The solutions of essential oil (0.5%, 0.8%, and 1.0% w/v) derived from Achillea millefoilum (EOAM) were evaluated for their repellent and insecticidal activity toward Acrobasis advenella, as well as their effects on the activity of selected larval tissue enzymes. The chemical composition of the EOAM analyzed by GC-MS showed that the main compounds included β-pinene, chamazulene, eucalyptol, and β-caryophyllene. Selection tests showed that larvae avoided inflorescences treated with 0.8% and 1.0% EOAM concentrations. The mortality of larvae increased with concentration and time of exposure to the EO, and the highest mortality rate was observed after 72 h. In addition, the impact of EO was examined on the activity of catalase (CAT), polyphenol oxidase (PPO), peroxidase (POX), α- and β-glucosidases, and acetylcholinesterase (AChE), i.e., important antioxidants, detoxification, digestive, and nervous system enzymes. A significant increase in CAT activity was found in larvae treated with 0.8% and 1.0% EOAM after both 24 and 48 h. Initially, treating larvae with 0.5% EO decreased β-glucosidase activity while increasing α-glucosidase activity. Moreover, 0.8% EOAM inhibited the activity of POX. These findings suggest that EOAM can affect important biochemical processes within the tissues of A. advenella larvae, making it a promising agent for the protection of black chokeberry crops.