Effects and attack characteristics of the native stem borer, Amphicerus bicaudatus Say (Coleoptera: Bostrichidae), on invasive Tamarix sp.

The biotic resistance hypothesis states that native herbivores may serve as evolutionary novel consumers to keep exotic prey from becoming highly invasive species. However, studies showing such occurrences are fewer in number compared to those supporting alternative hypotheses (e.g. enemy release.) We observed native stem-boring beetles (Amphicerus bicaudatus Say) feeding in high numbers on stems of exotic Tamarix sp. at Bonny Reservoir in eastern Colorado, USA. Eleven sites in were selected for further investigation. We used line transect and random survey methods to determine the percent occurrence and host preference of this herbivore for both Tamarix sp. and native Populus deltoides. Infested stems were dissected and gallery characteristics measured. Adult beetles were swabbed and fungal species cultured in an attempt to screen for plant pathogens. We found the beetle in each state sampled and in roughly 20% of Tamarix sp. surveyed. On the other hand, we found A. bicaudatus in only 3% of P. deltoides surveyed. The beetle was 8.6 times more likely to be found in Tamarix sp. than P. deltoides even though both tree species are available to the beetle in similar numbers. Although we found beetles in several live stems, it is unclear whether its presence causes stem mortality. We are currently conducting cage and inoculation experiments to see how A. bicaudatus and its associated fungi affect the survival and fecundity of Tamarix sp. If A. bicaudatus reduces Tamarix sp. fitness, this particular relationship will provide evidence for the biotic resistance hypothesis.