Performance of insect-resistant Bacillus thuringiensis (Bt)-expressing aspens under semi-natural field conditions including natural herbivory in Sweden

The intent with insect-resistant transgenic trees is to increase yield without adverse negative effects on non-target organisms. Thus, the assessment of these products is dependent on not only performance, but also on environmental effects realized in the field. With this study we transition from greenhouse evaluations to assessments under semi-natural field conditions with the objectives: (i) to assess the realized benefits (biomass production and resistance effectiveness) and (ii) to evaluate effects on the biotic interactions of two isogenic transgenic Bt (Bacillus thuringiensis, cry3Aa targeting coleopteran leaf beetles)-expressing aspens. We established an experiment with a total of 108 potted aspen plants (n = 36 for three different lines) in the field and applied an additional experimental nutrient treatment to reflect the effects of one aspect of environmental variability. Overall leaf damage was shown to be higher on the un-modified wild type (Wt) plants compared to plants from the high expression line (P = 0.003), and the same trend was also detected for the low expression line (P = 0.089). However, despite this advantage in resistance, biomass production was not significantly enhanced in insect-resistant plants. Also, whereas the high nutrient treatment showed an increase in biomass, it did not affect the amount of leaf damage inflicted on the plants. Presence/absence surveys of biotic interactions during the study showed that the number of plants that were scored for the presence of three insect orders and two leaf modifications was equal regardless of line treatment. Interestingly, one species included in the presumably targeted order, the leaf rolling beetle Byctiscus populi (Coleoptera) inflicted the insect-resistant plants with oviposition and feeding damage to a similar degree as the un-modified Wt plants.

► Insect resistant transgenic trees convey a resistance advantage in the field.
► Increased resistance did not translate into realized benefits in terms of growth.
► A presumably targeted herbivore caused oviposition damage on insect resistant trees.
► Nutrient availability induced growth of the trees but did not affect leaf damage.