I got an email a couple of weeks ago from Karen Harlan at Penn State New Kensington asking me about a deer tick “imposter.” She had heard about a small, soft bodied (“squishable,” was the way she phrased it, stressing that she would never undertake to voluntarily squish anything!) insect that had the shape, size, and coloration of the hard bodied (“unsquishable”) tick that carries, along with some other potential pathogens, the bacterium that causes Lyme Disease. Neither Deborah nor I had heard of this tick imposter, but we did a quick check and found several newspaper articles from last year about it. Interestingly, over the next couple of days we came across several current news stories about both ticks and their squishable mimics, so this discussion feels very relevant (Thanks, Karen!).
Let’s talk about some real ticks first: Pennsylvania is experiencing a population explosion in black-legged ticks (the tick formerly called the “deer tick”). This tick (scientific name Ixodes scapularis) is a small, common tick found throughout the northeastern and north-central parts of the United States. It is the transmission vector for a number of bacterial and viral pathogens including the bacterium that causes Lyme disease, Borrelia burgdorferi. For the last several years, Pennsylvania has led the nation in the number of human cases of Lyme Disease (last year (2014) there were 7487 confirmed human Lyme Disease cases in Pennsylvania up from 5904 cases in 2013).
There are a number of features of the black-legged tick’s morphology and life cycle that make it difficult to anticipate or detect. The life cycle of this species can involve combinations of over one hundred and twenty different potential hosts (fifty-two different species of mammals, sixty species of birds, and eight species of reptiles) and can stretch out over a two or even a three year period with staggered emergences of different instar stages during different months of the year. Also, early instars of this tick are extremely small and difficult to see! Above is a photo showing all of the life stages of the black legged tick and below is an idealized version of the life cycle that I described in a blog last year:
Eggs deposited in the fall in low, grassy or scrubby vegetation hatch the next summer into the very small, six-legged larva life forms. These tiny ticks typically seek out small hosts (like a white-footed mouse or a bird) but are able opportunistically to attach to larger mammals including humans. These larva, though, are not born with any of the pathogens associated with I. scapularis and are, thus, unable to transmit any of its diseases (a small piece of good news!). If these larvae feed on a host that is carrying one of I. scapularis’ bacterial or viral pathogens, though, that tick will become infected with that disease causing agent and will carry it and be able to transmit it throughout the rest of its life cycle.
After the larva has taken its blood meal it molts into the larger, eight-legged nymph life form. This molt often is delayed until the following spring. These nymphs, then, seek a host for their blood meal. These hosts are usually mammals ranging in size from white-footed mice to dogs to cats to deer to humans. Because of the timing of this nymph emergence the spring (May and June here in Western Pennsylvania) is a time of great risk for ticks bites (and disease transmission) for humans!
After the nymphs have taken their blood meals they molt into adults. These adults are especially abundant in the fall. These much larger ticks (size is all relative, of course!) typically attach to large mammals (like the list above). The female adult ticks take a large blood meal from their hosts and then use the energy from this feeding to make eggs. The adult male ticks attach to the same hosts, but do not feed (and, therefore, do not transmit pathogens at this stage). They are there to find a female and to mate! The males die shortly after mating and the females die after dropping off of their hosts to lay their fertilized eggs in the grassy and scrubby vegetation. Those eggs then overwinter and hatch in the summer to start the life cycle all over again.
So why have the number of Lyme Disease cases increased in recent years? Climate change does not seem to be a causative factor (warm winters do not seem to increase black-legged tick survival rates and cold winters do not seem to decrease them!). Most researchers looking at these ticks attribute their increases to increases in the most critical host in the black-legged tick’s life cycle: the white-footed mouse. Fragmentation of forest habitats and the optimal conditions of suburban ecosystems for these mice along with significant declines in their natural predators have led to great increases in their numbers. Black legged ticks, then, in their larval life stages are increasingly likely to find a white-footed mouse on which to feed and are, therefore, increasingly likely to survive to the next instar level. White-footed mice are also significant reservoirs for the Lyme Disease bacterium, so the ticks have a higher probability of assimilating and then passing on these bacteria.
The Pennsylvania Department of Environmental Protection collected and tested black-legged ticks and determined that 34% of them carried Borrelia burgdorferi. It is not known if this is an increase from previous years or not, but these data will provide a comparison baseline for future tick assessments.
Before I move on to the tick imposters, I want to stress a few things about ticks and Lyme Disease: black-legged ticks are not able to begin blood feeding (and consequential pathogen transfers) until they have been attached to a host for at least 36 hours. Careful examination for ticks and their rapid removal is a very good way to prevent contracting the Borrelia bacterium. Preventing tick attachment is an even better strategy to avoid Lyme Disease (wear long pants and long-sleeved shirts when out in the woods or fields, use DEET-based insect repellants on socks, pant legs, etc.). A thorough “tick check” after being out in a potential tick habitat is also a very effective way to reduce the chance of infection.
So who (or what) are the tick imposters? The ones that have gotten quite a bit of media coverage are small weevils called “billbugs” (great name, eh?). The most common billbug in Pennsylvania is the bluegrass billbug (Sphenophorus parvulus). Larvae of bluegrass billbug feed on bluegrass stems and roots and adults emerge from their overwintering hideouts in piles of leaf litter or weeds from late April through June. They are small (although they are significantly larger than black-legged ticks!), dark and, in a snouty, weevily way (all in the eye of the beholder), handsome. These adult emergences are especially abundant in wet springs and summers, and a bluegrass billbug walking up a pant leg or clustering on a shirt front could trigger a “TICK!” response in the pant leg or shirt front owner! Billbugs are harmless (except to bluegrass lawns and meadows) and, unfortunately for them, quite squishable!
A recent Penn State News article also talked about another tick imposter, the yellow poplar weevil. This 3 mm long, dark weevil has large antenna which can be mistaken for a extra set of tick-like legs. They have been emerging from their overwintering sites in the leaf litter in southwestern Pennsylvania throughout the month of June in order to mate and generate their tiny larvae which feed on the soft, inner tissues of yellow poplar leaves. Once again, their tick-like appearance is in the eye of the beholder. They can’t harm people, but they can do some considerable damage to yellow poplar trees. And, Karen, they are, like the billbugs, quite squishable!
So be careful of ticks but don’t panic if you find one, and go easy on the little billbugs and poplar weevils!