within 24 h after application, but uptake of 2,4-D continued to increase. Decreasing droplet size and/or increasing carrier volume increased GA₃ and 2,4-D uptake. Translocation to stem and roots was positively related to total uptake. A positive linear relationship between the logarithm of the total droplet/leaf surface interface area and 2,4-D uptake or translocation was found, but for GA₃ this relationship was quadratic. Potential mechanisms of the effects of spray application factors on foliar uptake are discussed. © 1999 Society of Chemical Industry     

C.d.a.–Controlled Droplet Application

Abstract

Efficiency of pesticide application can be improved if more consideration is given to where and when the active ingredient is most needed, and the optimum droplet size selected for a given target. Even when minimal volumes of spray are applied concentrate sprays are not justified as a greater proportion of the spray is collected on the target. Sprays with a narrow range of droplet size can be produced from ligaments thrown from centrifugal energy nozzles; droplet size being inversely proportional to the rotational speed.     

The influence of the extensional viscosity of very low concentrations of high molecular mass water-soluble polymers on atomisation and droplet impact

BACKGROUND: Water-soluble polymers are increasingly added to herbicide and pesticide formulations at very low concentrations (100-1000 mg L(-1)) in order to control the spray characteristics, notably to reduce spray drift and influence droplet bounce. The incorporation of polymeric adjuvants improves the efficacy of the spray solutions, thus enabling crop growers to maximise the performance of agrochemical sprays at lower dose rates of active ingredient. It is important to establish a fundamental understanding of how polymers influence the processes involved in droplet deposition.RESULTS: The shear and extensional viscosities of a series of high molecular mass (M(w)) poly(acrylamides) (M(w) approximately 10(6)-10(7)) have been determined at very low concentrations (100-1000 mg L(-1)). The polymer solutions demonstrated typical shear thinning characteristics under shear, and strain hardening behaviour under extension above a critical strain rate. The presence of the polymers was shown to increase the size of droplets produced in atomisation using an agricultural spray nozzle, as measured by laser diffraction. This was attributed to the increase in the extensional viscosity at the strain rates generated under pressure in the spray nozzle and was a function of both polymer concentration and M(w). In addition, the presence of polymer was found to have a significant influence on droplet bounce.CONCLUSIONS: The presence of very low concentrations of high molecular mass poly(acrylamides) significantly influences the size of droplets formed on atomisation and subsequent bounce characteristics. Large extensional viscosities generated above a critical strain rate are responsible for both processes.     

Tracing insecticide spray droplets by sizes on natural surfaces. The state of the art and its value

Recent trends in integrated chemical and biological pest control call for further refinement of the effectiveness of existing chemical pesticides by more efficient spray application. One aspect of improving spray efficiency is by the use of the correct range of droplet sizes. In this way the volume of spray, and the amount of active ingredient used, particularly in ultra-low volume techniques, can be further optimised, and when combined with correct application time, gives maximum biological efficiency with minimum disturbance of the environment. This necessitates a study of the size spectrum of droplets deposited on rigorously defined biological targets which in turn calls for an investigation of suitable tracer methodology. Additional details of the previously published fluorescent particle method are discussed, and its reappraisal shows that it is a unique method capable of estimating droplet sizes on all biological surfaces, especially insect larvae. The reversibly soluble fluorescent pigment method based on the relative balance between soluble and insoluble state of the pigment proves highly successful in imprinting fluorescent droplets on broad leaves, particularly on cotton.     

Factors affecting the biological efficiency of small pesticide droplets against Tetranychus urticae eggs

Using monosized oil-based droplets of dicofol against Tetranychus urticae eggs on 1 cm² discs of bean leaf, the effects of varying droplet diameter (18–146 μm) and concentration of active ingredient (0.5–40 g litre^?1) on egg mortality were investigated. A positive curvilinear relationship was found between the spacing of droplets required to produce 50% kill (LS₅₀) and the original diameter of the droplet, D, such that: where b varied from 0.65 to 1.44 and bore a U-shaped relationship to concentration. Thus, there was an optimum concentration of about 10 g litre^?1, with control efficiency reducing as much as nine times at the extreme concentrations, and even more from the smallest to the largest droplets. The theoretical, practical and economic implications are discussed.     

