Effect of adjuvants in insecticides in agricultural spraying

Adjuvants improve the performance of insecticides in agricultural spraying, enhance their characteristics and increase efficiency. However, your choice must take into account the needs of the product and the target you are trying to achieve, with the aim of improving the effect on pest control.

Brazilian agriculture is constantly evolving and its main challenge is to produce more and more food, with less environmental impact and without a significant increase in the cultivated area. In this model, the rational and efficient use of agrochemicals is essential, as it reduces reapplications and environmental contamination and improves agricultural spraying.

In Brazil, according to decree no. 4.074/2002, adjuvant means “product used in mixture with products formulated to improve their application”. All meanings of the term adjuvant have in common an improvement in spraying quality, as adjuvants can change the physicochemical properties of the spray, improving wettability, adhesion and spreading of droplets, contributing to better retention and penetration of the spray. active ingredient.

The use of adjuvants, in Brazil, is associated with the sale and use of fungicides, herbicides or insecticides from the same company that recommends them. However, research results have demonstrated that some adjuvants can enhance the efficiency of agricultural products, improving their efficiency, as long as this recommendation is made according to the characteristics of the application, the product to be applied and the biological target.

Adjuvants in agricultural applications

Adjuvants can be present in spray mixtures in two ways: incorporated into the agrochemical formulation (more common in emulsifiable concentrate formulations) or added to the spray tank before application. The classification is based on their activity, such as activators or utilities.

Activating adjuvants improve the activity of the agrochemical, increase the product's penetration rate and aid efficiency. The main ways to improve application quality are to reduce surface tension and retain moisture in droplets on the sheet surface. The main activating adjuvants are surfactants, vegetable oils, methylated oils, mineral oils, silicone derivatives and nitrogen fertilizers.

Utility adjuvants act as facilitators of agricultural spraying, reducing the negative effects of spraying and do not directly influence the efficiency of agrochemicals. They include compatibilizing agents, depositors, dispersants, drift reducers, defoamers, water conditioners, acidifiers, buffers, humectants, ultraviolet ray protectors and dyes. Although they do not have a direct influence on the action of the pesticide, their use can result in greater practicality in application, for example, the use of foam reducers in the water spray tank, which keeps the mixture foam-free while the sprayer is agitated.

Physicochemical characteristics of spray mixtures

For agricultural spraying, water is the basic element. However, due to certain characteristics of its molecule, a film forms on the surface of liquids, which is called surface tension. This same phenomenon occurs in spray droplets, so when there is only water, without the addition of adjuvants, more spherical droplets are formed and have less contact with the leaf, making coverage difficult.

One of the ways to measure surface tension is using a drop shape analyzer (goniometer). Recently, the Federal University of Santa Maria (UFSM) acquired this equipment and has carried out tests with several adjuvants available on the market. Table 1 presents some of these results based on the simulation of a 200 L/ha spray solution. The data obtained shows that the surface tension of water is around 72 mN/m and that some adjuvants can reduce this value.

The influence of surface tension in agricultural spraying mixtures can be seen in figure 1 with wheat leaves. In image A there is only water (71,8 mN/m) and in image B water + organosiliconates (22,8 mN/m). This difference in droplet behavior occurs due to the reduction in surface tension due to the addition of the adjuvant. Comparing the two drops, the more spherical one (image A, without the addition of adjuvants) is more exposed to environmental conditions (temperature and humidity), and has a reduced coverage on the leaf. However, the flatter droplet (image B, with the addition of adjuvant) has better wettability. Analyzing pest control, this drop has superior control potential, due to better protection and greater coverage of the insecticide.

Retention of agrochemicals

The leaf surface, made up of the cuticle, is the first barrier that products need to overcome. The leaf cuticle varies in composition and structure among different plant species. The addition of adjuvants can help products enter the plant or even protect those applied to the leaf surface, resulting in a greater quantity of active ingredient.

In a work by UFSM, carried out together with the University of Bonn, in Germany, through High performance liquid chromatography (HPLC), the amount of the insecticide chlorantraniprole was measured in wheat and corn plants, applied alone and in association with different adjuvants. To this end, an application was carried out with a spray volume of 200 L/ha and then the plants were cut and how much insecticide there was in each of the treatments was quantified.

The results of this work showed that in both corn and wheat, the addition of adjuvants increased the amount of active ingredient in the plants. This greater quantity of product in the plant can result in improved biological efficiency and in some cases can increase the residual period of insecticides.  

Effect of rain on insecticide applications

In the same work, images were taken with a scanning electron microscope, equipment that allows a view of the leaf's epidermis. The images show how insecticides behave alone and in combination with adjuvants when exposed to rain.

 In most of the images it was possible to visualize the crystals of the active ingredient, which indicates that although rain has a great influence on removal, not all of the product on the leaf ends up being removed by the impact of precipitation drops. Wheat leaves presented a greater amount of residue, due to their greater water repellency and their more vertical position, compared to corn leaves. 

The use of adjuvants can help with the quality of spraying, as these products, when used correctly, increase the useful life of agrochemicals and improve their efficiency. The interaction of spray droplets with the surfaces they want to reach continues to be one of the biggest challenges for improving the efficiency of pesticides.

It is currently possible to infer that the use of adjuvants is an important practice in improving the efficiency of agricultural pesticides. Its use helps control pests, as it improves the quality of application and increases the amount of active ingredient absorbed by plants.

 

Adriano Arrué Melo, Jerson Vanderlei Carús Guedes, Jonas André Arnemann, Clérison Régis Perini, Maiquel Pizzuti Pes, Lucas Hahn, UFSM

Article published in issue 206 of Cultivar Grandes Culturas.