Flea beetle (Chaetocnema tibialis Illiger) causes big damage on young sugar beet crop in hot and dry years: biting holes in the leaves leads to plant decay and reduced plant population. Treatment of seeds with neonicotinoid insecticides gave satisfactory results, but due to the possible negative effect on bees and development of resistance, these insecticides were banned on 27 April 2018 by the European Commission for all outdoor agricultural crops. Therefore, it was necessary to check the sensitivity of sugar beet flea beetle to insecticides that are licensed for use in the Republic of Croatia: chlorpyrifos, lambda-cyhalothrin and thiacloprid. Flea beetles were collected at nine locations in eastern Croatia. The experiments were performed according to methods of IRAC 011 (pyrethroids), IRAC 021 (neonicotinoids) and IRAC 025 (organophosphorus insecticides). The effectiveness of the recommended dose, double dose and 1/5 of the recommended dose of insecticides was determined in four repetitions. There are differences in efficacy between the active substances used, chlorpyrifos has the best efficacy. The pest has developed a certain degree of resistance to thiacloprid and lambda-cyhalothrin. The protocols used to assess the degree of resistance do not involve statistical analysis of the data and may lead to misinterpretation of the results. Populations categorized as resistant were subjected to statistical analysis and it was found that the reduced efficiency was not statistically significant. The methods need to be checked, and field research should be conducted regularly, as this is the basic tool that can detect the beginning of the development of resistance in a timely manner and implement anti-resistance strategies in a timely manner.