Dark Banquet (28 page)

Dr. Pellegrino gave a chuckle, but there was no humor in it. “It doesn't need to be a total extinction event. If bee deaths should reach 80 to 90 percent worldwide, I estimate that the earth's carrying capacity for human beings could be reduced, essentially overnight, from a maximum of twelve billion to about six billion—and we're at six point seven billion now.”

“So you think the result would be…?”

“I think the result would be widespread famine and economic collapse, on a planet where the kamikaze mentality has already turned religious extremists into tigers sharpening plutonium claws.”

“Okay…but why the huge effect—the lack of bee-pollinated crops?”

“That's part of it, Bill. We'd be reduced to harvesting wind-pollinated crops like wheat and corn. But just as important, certain organisms are keystone species—basically nature's cascade points. Should they go suddenly extinct, or should their numbers be greatly reduced, then the entire system is affected. The honey bee is one of those keystones. Knock them down, near to extinction, and our civilization is gone in five years. Without the honey bee, Rome falls.”

We sat there silently for a minute, watching the chess players clustered at tables near the park's southwestern entrance.

“Checkmate,” I muttered.

Pellegrino gave another humorless laugh. “You got that right.”
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Approximately one in three mite species belong to the suborder Prostigmata, and these are commonly known as harvest mites or scrub mites. Many of these species are relatively harmless as adults (feeding primarily on plant material) and some are actually beneficial—aiding in the decomposition of plant matter into humus—a vital soil component for the growth of plants. The problem is that somewhere between 2,500 and 3,000 prostigmatids (most belonging to the family Trombiculidae) have parasitic larval instars commonly known as chiggers.

Considering the grief that they cause, only a few species of chiggers count humans as their primary hosts. In that regard, most chigger/human encounters are accidental and generally end badly for both parties. Instead, a significant majority of chiggers parasitize nonhuman hosts, including many invertebrates (like arthropods) as well as every major group of vertebrates.

Chiggers, like their tick cousins, have a worldwide distribution, which means that you're just as likely to get bitten by a tick in Central Park as you are a chigger in Tunapuna, Trinidad, and although there are several species of chiggers in the United States (belonging to the genus
Trombicula
), the most commonly encountered is
Trombicula alfreddugesi
(while in England it would be
Trombicula automnalis
and so on).

Although chiggers and ticks do exhibit some similarities, the differences between them are significant enough that they should not be confused with each other.

Besides diet, one major difference between chiggers and ticks is size. Chiggers are nearly impossible to see with the unaided human eye unless they're clustered together (most are about four-tenths of a millimeter long, which is about one one-hundredth of an inch). Ticks, on the other hand, can be hundreds of times larger.

Unable to hop aboard their prey like the long-jumping fleas,
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both chiggers and ticks locate their hosts either by actively hunting for them or by lying in ambush and waiting for them to brush past.

What occurs next—bite preparation, the bite itself, and the actual mechanism of feeding—is another area where chiggers and ticks differ significantly.

When attacking humans, chiggers move rapidly to areas of the body where the skin is particularly thin, like the ankles, armpits, or the back of the knees. Unlike ticks, they are remarkably fast runners, although both use a similar combination of sensory stimuli (light, touch, and chemicals) to track their potential prey. Upon encountering regions of the body bound by tight-fitting clothing (like socks, belts, or the elastic bands found on underpants and bras), instead of traveling over the material, chiggers crawl under it—often choosing these areas to initiate their bites.

One of the misconceptions about chiggers is that they burrow deeply into the skin of their hosts where they embed themselves (like ticks), but this is where the two parasites couldn't be more different. Once chiggers find a suitable patch of skin (usually an epidermal pore or at the base of a hair shaft), they pierce the skin with a pair of short fangs called chelicerae. As these daggers work back and forth, muscular contractions inject saliva into the wound. This saliva contains strong digestive enzymes that produce two very different reactions in the areas adjacent to the bite. Within a few hours, the outer layer of the epidermis immediately surrounding the injection site responds to the corrosive spit by hardening into a strawlike structure called a stylostome (or histiosiphon). The stylostome, which soon extends down into the dermal layer, is formed (at least in part) by keratin, a waterproofing substance released from the hosts' own epidermal cells. As the chigger's saliva flows down the stylostome's central canal, the powerful enzymes within it reach the deeper layers of the epidermis and eventually spread into the dermis. Here the enzymes liquefy the surrounding connective tissue and the contents of nearby cells. This cellular soup is the preferred diet of chiggers and although blood cells may accidentally become part of the recipe, they are not true vampires. The rudest part of the chigger's feeding gig begins as the liquefied dermal stew is snorked up through the stylostome and into the parasite's muscular pharynx.

Since chiggers generally feed continuously for three or four days, humans usually scratch them off long before they're finished. Once displaced, chiggers cannot attempt to feed again and die without developing further.

Meanwhile, back at the bite site, the host's immune system reacts to the stylostome and foreign chemicals. The resulting inflammation produces some serious and prolonged itching, which can lead to secondary infection.

