Trophic barriers to fertilizer Cd bioaccumulation through the food chain: A case study using a plant-insect predator pathway

The objective of this study was to assess the uptake and subsequent transfe r of Cd and Zn from a soil amended with a single application (150 kg P ha(- 1)) of triple super phosphate fertilizer to wheat plants, aphids, and a pre dator and biocontrol agent of aphids, lacewings. The fertilizer amended soi l and wheat plants grown on this soil had elevated concentrations of Cd com pared to the controls, but similar concentrations of Zn. Aphids feeding on wheat plants on the fertilized soil had between three and seven times the c oncentrations of Cd and Zn observed in aphids feeding on the control plants . However, the lacewings showed no significant accumulation of Cd or Zn, an d no differences in larval performance were recorded. Changes in the availa bility of Cd and Zn in the soils and the transfer through the plant-insect pathway were monitored using isotope dilution, by labeling the soils with c arrier-free Cd-109 and Zn-65. Decreases in the specific activities for Cd i n the plants and aphids were observed for the fertilized soils compared to the controls, suggesting an increase in bioavailable Cd. On the fertilized soils the Cd:Zn ratio of the phloem-feeding aphids (0.008) was significantl y less than the host plants (0.025), indicating a reduced relative uptake o f Cd and a possible barrier for Cd along the soil-plant-herbivorous insect pathway-reducing uptake by phloem feeders and subsequently their predators.