Investigating the effect of starvation and various nutritional types on the hemocytic profile and phenoloxidase activity in the Indian meal moth Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae)

The defense mechanisms of the insects are based on involvement of the hemocytes and phenoloxidase. Hemocytes are the basic component of the cellular immunity and phenoloxidase as the part of prophenoloxidase (PPO) cascade is the component of both humoral and cellular defense. Nutrition as well as starvation and attack by any organisms can modify these parameters of the innate immunity. In the current study, the effects of the stresses imposed by the starvation or different types of diets were investigated on the important immunity aspects of the indian meal moth larvae. Results showed a decline in the total hemocyte count in hemolymph with the increase in the starvation duration. In the first test, 5th instar larvae were starved for three time intervals including 24, 48, and 72 h and then, the changes in hemocyte number and phenoloxidase activity were studied. In the second experiment, the larvae bred on four diets including diet (A) walnut, diet (B) pistachio, diet (C) pea and raisin, and diet (D) artificial diets were used. The total number of the hemocytes and percentage of each hemocyte were also considered. Larvae were kept in an incubator set at a temperature of 25 ± 1 °C with 45 % of relative humidity (RH), and a constant photoperiod of 14:10 h (L:D) during the tests. The number of the plasmatocytes, one the main immune cells was sharply decreased with prolongation of the starvation duration and finally, their number reached by 134.04 ± 25.25 mm3 of hemolymph. The number of the granulocytes was also decreased significantly 72 h post-starvation than other treatments. The prohemocytes as the stem cells were initially increased within 24 h, and they were decreased later. The oenocytoids as the key cells involved in the phenoloxidase activity were initially increased significantly within 24 h of starvation compared to the control, but they were decreased significantly after 48 and 72 h reaching the same amount as the controls. Results revealed that the types of the consumed diet influenced the number of cells and phenoloxidase activity. The highest total hemocyte count was related to the diet (C) pea and raisins (2158.18 ± 172.5 mm3), and the lowest was observed in the larvae fed on the pistachios (924 ± 78.33 mm3). The number of plasmatocytes, granulocytes, and oenocytoids was the highest in those larvae fed on the diet (C) pea and raisin and diet (D) artificial diet, respectively but the lowest numbers were observed for other treatments. The number of prohemocytes in the larvae fed on different diets did not differ significantly. The phenoloxidase activity was significantly reduced in the fifth instar larvae following starvation. The highest activity of phenoloxidase in feeding treatments was observed in those larvae fed on the artificial diet while the lowest activity was observed in the pistachio-fed larvae. Thus, the amount and type of the diet and the stresses including starvation can determine the immune response of the insects against the entomopathogens.