Time-response relationships for the accumulation of Cu, Ni and Zn by seven-spotted ladybirds (Coccinella septempunctata L.) under conditions of single and combined metal exposure

•Cu, Ni and Zn are at first efficiently accumulated by Coccinella septempunctata.•Mechanisms acted to prevent further accumulation of Cu and Ni after a short period.•Treatment with one metal had no effect on the accumulation of the other two.•Time-response relationships were altered by simultaneous metal treatment.•Alteration enhanced accumulation of all three metals after 15 d.

Accumulation, and therefore toxicity, of trace metals in invertebrates may be affected by potential interactive effects that can occur amongst different metallic elements. However, there is little data on the nature and effects of such interactions in terrestrial systems. This work reports the interactions among Cu, Ni and Zn during accumulation by the beetle Coccinella septempunctata. Test animals were treated with 500 mg kg−1 of each metal singularly and in combination for 15 d. The effects of treatment with a single metal had no effect on the baseline concentrations of the other two. Time-response relationships for Cu and Ni after treatment with one metal were curvilinear, demonstrating that the metals were initially accumulated, but after ∼8 d regulatory mechanisms became effective. This resulted in decreasing concentrations in test animals despite continued treatment. In contrast, the time-response relationship for Zn was linear. Treatment with metals in combination markedly altered the time-response relationships with all three metals showing a linear trend and the slope of the Zn relationship increasing significantly. After 15 d of exposure this had the effect of increasing the metal concentration in animals exposed in combination compared to those exposed singularly by 144% to 38.3 mg kg−1 for Cu, 141% to 27.5 mg kg−1 for Ni and 55% to 311 mg kg−1 for Zn. For all metals, differences amongst treatments were significant, indicating that inter-element interactions can enhance the concentration of trace metals in C. septempunctata.