Combined effects of a pyrethroid insecticide and azole fungicide on lepidopteran growth analysed with DEB-TKTD modelling

The co-application of insecticides and fungicides is common in crop protection. Several studies in bees have identified synergism between pyrethroid insecticides and azole fungicides for mortality and sublethal endpoints. However, there are no studies that detail this synergism in Lepidopteran species. Here, we conduct a mixture exposure with cypermethrin and the fungicide prochloraz to assess their joint effects on mortality and sublethal endpoints for the moth Mamestra brassicae. The effects of pesticide exposure on sub-lethal endpoints over the life-cycle was simulated using energy budget TK/TD models from the DEBtox family. The model describes how organisms acquire and use energy for maintenance, growth, and reproduction, and how toxicants impact these processes. The approach could reliably simulate growth and survival of control and exposed moth larvae. The threshold for effects on survival was 0.92 mg/kg for cypermethrin and 2.78 mg/kg for prochloraz, and the threshold for energy budget (growth) was 1.7 10− 5 mg/kg for cypermethrin and 0.002 mg/kg for prochloraz. Prediction of mixture effects using additive assumptions frequently underestimated observed effects, indicating a pattern indicative of synergism, especially in lower concentration cypermethrin exposures. The success of the overall experimental and modelling approach, support the further applications of the methods and models used here for pesticide testing for other lepidopteran species. This approach may help improve the ecological risk assessment for pesticide mixtures.