Does biotic resistance govern forest invasions by bark and ambrosia beetles?

The theory of biotic resistance states that community diversity promotes resistance to biological invasions. This theory has been widely explored for its ability to explain variation in habitat invasibility to non‐native plant species and while the theory holds in some systems, it does not in others. In the case of invasions by herbivorous insects, invasibility could be affected by diversity of plants and/or by diversity of native insects. While only a few studies have explored biotic resistance to insect invasions, limited evidence suggests that plant diversity can actually have a positive effect on invasibility via creation of niches for herbivorous insects though other studies of insect systems indicate that plant diversity has a negative effect on invasibility by diluting the density of hosts. Almost nothing is known about how native insect diversity affects resistance to invasions by other insects. Here we analyzed a unique inventory of native and non‐native Scolytinae/Platypodinae (bark and ambrosia beetles) across the conterminous USA. We assessed the correlates of geographical variation in numbers of both native and non‐native species per 50 × 50 km cell. We find that native tree diversity generally has positive effects on the richness of native beetle species, while the abundance of non‐native hosts promotes richness of non‐native beetles. We also observed that the effect of native beetle diversity on non‐native Scolytinae/Platypodinae species richness is either lacking or positive. These results indicate fundamental differences between plants and insects in the way native and non‐native species interact; while interspecific competition can exert a strong influence on plant invasions, it appears less important for insects. Results thus indicate that the biotic resistance hypothesis does not explain invasion patterns of bark and ambrosia beetles.