Every October I obsessively watch the weather, and this year I’ve been paying a lot of attention to the guys from Eye-on-the-Sky. The daily highs have struggled into the 40s, with quite a few days topping out in the 30s, and relatively few approaching 50. We’ve had a number of snow events, and fall-sport spectating has brought out coats and hats, scarves and gloves, boots and blankets. Biking to work has been unappealing.
People have lots of reasons to care about the weather, but mine is unusual: ticks. Most people avoid ticks like the plague, especially Black-legged Ticks (a.k.a. Deer Ticks), which is appropriate given the diseases they can carry. But I have the ignoble distinction of being one who goes looking for them. Since 2010 I’ve been trying to better understand how and why they are expanding their range, and 2018 has unfolded as a potentially rough year for them. A hot, dry summer followed by a cold fall with early snow could be something of a perfect storm with the potential to depress tick populations into 2020.
Looking for ticks is not a high-tech endeavor. It involves dragging a white cloth through tick habitat and stopping every 10 to 15 paces to inspect the cloth and remove any ticks found clinging thereon. But in order to be successful, we generally don’t do surveys unless the temperature exceeds 45° F, and the vegetation at ground level is no more than slightly damp. Collection efficiency drops off steeply in cold, wet conditions, mostly because ticks are cold-blooded and their ability to be active in such conditions is greatly restricted. So for tick collectors like me, this October has been dismal.
But for nearly everyone else, the weather extremes of 2018 have likely come with an underappreciated silver lining. To understand this a little better, it pays to think for a bit like a tick. Ticks are sit-and-wait predators. They park themselves on the tip of some low vegetation, and they wait for suitable prey to brush past them. This habit, called “questing,” is analogous to playing the lottery. Lots of us buy tickets, and then sit and wait to see if we win. A few do. Most don’t. And the more you spend on tickets, the better your chances. For a tick, every hour on the tip of a twig is like another lottery ticket. The more time spent, the better its chances at winning this version of blood sport. But the two weather conditions that predictably prevent ticks from engaging in their blood lottery are summer drought and winter cold.
Cold works against ticks in a relatively direct manner. Cold ticks are slow to move, less active, and they quest less. Additionally, they are incapable of laying eggs in temperatures below about 40° F. The effect of drought is less direct but no less important, and understanding their life cycle is helpful. A Black-legged Tick typically lives two years, and during that time it does not “drink” in the way that you and I would know it. Ticks can take on moisture either by feeding on blood or by absorbing moisture from their environment. Each summer tick larva hatch from eggs and begin questing for their first blood meal. If successful they get a big influx of moisture. They then molt into nymphs, wait out the cold winter weather, and try for their second “drink” the next spring. Successful nymphs molt into adults and try for a third meal again in the fall. Long periods spent questing on hot, dry days rob them of precious water, forcing a retreat to moist soil or leaf litter where they can slowly rehydrate. Time spent rehydrating is time away from questing, and their chances of winning the lottery plummet. You have to be in it to win it after all.
However, Black-legged Tick populations turn out to be rather resistant to fluctuations based solely on the weather. This can be understood by considering the activity patterns of the three different life stages. Nymphs are active in early summer, larvae are active in late summer, adults are active in spring and fall, and eggs may be laid in either spring or fall. All of that means that an intense drought in early June might have a dramatic impact on the nymphs, but since the larvae and the adults are inactive, the population as a whole is protected from utter devastation. Similarly, a very cold October might put the screws to the fall adults, but if the nymphs had a successful early summer, there will be plenty to molt into spring adults and the population marches on.
What has been unique about 2018 is that we had both a hot, dry summer, with reports of record drought, and an unusually cold fall. Thus, all three life stages will have faced difficult questing conditions in the same year. With a few caveats, this could depress their populations through 2019 into 2020. First, a warm December could allow the remaining adults a much-needed chance at finding a host and laying eggs. They are quite robust, and while the cold may keep them from seeking blood meals, it does not reliably kill them, and they can bide their time until the temperature rises. Second, the fact that on average they will have been less active this fall than in years past does not mean that they are not out at all. It only means that they are out less. There will still have been some adults out and about. While working on this article I was approached twice this by avid outdoors folk, both surprised to have found ticks under unexpectedly cold conditions. Third, the American Dog Tick (a.k.a. Wood Tick), and the Winter Tick (a.k.a. Moose Tick) have different activity patterns such that the arguments above will not apply strongly to them.
One final caveat: Fall 2018 adult Black-legged Ticks that are unsuccessful at finding a blood meal will overwinter unfed and emerge in the spring to try one more time. So, counter-intuitively, the hard conditions of 2018 might give a bump to the number of spring 2019 adults. If the fall had been mild, a larger proportion of the adults would have successfully fed, laid eggs, and died, leaving fewer adults to emerge in spring. Short winters, January thaws, and cool, wet summers will all promote the continued expansion of tick populations. Unfortunately, all of those are part of predictions for how our climate is expected to change.
I cannot say that I enjoy studying ticks in the same way that I have enjoyed other species I have had the privilege to spend time with, but I can say that I find them endlessly complicated and challenging, and therefore, interesting and engaging in their own special way.
Alan Giese is a professor of biology at Northern Vermont University – Lyndon.