A convertible glasshouse was established to study annual transgenic plants under near-field environmental conditions while simultaneously ensuring a high level of biological containment. This system can provide a useful step in the assessment of transgenic plants prior to open-field experiments. Two transgenic wheat lines (cv. Bobwhite) were investigated and compared with their corresponding non-transformed wildtypes with respect to plant performance, expression of the transgenic trait and interactions with antagonists. The first line expressed snowdrop lectin [Galanthus nivalis agglutinin (GNA)] for enhanced resistance to aphids, and the second one overexpressed the endogenous Lr10 gene to enhance resistance to leaf rust. Interestingly, 1000-kernel weight of Lr10-transgenic plants was significantly reduced, indicating that the overexpression of the Lr10 gene caused a significant fitness cost for the plant. GNA-transgenic plants expressed the lectin at levels too low to affect the target aphids. A detached leaf bioassay with Lr10-transgenic plants revealed an increased resistance to leaf rust. No differences in the performance of aphids or cereal leaf beetles on transgenic and non-transformed plants were recorded in the convertible glasshouse and in complementary glasshouse studies. Similarly, infection levels with powdery mildew did not differ between transgenic and non-transformed plants but Bobwhite plants were significantly more infected when compared with conventional Swiss spring wheat cultivars. Overall, the assessment revealed that for the plants investigated here, their genetic background had a stronger impact on the performance of a plant and its interactions with insect herbivores and pathogens than the expression of the transgene.