In addition to misconceptions about how chiggers feed, another myth concerns ridding yourself of the pests (or at least alleviating the itching they cause) by applying clear nail polish to the irritated skin. The truth of the matter is that the chigger has probably been scratched off already and nail polish has never been renowned for its therapeutic properties. Instead, it's recommended that the bite area be cleansed thoroughly. Following this, antihistamines and topical anesthetics can help to alleviate the itching, but even so, the welts and the urge to scratch the bites can sometimes continue for ten days or longer—basically until the stylostome has broken down and is reabsorbed by the body.

Although most chiggers do not get to complete their human meals, some chiggers lucky enough to find a nonhuman host eventually drop off (generally within three days) and burrow into the ground. There, they go through two more larval stages before a final molt results in an eight-legged adult mite.

Ticks are a much smaller group than mites (or even chiggers) and they're divided into two superfamilies: the Ixodoidea (or hard ticks) and the Argasoidea (or soft ticks). Ticks are obligate blood feeders, and as such, their feeding habits are far more specialized than those seen in mites. Ticks feed solely on vertebrate blood, and they parasitize mammals, birds, reptiles, and amphibians, which basically means that they pester every major group except fish. Ticks are a huge problem for humans even though we are not the primary hosts of a single tick species.

Hard-bodied ticks, ixodids (which cause the most grief for humans), range from 1.7 to 6.1 millimeters in length and soft-bodied ticks can get even larger yet (3.6 to 12.7 millimeters). Amazingly, when their bodies are bloated with blood, ticks from both groups can reach lengths of between 20 and 30 millimeters.

In the United States and elsewhere, tick control has focused on hard ticks since these parasites are responsible for the transmission of eleven different diseases to human hosts (which places them second only to mosquitoes in the variety of diseases that they transmit to us). There are approximately eighty species of hard ticks in the United States, twelve of which are problematic for humans. Of these, three species pose serious problems.

The black-legged tick, or deer tick
(Ixodes scapularis),
is responsible for the transmission of three human diseases, including Lyme disease. Two less frequently observed afflictions are babesiosis (a malaria-like infection that attacks red blood cells), and human granulocytic ehrlichiosis (a bacterial infection that is analogous to anaplasmosis—a form of “tick fever” in cattle). The primary reservoir for the Lyme disease bacterium,
Borrelia burgdorferi,
is the white-footed mouse
(Peromyscus),
which is apparently not sickened by the infection.
Borrelia
is transferred to the ticks when they obtain a blood meal from the mouse, and Lyme disease can result when the infected tick transfers the bacterium to other animals, like deer, dogs, and humans.

The American dog tick (sometimes called the wood tick),
Dermacentor variabilis,
is the primary vector for Rocky Mountain spotted fever, a potentially fatal disease caused by the bacterium
Rickettsia rickettsii.
Named for the area where the disease was first diagnosed, and for the characteristic spotted rash that occurs in places like the palms and soles of the feet, Rocky Mountain spotted fever begins with flulike symptoms that worsen as blood vessel linings are attacked and major organ systems suffer the consequences.

Finally, the Lone Star tick
(Amblyomma americanum)
is so named for the distinctive, roughly star-shaped silver marking on the dorsal surface of the female's body (the male has white markings along the posterior edge of its body). The Lone Star is a major concern because it transmits a “Lyme disease–like illness.” This is no surprise to those who study ticks since the bacterium they transmit
(Borrelia lonestari)
is closely related to
Borrelia burgdorferi,
the spirochete that causes Lyme disease. Research on the Lone Star tick has been stepping up recently, mainly because of the way the tick is sweeping into the northeastern United States. In fact, in many places (like Long Island) it is swiftly replacing the black-legged tick as the species most commonly encountered by humans. A more aggressive predator than the black-legged tick, the Lone Star actively tracks its potential host rather than waiting for it to pass by.

According to entomologist Tamson Yeh, the Lone Star tick presents additional problems for both integrated-pest-control specialists and the public.

“In the past, tick-free zones could be set up in parks and playgrounds. We did this by cutting back on brush and building mulch or woodchip buffers between areas of woods and lawn. But since the Lone Star ticks are more mobile, they have no problem crossing these buffers.”

Interviewing Dr. Yeh at her office in Riverhead, New York, I also learned that while the black-legged tick is primarily a forest dweller, the Lone Star prefers its environments hot and dry with plenty of open spaces.

“Considering the changes in local moisture patterns, it's a nobrainer that this particular tick is becoming prevalent in the northeastern United States.”

I shifted in my seat. “By changing moisture patterns, are you referring to global warming?” Considering the media bombardment surrounding this catch phrase, I felt slightly uncomfortable now that I'd finally used it in a complete sentence.

Dr. Yeh hesitated. “Yes, global warming is a consideration—but it's more than that. When you cut down a wooded area and throw down a bunch of houses, lawns, and concrete, things are going to get hotter and drier. Humans are changing vegetation patterns, and the urban environments they're creating are just what the Lone Star tick thrives in. Plus, more people equals more contact with ticks.”