Combination of functional plants conserves predators, repels pests, and enhances biological control of Aphis spiraecola in apple orchards

Functional plants possess the ability to attract natural enemies or repel pests, and they play an important role in ecological regulation. While most studies have focused on the impacts of individual plants or plant combinations with similar functions on pests, there is a scarcity of research on the effects of combining plants with different functions, particularly in the case of orchard pests. In this study, we combined plants (Cnidium monnieri (L.) or Brassica napus L.) that attract predators with plants (Ocimum basilicum L. or Mentha haplocalyx Briq.) that repel pests to determine if these combinations can enhance biocontrol of Aphis spiraecola Patch on apple trees. All four combinations reduced the A. spiraecola population on apple trees. B. napus & M. haplocalyx achieved the highest reduction rate (88.58%), followed by C. monnieri & O. basilicum (88.06%), B. napus & O. basilicum (86.11%), and C. monnieri & M. haplocalyx (79.41%). Cage exclusion tests on 29 June and 4 July 2023 showed that biocontrol services index of all combinations was significantly higher than that of the control. Rubidium marking test showed that 61.90%–71.43% of Chrysoperla sinica (Tjeder) individuals in trees had transferred from functional plants. Behavioral responses and field trapping indicated that compounds such as cis-citral, nepetalactone, caryophyllene, beta-pinene, and eugenol, which are present in repellent plants, had significant repellency against A. spiraecola. The use of functional plant combinations in apple orchards can enhance biocontrol of A. spiraecola by promoting predators and by repelling aphid. This supports the potential of functional plant combinations in agroforestry ecosystems for pest biocontrol.