Stable isotope analysis has proven to be an invaluable tool for unravelling feeding relationships, since the nitrogen and carbon isotopic signatures in animal tissues are a reflection of their diets. We conducted a preliminary investigation of the feasibility of stable isotope analysis to identify the species identity of hosts of the black-legged tick, Ixodes scapularis. Engorged larval black-legged ticks were collected from wild-caught white-footed mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus) by holding animals in the lab until feeding ticks dropped off. Some animals were fed a diet high in corn which, as a C4 plant, is enriched in 13C relative to C3 plants, in an attempt to track this high 13C signal into ticks and further into spiders that consumed them. Fur samples were also collected. Ticks were allowed to molt under field conditions. Molted nymphs were batched in 3 to 5 tick groups and analyzed in a Thermo Delta Advantage mass spectrometer in continuous flow mode connected to a Costech Elemental Analyzer via a ConFlo IV at Union College in Schenectady, NY. We also created a reference library of isotopic signatures of fur samples of 19 species from our museum collection.
Results/Conclusions
Expected enrichment of 15N was observed between the fur samples and the molted nymphs with an average d15N of 3.71‰ ± 0.21 (SE). Fur and tick isotopic signatures of mice (n=6) and chipmunks (n=5) were indistinguishable (p>0.05). However, ticks from corn-fed mice and chipmunks had significantly higher 13C content than those that fed on conventional animals (d13C = -22.69‰ ± 0.15 vs. -23.91‰ ± 0.17, respectively). The isotopic signatures of spiders that had eaten the corn-labeled ticks were not distinguishable from those that had eaten other ticks. Our small library of museum fur samples (n=1 per species) revealed considerable variation in isotopic signatures, with the expected enrichment in the heavier isotopes in predators vs. herbivores. Species we identified as primarily carnivorous to omnivorous (n=10) had significantly higher d15N than those identified as primarily herbivorous/omnivorous (n=9) (d15N=7.79‰ ±0.77 and 4.62‰ ±0.74, respectively). Our small study indicates that it may be difficult to identify the host species of field collected black-legged tick nymphs using stable isotope analysis alone. However, it is possible that host guild could be established based on the isotopic signatures of field collected ticks. Certainly this question warrants more study using ticks fed on a variety of host species.