A major challenge facing ecologists and land managers is being able to predict when exotic species either become invasive or persist at low numbers. One potential explanation is the biotic resistance hypothesis: native competitors and/or predators present in certain areas may keep exotics in check. Studies investigating this hypothesis are fewer in number compared to those investigating the enemy release hypothesis. In 2008, we observed native stem-boring beetles (Amphicerus bicaudatus Say) feeding on stems of exotic Tamarix sp. at Bonny Reservoir in eastern Colorado, USA. Eleven sites along four major rivers in the states of Colorado, Nebraska, and Kansas were selected for further investigation. At each site, we selected up to 60 trees of both Tamarix sp. and native cottonwoods (Populus deltoides) to record the percent occurrence and host preference of this herbivore. Individual trees were thoroughly examined, and if signs of infestation were detected, we recorded the tree as being a host. Samples of infested stems were collected in order to confirm the presence of A. bicaudatus, to collect and catalog beetles, and to measure gallery characteristics.
A. bicaudatus was found at 9 sites representing each state and river sampled. Across all sites we found evidence of this insect in 112 out of 579 (19%) Tamarix sp. individuals surveyed. On the other hand, we found 13 occurrences from 480 (3%) cottonwoods surveyed. A. bicaudatus was 8.6 times more likely to be found in Tamarix sp. than cottonwood. Occurrence of A. bicaudatus on Tamarix sp. ranged from 0% to 86% per site. Our study shows that this native herbivore highly prefers Tamarix sp. over cottonwoods. The range of A. bicaudatus is limited to east of the North American continental divide where it has been reported as a pest in other exotic woody plants. The pressure exerted by A. bicaudatus on Tamarix sp. in the central U.S. may help explain why populations of the plant in this area rarely occur at the high densities found in similar habitats west of the continental divide. Follow up experiments which examine how A. bicaudatus affects the survival and fecundity of Tamarix sp. are needed in order to validate whether this particular relationship contributes to the biotic resistance hypothesis.