Insect responses
Insect family richness was not associated with invasion in B. arvensis gradients and only marginally associated with invasion inB. tectorum gradients, where insect richness slightly increased with increasing invasion (Figure 1b, Appendix S1: Table S1). However, insect community composition differed significantly with invasion in all three gradients (Figure 2b, Appendix S1: Table S2). Further, insect functional composition was correlated with invasion in B. arvensis gradients in Montana and B. tectorum gradients in Wyoming (Figure 3b, Appendix S1: Table S3). At the functional group scale, leaf-chewing herbivores and sap-sucking herbivores were negatively correlated with B. arvensis invasion in Montana (Appendix S1: Figure S3b, Table S4), while parasitoid and sap-sucking herbivore abundance were positively correlated to B. arvensisinvasion in Wyoming (Appendix S1: Figure S3b, Table S4).
Total insect biomass was not related to invasion across any gradient type or year (Figure 4a; Appendix S1: Figure S4; Table S5). However, total plant damage due to insect herbivory differed marginally or significantly between native and invasives within every invasion level. Invasion level rarely affected average herbivory, except in 2021 when plant herbivory at the 0% invasion level was marginally lower than plant herbivory at the 100% invasion level (Figure 4b, Appendix S1: Table S6). Overall, in 2021, average total plant herbivory on invasive annual bromes was 0.84 ± 0.14 %, compared to 4.06 ± 0.78 % on native plants, and in 2022, herbivory on bromes was 1.02 ± 0.29 %, compared to 4.36 ± 0.65% on natives.