Spray impaction,retention and adhesion: An introduction to basic characteristics

Droplets, falling under gravity through air that is not moving relative to the target, will impact on any object in their path, while charged droplets will be drawn to objects of earth potential along paths normal to the lines of equipotential; thus near the catching surface, they will move directly towards it. If the air is moving relative to the target, it will tend to move the droplets with it. The greater the drag to mass ratio of a droplet, the more rapidly any initial motion it has through the air will cease, and it will move through the air only very slowly under the effects of gravity and any electromagnetic potential. Air flowing past an object is able to change its path rapidly, but droplets moving with the air are less able to do this. Their ability to avoid impact increases with decrease in droplet size and wind speed, and with increase in the size of the catching surface. Thus small smooth stems catch big droplets in a high wind efficiently, but large smooth branches in a light wind will not catch many small droplets. Artificial cylinders and ribbons are poorer at catching droplets than natural surfaces, which are rarely smooth and often hairy. Hairs or spikes on a surface greatly increase the catch efficiency of droplets carried in the wind. A droplet several hundred micrometres in diameter is so dominated by gravity that it will fall in a near vertical path even in a moderate wind, impacting on any horizontal surface that obstructs its path. Its chances of reaching a vertical stem are negligible unless it runs off or splashes from a near horizontal surface. Conversely, a small droplet will be carried almost horizontally in any wind and is most likely to impact on vertical surfaces or flapping leaves. It has a much greater chance of getting inside the canopy without being caught because most leaves are near horizontal, and once there, it must rely on the turbulence induced by the wind for transport and impaction on undersurfaces or hairs. Because turbulence is reduced as the droplet nears the ground, it is very difficult to catch droplets on the lower parts of the crop within the canopy. To bounce, a droplet must have enough surplus kinetic energy to rebound clear of the surface, allowing for the energy losses in deforming the droplet in the bounce process; moreover, the surface must not be significantly wetted by the drop. Thus the droplet must be moderate to large in size, must be moving rapidly relative to the surface, and must have a high surface tension to contain it as a droplet, even at its extreme deformation. Surface condition is of great importance; the presence of hairs and the type of roughness affect the probability of maintaining an air film between the surface and the droplet. In general, droplets below 150 μm diameter are unlikely to bounce, but adding small amounts of surfactant to the droplet formulation can increase this size by several times. Any one plant leaf can vary considerably over its area because of age, abrasion and local surface shape. A film of water on a wet surface ensures an air film is maintained and the droplet will bounce.     

A review of the effects of humidity, humectants, and surfactant composition on the absorption and efficacy of highly water-soluble herbicides

It is well known that environmental conditions have an important influence on herbicide efficacy. In particular, the effect of humidity on herbicide uptake has been attributed to changes in cuticle hydration and droplet drying. As early as the 1950s, it was hypothesized that humectants such as glycerol would enhance herbicide uptake by not letting droplets dry, thus maintaining the herbicide in solution, and hence making it available for uptake. Shortly thereafter, evidence was found to support this hypothesis and humectants were used successfully in warm, dry areas to increase herbicide efficacy. However, by the mid-1980s, there was little use of humectants as research on humectants gave way to investigations on the effect of ethylene oxide (EO) content on surfactant performance to improve herbicide uptake and efficacy. While ethoxylated surfactants effectively increase the uptake of both lipophilic and hydrophilic herbicides, the suggestion that long EO chains have humectant properties is misleading, since the studies that led to this suggestion were performed at high humidity, which would prevent rapid droplet drying. Furthermore, current evidence suggests that highly water-soluble, ionic herbicides may be more sensitive to low humidity and rapid drop drying than lipophilic herbicides. Therefore, an overview is presented on the interaction of water-soluble herbicides with surfactants, the cuticle, and humidity, with particular emphasis on the impact of low humidity and humectants on herbicide uptake. It was found that when one focuses on research performed at low humidity the importance of humectants emerges, which is not in keeping with what is now commonly accepted.     

