With two daggers extending from each foot, ticks are well armed. And they are endlessly patient. Swaying on a blade of orchard grass, a tick extends its questing legs and waits. Night and day, through rain and drought, the tick remains poised to smell the coffee and leap. Well, more accurately, the tick waits to sense butyric acid and drop. Butyric acid wafts from the skin of a passing mammal, and with that fragrance the tick drops (hopefully onto the fur) and begins scurrying toward warmth. Three times the tick must stalk and feed from a vertebrate – first as a larva, again as a nymph, and finally as an adult.
Globally, there are about 400 named species of ticks. In northern New England, we have around a dozen. Ticks are a type of mite, a considerable group of 20,000 species. Some species are named for their favorite hosts – there are bird, woodchuck, rabbit, and moose ticks. None of the local ticks specialize in humans, but Homo sapiens are worth a try in a pinch. In fact, you may personally have had intimate experience with the dog tick, lone star tick, or the tiny black-legged tick. The local variety of the black-legged tick, Ixodes scapularis, is known as the deer tick.
Deer ticks are not much to look at; or rather, they are hard to see at all. Emerging from an oval egg below the leaf litter, a minute, pinkish larva ascends to a low perch in the thatch to wait with a pair of outstretched legs. White-footed mice (Peromyscus leucopus) are the first hosts sought by larvae. With a liquid lunch from a spot behind the mouse’s ear, the tick swells, molts, and acquires two more legs (eight total). With that meal, the tick may also become an unwitting delivery agent for the Lyme disease spirochete.
The next stage of life, the adolescent nymph, is the most dangerous to humans. This potentially disease-bearing nymph is the size of this lowercase “o” and doesn’t even tickle as it seeks the second meal of its life. Nymphs are most active in mid-summer, when more people are active out-of-doors. With that coincidence comes the greatest risk for exposure to Lyme disease.
As an adult, the female deer tick is about the size of this capital “O.” The male is smaller and does not increase much in size when he feeds. Both adults have a distinct red tone. For the last meal of their lives, these ticks favor white-tailed deer.
Dropping onto a deer and clambering through the hair, a tick hones in on a patch of skin. The head and body of the tick are fused. In front of the head is the false head or capitulum, bearing the biological equivalent of a Sawzall. The palps look something like a matched pair of sawfish beaks. The tick excavates a gouge, and blood flows freely with the help of anti-coagulants. The deer tick also concentrates the anti-inflammatory agent prostaglandin from the gushing blood and is able to store it with its own saliva. Using the prostaglandin, and other enzymes, the tick can feed for a week on red blood cells before finally engorging and dropping free. Needless to say, this is a site just ripe for infection. Opportunity for a front-row seat at a fresh wound has encouraged many diseases to piggy-back on the hard work of parasites.
Nymphs overwinter, becoming active again when the temperature rises above 45 degrees Fahrenheit. As an adult, the tick will pass its second winter in the leaf litter, and there, in early spring, the female deer tick will lay a cluster of up to 3,000 eggs.
More ticks survive when the winter is warmer, and when spring and summer are wetter. The El Niño phenomenon makes the news fairly regularly due to its pronounced effects on the winter weather of California. However, in New England, a stronger predictor of medium-term climate shifts is the North Atlantic Oscillation. Warmer sea surfaces in the North Atlantic permit storms to track along the coast, bringing spring and summer rain.
Susan Subak, from the School of Environmental Sciences in Norwich, England, examined the population changes in Ixodes ticks in seven states from Maryland to Massachusetts. Subak notes that ticks cannot regulate water loss and slowly dry out as they quest. Nymphs survive better when more moisture is available. Thus, a summer drought reduces the tick population.
The Institute for the Study of Earth, Oceans, and Space at the University of New Hampshire reports that two respected long-term climate models project significant warming, ranging from 6-10˚F, and increased precipitation for New England over the next century. Warm and wet. Good for growing all sorts of things, including bountiful ticks.