[Background] Herbivore injury has a direct effect on the growth and performance of host plants through photosynthetic suppression.However,changes in photosynthetic activity of host plants affected by mutualism between honeydew-producing hemipterans and tending ants remain poorly understood.[Method] The effects of interaction between an invasive mealybug Phenacoccus solenopsis and its tending ant Paratrechina longicornis on chlorophyll fluorescence characters of infested cotton Gossypium hirsutum leaves were observed through a chlorophyll fluorescence measurement system under greenhouse conditions.[Result] P.longicornis numbers increased with P.solenopsis numbers.P.solenopsis numbers with ants were lower than those without ants in the latter period.C hanges in light utilization efficiency were induced by P.solenopsis feeding of infested cotton leaves.After P.solenopsis feeding for 20 d,the light utilization efficiency compared with the control was reduced by 37.0%and 53.5%for without-and with-ant treatments,respectively.Changes in maximum relative electron transport rate were also induced by P.solenopsis feeding,and the influence was more obvious at the end of the experiment or with tending ants.However,the light saturation coefficient describing the resistant capacity of a sample to glare was not influenced by P.solenopsis with or without ants.[Conclusion and significance] The interaction between P.solenopsis and P.longicornis had negative effects on the photosynthetic activity of cotton leaves,could have been caused by P.solenopsis feeding rate and fecundity stimulated by P.longicornis.