Saturday, August 8, 2015

Tick Seasonal Patterns

Note: This program first aired on August 8, 2015.

Ticks, though nearly universally reviled for their parasitic eating strategy and disease carrying potential, have a pretty amazing life history. A little knowledge about their seasonal cycles can even help you understand why they show up when they do, why they seemingly disappear for a while and when you need to be most concerned about having them around.

Here in Maine, the ticks we are most worked up about belong to the genus Ixodes. There are several species, only one of which carries the most common tick borne illness in this area, Lyme Disease. Ixodes scapulara is the deer tick or black legged tick, and that is the one to keep track of, so it’s the one we will use as our example of tick life history.

In Maine the deer tick’s life can take upwards of 2 years to complete. It starts as an egg, hatching in August or September, essentially the end of the growing season. What emerges from the egg is called the larval stage; they are tiny, very hard to see, but because they are larval, you don’t need to fly into a panic if you some how notice you have been bitten by one. Larval tick hatch from eggs in a pure state—meaning, even if their mother was carrying the bacteria that causes Lyme disease, the bacteria does NOT carry over into the eggs. Newly hatched larval ticks don’t have the bacteria in them yet. So if you get bitten by one a. you probably won’t notice because they are so small, and b. it doesn’t matter for Lyme disease, since they can’t transmit it to you anyway. In late summer/early fall these larval ticks are primarily feeding on rodents like white footed mice, shrews and chipmunks, and it is from these animals that they first contract the Lyme causing bacteria. White footed mice are the primary reservoir for the disease, not deer like so many people think. After the larval ticks have fed in the fall, they over winter engorged in the leaf litter and eventually molt.

The next spring the ticks emerge as newly molted nymphs. The nymphs need to eat in the spring, and when everyone freaks out about the sudden flush of ticks as soon as things warm up, these are the ticks they are freaking out about. As we just learned, these nymphs could have acquired the Lyme causing bacteria from their first meal the fall before as larvae, and it is from getting bitten by these nymph stage ticks that most humans pick up the Lyme causing bacteria. After these nymphs feed in the spring, they molt into adults.

Adult females need to eat again before they can lay eggs, and it is typically fall before they do their biting. They over winter fully engorged, and emerge in the spring to lay their eggs, thus completing their life’s work. So to recap: larval ticks feed in the late summer, nymphs in the spring/early summer, adult females later in the fall. Only the nymphs and adults can transmit disease, with nymphs doing the lions share of infecting. The other commonly encountered ticks here in Maine are wood ticks, the adults of which are the ones you might find trying to bite you in late spring or early summer. Now that it is August, I hardly see any of these. Now I am just waiting for fall and the emergence of the adult deer ticks. It seems strange to mark the passage of the seasons by the different life stages of a creature so despised, but knowing the rhythm of the ticks not only empowers me but adds yet another layer of understanding, another layer of connection between myself and the rest of nature.


Comprehensive list of ticks in Maine from Maine Medical Center Research Institute: