It has been reported that insecticide-detoxifying enzymes such as glutathione S-transferases (GST) and esterases are affected by microbial infections in hemipteran insect vectors. The total protein content, and GST and α- and β-esterase activities were quantified in ‘Candidatus Phytoplasma ulmi’-infected and uninfected adults of Amplicephalus curtulus Linnavuori & DeLong (Hemiptera: Cicadellidae) at 25, 35, and 45 days after the acquisition access period (AAP) in the head-thorax and abdomen sections. The total protein content was lower in phytoplasma-infected leafhoppers 25, 35, and 45 days after the AAP. Thirty-five days after the AAP, the GST and β-esterase activities had increased (26 and 69%, respectively) compared to the control. However, 45 days after the AAP, the phytoplasma-infected leafhoppers displayed lower GST (87%) and β-esterase (253%) activities than the uninfected individuals. On the other hand, the α-esterase activity proved to be unaffected by the phytoplasma infection. Forty-five days after the AAP, females had a higher phytoplasma titer (46%) in their head-thorax than in their abdomen sections, whereas males showed a higher titer in their abdomens (75%). In addition, the GST and β-esterase activities in the abdomen were affected negatively by 96–98% as a result of the increasing ‘Ca. Phytoplasma ulmi’ titer. These results indicate that an infection of ‘Ca. Phytoplasma ulmi’ alters the metabolic activities of A. curtulus.