THE INFLUENCE OF SIZE OF SPRAY DROPLETS ON THE HERBICIDAL ACTIVITY OF DIQUAT AND PARAQUAT

Summary. Single droplets ranging in size from 250 to 1000 μ diameter and concentrations of diquat and paraquat over the range 0–09–0.75% ion were examined for herbicidal activity. Size of droplet and concentration of herbicide in the droplet were shown to have a marked influence on activity. An increase in droplet size above 250 μ increased herbicidal efficiency. An optimum was reached between 400 and 500 μ while activity fell off with a further increase to 1000 μ.     

Factors Influencing Aerial Insecticide Application to Forests

A successful aerial insecticide application is one that provides the desired degree of pest control at an economic cost, with little environmental impact. This paper discusses a broad range of factors that affect treatment efficacy and environmental impact from aerial insecticide application to forests. Efficacy is affected by parameters such as the choice of active ingredient and its application rate, the volume application rate, tank mix characteristics, the spray droplet size spectrum, and the timing of, and meteorological conditions during an application. Environmental impact is influenced by aspects such as active ingredient specificity, meteorological conditions during the application, avionics use and buffer zone width. Key differences between insecticide applications in forestry and agriculture are pointed out.     

Some Thoughts on the Concept of ULD (Ultra Low-Dosage) Spraying

Applications at ultra-low dosage and volume with small, uniform and non volatile droplets were developed when it appeared that their effectiveness was based on a redistribution of the active ingredient on the plant tissues originally not covered by the compound. Preference has been given to a droplet size of 70 microns provided by rotating devices. Volumes of 1 to 8 lit./ha. seem sufficient to control fungi and less mobile insects and 0,1 to 1 lit./ha. for mobile insects. However, dosages can still be reduced according to the strict requirements.
The drift hazards of non volatile compounds appear to be less than those existing for aqueous preparations. A careful study of the formulations could further reduce the dangers of environmental contamination and it appears that ultra low-dosage applications merit special attention.     

The influence of molasses on the physical properties and sprayability of two pesticide formulations

The effects of molasses on some of the physicochemical properties and on the sprayability of a carbaryl wettable powder (w.p.) and triazophos emulsifiable concentrate (e.c.) were studied in the laboratory and the field. When sprayed in the laboratory on bean leaves, the deposit obtained from either formulation decreased as the percentage of molasses was increased. In an aerial application field trial on cotton, 10% molasses gave a smaller deposit of active ingredient than obtained in the absence of additive, while 20% molasses gave a greater deposit, with either formulation. Molasses had an adverse effect on some properties of the spray mixtures, which resulted in lower deposits when spraying was done at high humidity in the laboratory. This effect was offset when spraying was done at lower humidity in the field, because molasses improved the impaction of droplets by increasing their density and by preventing the complete evaporation of small droplets.     

飞防助剂对植保无人飞机喷施除草剂雾滴分布的影响

通过仪器分析结合田间试验方法, 研究了不同飞防助剂对3种剂型除草剂沉积率?雾滴粒径?物理指标及田间药效, 系统地分析飞防助剂对植保无人飞机喷施除草剂的增效作用及增效机理?结果表明, 4种飞防助剂均有显著的增效作用?其中迪翔对3种除草剂的作用效果最为显著, 可降低雾滴谱相对宽度至0.62, 使雾滴均匀分布; 增加沉积率, 可最多增加32.2百分点; 同时可显著降低3种除草剂的药液表面张力, 增加药液黏度及扩展直径, 延长其干燥时间?迈飞对乳油作用效果较好, 在增加药液扩展直径方面优于其他3种飞防助剂?总体比较4种飞防助剂对除草剂的增效作用, 大小依次为迪翔>迈飞>烈鹰>Greenwet 3710; 3种剂型除草剂中, 飞防助剂对乳油增效作用最显著, 增效可达23百分点?     

