Millardet is chiefly remembered for his work dealing with plant pests.

In the 1860s the vineyards of France were infested by the destructive Phylloxera, an aphid-like pest inadvertently introduced to Europe from the United States.

Millardet and fellow botanist Jules Émile Planchon (1823-1888) controlled the infestation by using American grape vines that were resistant to Phylloxera as grafting stock.

Munson, was instrumental in identifying and provisioning the American rootstock that was resistant to Phylloxera and suitable for French growing conditions.

He was also responsible for protecting grape vineyards from downy mildew fungus (Plasmopara viticola).