G.P. FARINÓS, M. De la POZA, P. HERNÁNDEZ-CRESPO, F. ORTEGO and P. CASTAÑERA
CSIC, CIB, Dpto Biología de Plantas, Velázquez 144, 28006, Madrid, Spain

The greater specificity of Cry1Ab toxin with respect to conventional insecticides to control corn borers, and the likely reduction of the latter under a transgenic crop system should result in a more favourable environment for the non target-arthropods in the crop. However, it has been reported from laboratory experiments that Bt-maize may have a negative impact on some natural enemies by a combined effect of Bt exposure and nutritional deficiency caused by a reduction in the quantity and quality of their food supply. Laboratory experiments are very useful to point out possible deleterious effects of Bt-maize, but they must be verified with field experiments.
In 2000 we have started a three years field-study to assess, in a commercial field (about 5 ha) near Madrid (Spain), the potential impact of Bt-maize on the abundance and diversity of non-target arthropods. Three treatments with three replicates (= 0.6 ha/plot) were arranged in a completely randomised block design: Bt-maize (cv. Compa CB) was compared with the isogenic cv. Dracma under conventional farm practices, with or without Imidacloprid treatment. Two sampling techniques (visual counts and pitfall traps) were used to compare their effects on non-target arthropods that comprise maize-based food webs from late June to the end of September.
Visual surveys reveal that Orius spp. and spiders were the most abundant predators on the maize plants. A common pattern of the frequency of Orius on plants was found in both years, with few specimens at the beginning of the summer and a big increase in September. Conversely, spiders were present in the plant all through the summer.
Ground beetles and spiders were the most abundant poliphagous predators found in pitfall traps in all treatments. Five carabid species accounted for 98% of the total beetles collected. Similarly, six species of spiders accounted for 97% of their total number.
No detrimental effects associated with the transgenic cultivars have been detected so far, although no firm conclusions can be drawn until these studies are completed.