Botrytis cinerea and its Resistance to Dicarboximide Fungicides1

In all vineyards in the Pfalz area (FRG) where the dicarboximide fungicides have been applied, resistant isolates can nowadays be found. These do not differ from the sensitive ones in uptake of the active ingredient of the dicarboximides or in the number of nuclei in spores or hyphal cells. The percentage of resistant strains decreases in the population when there is a long gap between applications of the fungicides. As a possible explanation, we have observed tendencies to resensitization in all resistant isolates. The degree of resensitization varies according to isolate.     

Relationship between the Spray Droplet Density of Two Protectant Fungicides and the Germination of Mycosphaerella fijiensis Ascospores on Banana Leaf Surfaces

The effect of fungicide spray droplet density (droplet cm^-2), droplet size, and proximity of the spray droplet deposit to fungal spores was investigated with Mycosphaerella fijiensis ascospores on the banana (Musa AAA) leaf surface for two contact fungicides: chlorothalonil and mancozeb. When droplet size was maintained at a volume median diameter (VMD) of 250 μm while total spray volume per hectare changed, M. fijiensis ascospore germination on the leaf surface fell below 1% for both fungicides at a droplet deposit density of 30 droplet cm^-2. At a droplet deposit density of 50 droplet cm^-2, no ascospores germinated in either fungicide treatment. When both droplet size and droplet cm^-2 varied while spray volume was fixed at 20 litre ha^-1, ascospore germination reached 0% at 10 droplet cm^-2 (VMD=602 μm) for both fungicides. At lower droplet densities (2–5 droplet cm^-2 VMD=989 μm and 804 μm respectively), ascospore germination on the mancozeb-treated leaves was significantly lower than on the chlorothalonil-treated leaves. The zone of inhibition surrounding a fungicide droplet deposit (VMD=250 μm) on the leaf surface was estimated to extend 1·02 mm beyond the visible edge of the spray droplet deposit for chlorothalonil and 1·29 mm for mancozeb. The efficacy of fungicide spray droplet deposit densities which are lower than currently recommended for low-volume, aerial applications of protectant fungicides was confirmed in an analysis of leaf samples recovered after commercial applications in a banana plantation. Calibrating agricultural spray aircraft to deliver fungicide spray droplets with a mean droplet deposit density of 30 droplet cm^-2 and a VMD between 300 and 400 μm will probably reduce spray drift, increase deposition efficiency on crop foliage, and enhance disease control compared to aircraft calibrated to spray finer droplets. © 1997 SCI.     

Spider webs are efficient collectors of agrochemical spray

Spiders are important arthropod predators in natural and agricultural environments. This laboratory study aims to determine the uptake of agrochemical spray by orb webs. Orbs of Araneus diadematus were exposed under controlled conditions to sprays of variable pressure, volume, forward speed and droplet size. Using a tracer dye we determined, by photospectrometry, the amount of spray collected under different conditions which enabled us to calculate the exact volume collected by webs. The spraying treatment simulated four different field practices as well as drift to the field edge. In all cases webs were excellent collectors of spray. Regarding the pure area of the threads, they collected one order of magnitude more of the spray than paper strips did. Moreover, webs were especially efficient in collecting small droplets. We conclude that orb webs (and the spiders who ingest them) are severely exposed to agricultural sprays. Furthermore, due to the uptake of drift, even webs in hedgerows and at field margins may receive high levels of pesticides. The risk of exposing these beneficial arthropods to spray might be reduced by better control of droplet size.     

