The impacts of different doses of the plant growth regulator gibberellic acid (GA(3)) in diet on the number of total and differential hemocytes, frequency of apoptotic, and necrotic hemocytes, mitotic indices, encapsulation, and melanization responses were investigated using the greater wax moth Galleria mellonella L. (Lepidoptera: Pyralidae) larvae. Total hemocyte counts increased in G. mellonella larvae at all treatment doses whereas GA(3) application had no effect on the number of different hemocyte types. The occurrence of apoptosis, necrosis and mitotic indices in GA(3) treated and untreated last instars were detected by acridine orange or ethidium bromide double staining by fluorescence microscopy. While the ratio of necrotic hemocytes increased at all GA(3) treatments, that of late apoptotic cells was only higher at doses >200 ppm when compared with untreated larvae. The percentage of mitotic index also increased at 5,000 ppm. Positively charged DEAE Sephadex A-25 beads were used for analysis of the levels of encapsulation and melanization in GA(3) treated G. mellonella larvae. At four and 24 h posttreatments with Sephadex A-25 bead injection, insects were dissected under a stereomicroscope. Encapsulation rates of larval hemocytes were dependent on the extent of encapsulation and time but not treatment groups. While the extent of melanization of hemocytes showed differences related to time, in general, a decrease was observed at all doses of GA(3) treated larvae at 24 h. We suggest that GA(3) treatment negatively affects hemocyte physiology and cell immune responses inducing cells to die by necrosis and apoptosis in G. mellonella larvae.