Different pistachio cultivars impair hemocyte frequencies in diapausing and nondiapausing larvae of pistachio seed chalcid, Megastigmus pistaciae (Hymenoptera: Torymidae)

Cellular immunity with the activity of hemocytes is one of the important phases of innate immunity in insects. The role of hemocytes is key in dealing with the stresses and contaminations introduced to the insect hemolymph. These cells react by changing their number, shape, and type during stress. Identifying hemocytes and their diversity is the first step in immunological studies. Humoral defense usually starts with a little delay after cellular immunity, which is associated with the activity of phenol oxidase and antimicrobial peptides. The present study was conducted to investigate the immunology of diapausing and non-diapausing larvae of Megastigmus pistaciae fed on different varieties of pistachio. The pistachio seed chalcid, M. pistaciae, has been one of the key and destructive pests of pistachio in recent years. After collecting infected fruits and transferring them to the laboratory, hemolymph of larvae was extracted and hemocytes were identified after staining with Giemsa solution. Four types of hemocytes were seen in the hemolymph of larvae: prohemocytes, plasmatocytes, granulocytes and oenocytoids. The morphometric sizes of the cells were determined by light microscopy. Differential hemocyte count in hemolymph of larvae, pupae, and adult was determined. The frequency of granulocytes in the larvae was the highest (more than 35±2.4%) and higher than the frequency of granulocytes in pupa and adult. In the study of changes of blood cells in diapausing and non-diapausing larvae feeding on different cultivars, results showed that the hemocyte density of non-diapause larvae that fed on all cultivars was significantly higher than the number of diapause larval hemocytes. In addition, the total hemocyte count, granulocytes, oenocytoids, and prohemocytes in larvae which fed on Kaleghochi and Shapasand cultivars was higher than similar cases on larvae fed on Khanjari, Akbari and Nokhodi cultivars. Changes in phenol oxidase activity in different treatments were similar to changes in blood cells. On the other hand, the activity of this enzyme was higher in non-diapausing larvae than diapausing larvae and more than on larvae that fed on Kaleghochi and Shapasand cultivars as compared with other cultivars. Identification of hemocytes and their changes in diapausing and nondiapausing of pistachio seed chalcid, M. pistaciae, by feeding on different pistachio cultivars, is done for the first time and could be the basis for further studies to identify the immunological characteristics of this pest. It seems that diapause and type of the diet can determine the immune response of the insects against the entomopathogens. Additional research in this field can be effective in the microbial control methods of this pest.