Post #3: Spatial Variation of Ticks

Hi again! After a short turnaround, I am ready to present my findings about the spatial variation that we’ve noticed in Ixodes scapularis from 2010 to 2019.  As an added bonus, I’ll talk about the patterns we’ve noticed in tick’s responses to climatic variables, although I plan on delving into that as an expansion on this summer’s project.

Almost every summer since 2010 (we didn’t collect in 2014 due to a gap in funding), we have visited a variety of sites around the Williamsburg/Newport News area. Most of these are parks (e.g. Sandy Bottom Park, Newport News Park), although some are in more urban areas (e.g. we sample a plot behind Boo Williams Sportsplex) and some are more rural (e.g. Dragon Run State Forest). At this point, we consistently sample 13 sites, which we break down into several plots throughout the site. We sample each plot by dragging a square piece of cloth on the forest floor for 30 m north to south and then another 30 m east to west.

Going into this project, I hypothesized that ticks would prefer to be in more forested areas, where they can easily find hosts. This was confirmed by some of the literature that I read in preparation. A 1995 study by Ostfeld et al. involved dragging for ticks in multiple habitat types, and it turned out that ticks were found most often in forested patches. With this in mind, I set out to do my analysis. I had two main questions.

Are there sites where we collect ticks more often?

I used Figure 1 on this document (MonroeGraphsBlog2) to help answer this question.

To answer this question, I thought it was most appropriate to look at the proportion of years that we found at least one tick. There were some sites where we found nymphs at a significantly higher rate than others; we found nymphs significantly more often at William and Mary and the Colonial Parkway than at Sandy Point State Forest, Green Springs, and Freedom Park. We also detected nymphs at Waller Mill significantly more often than at Sandy Point. We also saw significant differences in adults. The Colonial Parkway and Newport News Park had more tick encounters than Green Springs and Freedom Park. On top of Green Springs and Freedom Park, the urban sites in and around Newport News had a significantly higher rate of successful collections than Sandy Point, New Quarter Park, Dragon Run State Forest, and Sandy Bottom Park. Interestingly, no plots had a significantly higher rate of encounters.

I think that much of the explanation for these patterns lie in host availability. We probably see ticks more often in these places because they are, in one way or another, suitable for the ticks’ hosts (rodents for larvae and nymphs and deer for mating adults). This is definitely something I need to look into further, and looking at landcover data for each site could be a nice extension to the work I did this summer.

Next, are there sites where we collect a larger number of ticks?

Figure 2 accompanies this question.

This question required taking the average number of ticks that we’ve collected at each site over our study period. It turns out that there aren’t a lot of significant differences. We found, on average, more nymphs at Waller Mill and the Colonial Parkway than at Dragon Run, Freedom Park, Green Springs, Newport News Park, and Sandy Point. Again, no individual site prevailed as some gold mine of ticks.

Again, I think the answer is somehow related to where ticks are most likely to encounter hosts. I think in this analysis, it’s easier to see the rural/urban divide between where we don’t/do see ticks. The Waller Mill and the Colonial Parkway sites are both off of major roads (for those of you who don’t know, Waller Mill Park is only a couple minutes away from I-64). Obviously, the two state forests are more secluded, and some of the other sites are a little more removed from the cities as well. Perhaps these spatial differences are really important in determining where host species can be found—I’ll keep you posted in my conclusion.

I wanted to give you all a taste of the temperature and humidity patterns that I had noticed as well (Figures 3-6), even though I won’t be putting them on my poster. Much like the Julian date analysis that I talked about last post, it’s a little too demanding for this summer. However, in all 4 graphs we see a kind of quadratic pattern. I’m not sure why—most of the papers I read described more linear relationships (Vail and Smith, 1998). I’d love to wax about why, but that’s for another time.

I am looking forward to reviewing the literature and search for explanations for my findings! Look for a conclusion post in the upcoming days.

References:

Vail SG and Smith G (1998). Air temperature and relative humidity effects on behavioral activity of blacklegged tick (Acari:Ixodidae) nymphs in New Jersey. JMedEntomol 35(6):1025-1028.

Ostfeld RS, Cepeda OM, Hazler KR, and Miller MC (1995). Ecology of Lyme disease: Habitat associations of ticks (Ixodes scapularis) in a rural landscape. Ecological Applications 5(2):353-361.

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