Toxic honeydew produced by Flatidae Epormenis cestri in Uruguay has been shown to cause among honeybees (Apis mellifera) colonies a massive larva death called “River disease”, but the intrinsic mechanisms are still unknown. Because fat body cells, oenocytes and trophocytes, are known to regulated larvae metabolism, and to be affected by xenobiotics, we tested whether apoptosis of these cells can be an underlying cause of larvae death. Ten colonies were divided into two groups and fed with common honey or toxic honeydew obtained from colonies affected by “River disease”. Five-day-old larvae were collected and processed for histology and immunohistochemistry for caspase-3. The area, diameter, and immunostaining area in oenocytes and trophocytes were measured. The oenocyte and trophocyte cellular area decreased in the treated group (p=0.002; p<0.001 respectively) compared to the control group. The diameter of oenocytes (p=0.0002) and trophocytes (p<0.0001) decreased in the treated group. Caspase-3 was detected in cytoplasm in the control group but in the cytoplasm and nucleus in the treated group. The caspase-3 immunostaining area increased in oenocytes (p<0.002) and trophocytes (p<0.0001) of the treated group. The ingestion of toxic honeydew altered the morphology, localization and immunoexpression of caspase-3 in fat body cells, which suggests that the deregulation of the apoptotic mechanism affected the normal development in A. mellifera larvae.