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small-bodied hosts, then molt and become nymphs. The nymphs overwinter and feed on small-bodied hosts the next spring and summer, molt again, and become adults. LD is maintained in this system because larvae feed on hosts that have already been infected by nymphs. The phenology hypothesis is based on the idea that in the southern United States the climate is warmer, and the tick life cycle is sped up, and larvae often feed on hosts that have not yet been infected by nymphs. We sampled ticks at all of our field sites, using standardized techniques, so our results would be completely comparable. We’re still waiting to analyze the rest of this year’s data, but so far we’ve found that southern juvenile ticks had much the same phenology as did juvenile northern ticks. That is a challenge to the phenology hypothesis. The data from our tick surveys and our counts of ticks on hosts also helped us test the black-legged tick abundance hypothesis, which is the suggestion that while black-legged ticks do occur throughout the south, they never reach high enough numbers to sustain LD in host populations. Like many researchers before us, we had great difficulty collecting ticks in the south, even using the same sampling techniques that were highly successful in the north. But while we were trying and failing to collect southern ticks, the small mammals – and especially the skinks – had plenty of ticks on them. Clearly there are substantial numbers of ticks in the south; the impression that black-legged ticks are rare in the south seems to prevail, at least in part, because they’re difficult to sample there. Obviously, the tick-collecting techniques we used successfully in the north don’t work as well in the south, and that must
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Figure 4. Five-lined skink (Plestiodon fasciatus) with four black-legged tick (Ixodes scapularis) nymphs attached around the front limbs. Photo courtesy of Graham Hickling.
be explained. The black-legged tick genetic hypothesis is based on the previous findings that there are genetic differences between black-legged ticks in different parts of the country, resulting in somewhat separate subpopulations. Maybe the southern black-legged tick subpopulations don’t transmit the Borrelia infections as effectively, and maybe they prefer lizard hosts over mammal hosts. Our tick-collecting techniques involve sampling in areas where we would expect to find ticks searching for mammals, whereas ticks searching for lizards might be someplace else. Other team members are currently looking at the possible impact of genetic differences in tick subpopulations on host-searching behavior. LD is an important health concern in Europe as well, where they have different Ixodes ticks and different Borrelia. In Europe wall lizards (genus Podarcis) often host ticks, and it appears that wall lizards are reservoir
competent for European Borrelia. Therefore, in Europe, lizards don’t reduce LD risk; rather, they may increase it. The two common wall lizard species in Europe have been introduced into numerous places in North America, including Long Island! Wall lizards are well named; they co-exist very well with people in rural, suburban, and fully urban areas and are often seen on walls. For example, there are now large and stable wall lizard populations in New York City. There is no immediate concern that these lizards will spread LD into our cities, I personally have seen and captured many hundreds of wall lizards from most of the introduced populations, and have never seen a tick on any of them. However, wall lizards do live here in some areas where black-legged ticks occur, and where LD cases are common. We decided to investigate the potential role that the introduced wall lizards might play as they spread into more areas in the United States with black-legged ticks and LD.