Sampling and selection of butterfly indicators for general surveillance of genetically modified maize in north-east Spain
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Genetically modified (GM) maize has been cultivated commercially in Spain since 1998. Although long-term environmental monitoring to detect unexpected environmental effects of GM crops (General Surveillance, GS) is compulsory in the EU, GS currently has a very low capacity to detect adverse effects on the environment. This study aimed to increase the feasibility of GS of GM maize expressing insect-resistance (Bt) and herbicide tolerance (HT) traits by using butterflies as models. Butterflies (Lepidoptera: Papilionoidea) were sampled using transect-counts in three differentiated maize-growing regions in north-east Spain. Five transects were established per region and sampled three times per season in two consecutive years. Transects were 300 m long, including 100 m sections in field margins, alfalfa (Medicago sativa) and non-crop vegetation. In addition, butterfly larvae were sampled during maize anthesis in field margins in Lleida region and distribution of larval host plants in maize agroecosystems was assessed in the three regions. Field data and literature were used to construct a step-by-step selection process to identify appropriate butterfly indicators for monitoring effects of GM maize cultivation. In addition, suitable multispecies indicators were constructed. The required sampling effort to detect effects using these butterfly indicators was estimated by prospective power analysis. We identified 41 butterfly species, including three protected species. Most species were potentially exposed to GM maize cultivation effects because their larval host plants were present in maize fields, margins and neighbouring habitats. We identified larvae of four butterflies in maize field margins, the most abundant of which was Carcharodus alceae. It would be possible to detect a 30% population change by sampling its host plants Malva spp. in 35 to 95 site pairs. When we applied the selection procedure, the most appropriate species for monitoring depended on the region considered. Across regions, the sampling effort using selected indicators was lowest for multispecies groups (i.e. 15–32 site pairs for butterfly abundance) and for the single species Pieris napi and Polyommatus icarus (24–84 and 27–87 site pairs respectively). These indicators could be monitored through existing butterfly monitoring schemes as part of a wider environmental monitoring in agricultural regions to assess impacts of agri-environmental management.