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A Phylogenetic Strategy for Identifying a Biological Control Agent for Non-Native Populations of the Brown Treesnake (Boiga irregularis)
Dr. Jonathan Richmond | United States Geological Survey
The ecological success of invasive brown treesnakes (Boiga irregularis) on Guam is related to a lack of natural population controls in the non-native environment. While certain techniques have successfully reduced the number of snakes entering ports and other critical infrastructure, limited research has explored the prospects for biological control as an additional management tool. This project proposed to use the phylogenetic relationships of B. irregularis populations spanning the native species’ range as an evolutionary map for targeting native populations to survey for parasites. The rationale was that once parasites could be characterized in native populations representing a range of phylogenetic distances to the Guam snakes, it would be possible to develop a candidate list of parasites for experimental studies on biocontrol efficacy. The objectives of this project addressed these endeavors by (1) genetically identifying the source population(s) for introduced Boiga on Guam, (2) providing preliminary phylogenetic data upon which to base decisions for parasite prospecting, (3) testing the longstanding presumption that Guam B. irregularis are parasite free, (4) assessing whether parasites from the source population persist on Guam and (5) providing an initial characterization of metazoan and protozoan parasites in B. irregularis populations within its native range.
DNA sequence data from five genetic markers and Bayesian modeling were used to infer a phylogeny for B. irregularis populations occurring over much of the native range. These same data were used to compare indices of genetic diversity in the native and introduced range, as low genetic variation on Guam may indicate a reduced immunological capacity to respond to infection. Parasites were harvested from field-captured snakes at two locations, one representing the putative source of the Guam population and one representing ‘mainland’ New Guinea, and parasites were identified to the lowest taxonomic rank possible based on morphology. Parasite surveys were also conducted on Guam to test whether invasive B. irregularis continued to harbor parasites from the source population.
A single mtDNA haplotype and limited nuclear genetic variation were recovered in 24 treesnakes captured across Guam, suggesting that this population was founded by a small number of individuals from a single source location. DNA sequence data verified the source in the Admiralty Islands off the north coast of New Guinea. Contrary to previous assertions, Guam snakes do harbor helminth parasites, although prevalence, infection intensity, and species diversity is low. Papua New Guinea populations were heavily infected with a variety of helminth and haemoparasite species, none of which were recovered from treesnakes on Guam. The results satisfied the proof-of-concept necessary to pursue further lines of investigation on biocontrol.
Eradication of B. irregularis on Guam would have tremendous ecosystem, health and economic benefits, and would reduce the risk of further introductions into other non-native areas. Benefits specific to biological control include reduced human intervention and therefore reduced cost to sustain management, given that an effective bio-control agent would self-deploy. The evolutionary approach to using parasites for treesnake management demonstrates the potential of this work to broaden the scope and efficacy of control tactics already in place on Guam.