Lyme Disease

We are now in the peak time of Lyme disease danger (May to July). By now we all know to spray our pants or legs with deet, wear light colored clothes, do tick checks, and stay out of the woods unless you're on a trail that doesn't bring you in contact with low vegetation. This post is not about preventing or treating Lyme disease, it's about how tightly this miserable disease is tied into the life of the forest.

 The carrier of this dangerous disease is the tiny deer tick, which has three separate life stages: larva, nymph, and adult. Newly hatched larvae emerge in mid-summer. They are the size of the period at the end of this sentence and are 99% free of Lyme disease bacteria. These tiny bugs can't move around very much so they need an animal to pass close by in order for them to latch on. At this point the tick can't afford to be picky so it will latch onto almost any mammal, bird, or even reptile that passes by. After drinking blood for three to five days, it drops off and in the course of a month changes into a nymph which remains quiet in the leaf litter throughout the winter and early spring. The nymphs are the size of a poppy seed and they are active now. They climb onto low vegetation to wait for a passing meal. At this time between a quarter and a third of the ticks carry Lyme disease. After feeding for the second time in their life, the nymph lies low and transforms into an adult. The peak time for adult activity is midautumn. Now the size of a sesame seed, the tick can climb up to a meter high on plant to wait for passing large animals like the deer that give the tick its name. Female ticks latch on for another blood meal while males search out females to mate. At this time 50-75% of the ticks carry Lyme disease. The well fed females drop off and live through another winter laying their eggs early the next spring and starting the cycle again.

That all seems very straightforward, but things get complicated quickly. Bambi has gotten a bad rap regarding deer ticks-it's actually the king of the Magic Kingdom Mickey Mouse that is the problem. It turns out that white footed mice are carriers of the Lyme disease bacteria. There are lots of white footed mice running around in the woods so a larval tick has a great chance of sucking on a mouse and picking up Lyme disease. The chance increases again if a nymph attaches to a mouse. The poor deer, like us, are not normally carriers of the disease, they are victims and, unfortunately, public transportation for adult ticks.

We often hear how winter weather can affect tick populations. A snowy winter like the last one is good for dormant ticks because the snow cover protects them from drying out and dying due to frigid winter winds. A less well known factor in tick production are, believe it or not, oak trees.

Oaks have evolved a strategy to beat acorn predators by "randomly" producing boom and bust years for acorns. Whole forests of oaks will produce very few acorns in some years which starves out squirrels, chipmunks, and white footed mice. After a bust year reduces acorn predators, a boom year swamps whatever predators are left with extra acorns leaving many to germinate and make new oaks. Of course, a bumper year also allows the predators to bounce back quickly which is OK with the oaks since white footed mice also eat bugs that attack the trees.

So a bumper year means the next spring there are more white footed mice for baby ticks to bite increasing the chance they will end up carrying Lyme disease. Here's where the deer public transportation system comes in. Normally deer eat tender branches of woody plants. Given their druthers they would prefer to nibble on maple trees rather than oaks because oaks have developed additional defenses for their leaves and branches. In a non-boom year for acorns, you are likely to find a lot of deer ticks in maple forests. Unfortunately, deer love to eat acorns so in a boom year, they and their adult tick riders spend more time in the oak woods. There the female ticks drop off and lay their eggs right in the path of the coming white footed mouse population explosion creating the "perfect storm" for a major outbreak of Lyme in humans.

Now if this wasn't bizarre enough. Let me tell you why gypsy moths are your friend in the fight against Lyme disease. As an invasive species gypsy moths have been able to exploit a loop hole in the oak trees defenses. Normally, gypsy moths are kept in check by, you guessed it, white footed mice which feast on late stage gypsy moth caterpillars, but when an acorn bust causes the mouse population to crash, the moths can defoliate oak trees enough to kill them further suppressing the mouse population and allowing more trees to be killed. Now there are a gazillion baby ticks out there so the population is not going to be wiped out by gypsy moths, but what happens is without mice to latch onto, the larval ticks fasten onto things that don't carry Lyme disease as often as the mice reducing risks for humans.

So what's the answer for controlling ticks?  Should we release more thousands of feral cats into the wilderness to eat the mice? According to Richard Ostfeld who has spent his entire career studying Lyme tick ecology, there are three answers. One is to know tick ecology and avoid places that are having large tick outbreaks. Second is to promote biodiversity. A woods that has an uninterrupted food web is a tick smorgasbord that gives them a chance to suck on something that isn't carrying the Lyme bacteria. For example, cats and humans have made things more difficult for ground nesting and ground feeding birds which are alternate tick hosts that don't carry Lyme disease. And third, deet and tick checks should always remain part of your woodsman's arsenal.