Which type of adjuvant increases the viscosity of spray mixtures?

Thickeners are designed specifically to increase the viscosity of spray mixtures. By enhancing viscosity, they help in reducing drift and improving the deposition of the spray on target surfaces. This is particularly important in agricultural applications where achieving coverage on the intended areas can lead to better efficacy of the pesticide or herbicide being applied.

In contrast, extenders are used to increase the volume of the spray mixture without affecting its viscosity, while stickers create a film that helps in adhering the pesticide to the target surface, enhancing its effectiveness over time. Plant penetrants, on the other hand, are meant to facilitate the uptake of the active ingredient into plant tissues, increasing its absorption but not necessarily affecting the viscosity of the mixture.

Thus, thickeners are the correct choice as they fulfill the specific role of modifying the physical properties of the spray solution to optimize application performance.