雾滴大小与施药液量对草甘膦在空心莲子草叶片沉积的影响

为提高草甘膦防治空心莲子草Alternanthera philoxeroides时药剂的有效利用率,用丽春红S为示踪剂研究了草甘膦药液在空心莲子草叶片的沉积特性。结果表明,用体积中径(VMD)149.5~233.7 μm的雾滴喷雾,草甘膦在空心莲子草叶片上的沉积量在体积中径为157.3 μm时最多,随着雾滴体积中径增大,沉积量减少。雾滴体积中径157.3 μm与施药液量339 L/hm2处理的沉积量是雾滴体积中径233.4 μm与施药液量694.5 L/hm2处理的1.54倍。施药液量超过382.5 L/hm2时,草甘膦药液的流失明显增多。800 mg/L草甘膦药液在空心莲子草叶片上的最大稳定持留量约为 4.92 μg/cm2。结果表明,喷雾施药时采用小雾滴和较低施药液量,可大幅度提高草甘膦在空心莲子草上的沉积量。     

The kinetics of insecticide action. Part III: The use of stochastic modelling to investigate the pick-up of insecticides from ULV-treated surfaces by larvae of Spodoptera littoralis boisd

The pick-up of pesticide by lepidopterous larvae walking across ultra-low volume pesticide treated surfaces has been stochastically modelled and simulated using a computer. The duration of the random walk was considered as a series of small, discrete time intervals during which the mode of pick-up varied, depending upon the behavioural state (resting, feeding or walking) of a larva. Estimates can be obtained for the total mass picked up by the larva (gross) and the net amount eliminated, the internal exposure to insecticide, and the proportion of insects knocked down and killed with increasing time. The results of the simulation of Spodoptera littoralis larvae confined to permethrin-treated surfaces were in agreement with experimental data, and suggested that a high pick-up of insecticidal droplets resulted from adhesion of the pesticide to larvae following encounter. However, the availability of drops at the leaf surface decreased with time, probably as a result of droplet spread and subsequent uptake by the cuticular waxes. Droplet stability on leaf surfaces may be related to droplet size.     

Surfactant‐induced deposit structures in relation to the biological efficacy of glyphosate on easy‐ and difficult‐to‐wet weed species

BACKGROUND: Typical active ingredient (AI) residue patterns are formed during droplet drying on plant surfaces owing to the interaction of spray solution characteristics and leaf micromorphology. Currently, comparatively little is known about the influence of AI deposit patterns within a spray droplet residue area on the penetration and biological efficacy of glyphosate. A scanning electron microscope with energy dispersive X‐ray microanalysis has been used to characterise residue patterns and to quantify the area ultimately covered by glyphosate within the droplet spread area. RESULTS: The easy‐to‐wet weed species Stellaria media L. and Viola arvensis L., as well as the difficult‐to‐wet Chenopodium album L. and Setaria viridis L., differing in their surface micromorphology, have been used. Rapeseed oil ethoxylates (RSO 5 or RSO 60) were added to glyphosate solutions to provide different droplet spread areas. Addition of RSO 5 enhanced droplet spread area more than RSO 60, and both caused distinct glyphosate residue patterns. The biological efficacy of treatment solutions showed no significant correlation with the area ultimately covered by glyphosate. CONCLUSION: The results have implications on herbicide uptake models. This study shows that droplet spread area does not correspond to the area ultimately covered by glyphosate, and that the latter does not affect glyphosate phytotoxicity. Copyright © 2009 Society of Chemical Industry     

A Rapid Method of Calculating the Downwind Distributions from Aerial Atomisers1

There are many diverse views about the effects of wind on aircraft sprays. Some operators wish to spray only when the air is still, others will not spray unless the wind speed is at least 3 m/s. The basis of this difference is the fear of, as against the use of, the downwind movement of droplets. This note uses a simple model of droplet behaviour in a wind to try to predict the deposit distributions downwind of a spraying aircraft. The results demonstrate that the level of turbulence caused by the wind blowing over the earth's vegetation reduces the downwind distance most small drops travel before being captured. Trees create more turbulence than cereal crops, which create more turbulence than bare earth or short grass. The deposit density at a given distance downwind reduces with decrease in aircraft flying height, wind speed and smoothness of the canopy. It remains roughly constant with increasing droplet size until a critical diameter is reached when it falls rapidly with increase in droplet size. This critical size depends upon windspeed and turbulence intensity. A downwind buffer zone between the spray area and a sensitive crop is desirable and its minimum size can be determined using the techniques discussed in the paper.