Abstarct: Cabbage willow with the scientific name of Plutella xylostella (Lepidoptera: Plutellidae) is one of the important pests of the family of Brassicaceae or Cruciferae, which annually causes severe damage to cabbage varieties such as cauliflower, Cabbage, Broccoli, Kohlrabi, as well as Rapeseed, Turnip, Mustard, Garden radish and Garden cress. Control of this important pest in our rapeseed fields is mainly done using various chemical methods and the pathogenic bacterium Bacillus thuringiensis. Studies on the interaction between pathogens and insect defense factors have been growing significantly in recent years because the results of this research can be effective in adopting appropriate microbial control methods against fruitful pests. Insect hemolymph contains the most important defense components, namely hemocytes. Blood is involved in the transport and storage of food and defense against microorganisms. The success of the immune system depends on the number and type of blood cells. Sometimes the good functioning of blood cells prevents the development of infection in the body. In the present study, the effect of B.thuringiensis pathogenicity and temperature stress on homocyte abundance and phenol oxidase activity was investigated. First, the sub-lethal concentration of bacteria was determined using a bioassay test. For this purpose, 300 larvae of cabbage willow in 4 replications with canola leaves infected with different concentrations of bacteria including 20, 30, 50, 100 and 200 ppm were placed in petri dishes and larval mortality was recorded after 24 hours. Immunological tests were performed after determining the concentration of 40 ppm as the sub-lethal concentration. In the first experiment, rapeseed leaves were treated with sub-lethal concentrations of bacteria and larvae aged 2 , 3 , 4 , 5 blood sample were taken at 12 and 24 hours after feeding on the leaves. In the second experiment, the larvae were exposed to temperatures of 4 and 30 degrees Celsius for 12 hours and temperature stress was examined on their homocyts. The result showed that feeding on canola leaves infected with bacteria after12and 24 hours caused a significant increase in total cells, plasmocytes and granulocytes and phenol oxidase activity compared to the control, which was more significant in larvae that tolerated high temperature stress. Certainly, additional research is needed to be able to evaluate the possible use of microbial control methods against cabbage willow by recognizing the safety characteristics of susceptible and resistant populations.