An auxin acid-catalyzed stable microemulsion pesticide formulation is provided that is clear and stable under a wide range of temperature conditions encountered during distribution, storage, and use of the pesticide formulation. A precursor for the microemulsion includes an auxin ester, a protox inhibitor, and an auxin acid stabilizing agent as active ingredients. Inactive ingredients in the precursor include a microemulsifier, a stabilizing co-emulsifier, a dispersant, and a solvent present in amounts such that when the precursor is added to water of dilution to form a concentrate or a ready-to-use formulation, the clear and stable microemulsion that is spontaneously formed has oily active ingredient-containing nano-sized particles no larger than about 1μ dispersed in the water.