Impact of farm-scale Bt maize on abundance of predatory arthropods in Spain

Transgenic maize expressing the Cry1Ab toxin from Bacillus thuringiensis (Bt-maize) has been grown in Spain since 1998, representing about 5% of the total maize area. As part of a Spanish specific monitoring program for Bt maize, a farm-scale study was initiated in 2000 to assess the potential impacts of Bt maize on predatory arthropods. The trials were conducted at two maize growing areas (Lleida and Madrid) over 3 years. In each locality three treatments (each with 3-4 replicates of 0.4-0.7 ha) were arranged in a completely randomised block design, Bt-maize (cv. Compa CB) being compared with the isogenic cv. Dracma under conventional farm practices, with or without the insecticide (imidacloprid) seed treatment. Predator abundance was monitored from late May to mid September in Lleida and from mid June to mid September in Madrid by visual surveys and pitfall traps. Anthocoridae, Coccinellidae, and Araneae represented about 90% of the total number of predators recorded in visual samplings whereas Carabidae and Araneae were the two prevalent predator groups (85-90% of the total predators) collected in pitfall traps. Their abundance varied from year to year and between locations, but no clear tendencies related to Bt maize was recorded. Insecticide treatment reduced anthocorid numbers occasionally but no consistent effects were found for the rest of predators. Our data suggest that Bt maize is compatible with the naturally occurring predators.