Which type of adjuvant is designed to increase the viscosity of spray mixtures?

The correct choice is thickener, as this type of adjuvant is specifically formulated to increase the viscosity of spray mixtures. Increasing the viscosity of a spray can enhance the adherence of the pesticide to plant surfaces, improve coverage, and reduce the likelihood of drift during application. Thickeners play a vital role in ensuring that the liquid remains on the target surface longer, promoting better effectiveness of the pesticide.

Sticker adjuvants are primarily designed to enhance the adhesion of the pesticide to the surfaces they are applied to, which can help in preventing wash-off from rain or irrigation but do not specifically increase viscosity.

Extender adjuvants work to prolong the effectiveness of a pesticide application over time, often by slowing down the degradation of the active ingredients, but like stickers, they do not function by altering the viscosity.

Surfactants, although important for improving the spreading and wetting properties of a spray solution, do not increase viscosity. Instead, they reduce the surface tension of the solution, which helps the pesticide to spread more evenly over the target surface.

Thus, thickener is the right choice as it directly contributes to increasing the viscosity of spray mixtures, enhancing the application process and efficacy of the pesticide.