Sabry, H., W.A.Z. El-Medany, H.S. El-Tahawe and E.M. Abd El-Azeem. 2023. Biochemical Evaluation of Acremonium sp. as Biological Control Agent Against the Spiny Bollworm, Earias insulana by Scanning Electron Microscopy. Arab Journal of Plant Protection, 41(2): 140-145. https://doi.org/10.22268/AJPP-41.2.140145 Ultrastructure and physiological effects of the biological control agent Acremonium sp. on larvae of the spiny bollworm (SBW) Earias insulana (Boisduval) (Lepidoptera: Noctuidae) were investigated. The isolate of Acremonium sp., EZ1 (MN25101) was applied on the 4 th instar larvae and pupae of the spiny bollworm E. insulana, with two ml spore suspension of different concentrations of 6x106 , 6x107 and 6x108 spores/ml mixed with four gm of artificial diet. Scanning electron microscopy (SEM) was used to investigate sporulation potential and the extent of damage to the growth rate of Acremonium sp. on the exoskeleton of E. insulana larvae and pupae five days after inoculation, as compared to the untreated control. Treated larvae showed varied level of cuticle damage. Surface of infected pupae showed varied stages of mycelial growth. The biochemical parameters investigated were: carbohydrate hydrolyzing enzymes activity (trehalas, invertase, and amylase), the total soluble protein, acetyl choline esterase, aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) activities. High significant reduction in amylase, trehalase and ALT activities in treated larvae was observed. On the other hand, high significant gradual increase in the amount of total soluble protein and the acetyl choline esterase activity were observed. At the same time AST activity was slightly increased, whereas invertase activity was not significantly different from the control. Scanning electron microscopy clearly showed the ability of Acremonium sp. fungus to damage the cuticle of both larvae and pupae of E. insulana by spore's germination. In addition, it disturbed the activity of many important enzymes, thus it can play a vital role in the control of the target pest in a safe manner without polluting the environment. Keywords: Earias insulana, Acremonium sp., scanning electron microscopy, AST, ALT.