[Objectives]  The cotton bollworm, Helicoverpa armigera (HÜbner), is one of main pests of peanut crops in Shandong Province. The widespread planting of Bt cotton at the end of 1990’s has effectively controlled the cotton bollworm in peanut fields. However, as planting of Bt cotton has gradually decreased in recent years, cotton bollworm populations in peanut fields are once again rising yearly. [Methods]  Numbers of adults, eggs and larvae of H. armigera in peanut fields in four county-level cities (Longkou, Zhaoyuan, Laiyang and Wendeng where no Bt cotton is planted), were monitored and trends in the cotton bollworm population between 2000 and 2014 analyzed. [Results]  There is a significant negative correlation between the area planted in Bt cotton and the occurrence of cotton bollworms in peanut fields, and between the area of Bt cotton and the amount of cotton bollworm eggs and larvae. [Conclusion]  Reduction in the planting of transgenic cotton is a major factor contributing to the increase in cotton bollworms in peanut fields. In addition, with the decline in planting of Bt cotton, the area infested by cotton bollworms in corn and peanut fields has increased yearly. If no effective measures are taken, there may be future outbreaks of the cotton bollworm.