[Public Health] The Arachnid Phantom of the Opera

Way back in 1989, early in my career as the U.S. General Services Administration’s regional entomologist in Washington, D.C., I came across a medium-sized brownish spider that I didn’t recognize in a back stairwell of my office building. It had a small body and very long legs, and didn’t match any of my mental flash cards for the local urban fauna. But I was in a rush, and taking care not to step on it (my official duties hadn’t quite sunk in yet), I passed it by.

Many years later, I realized this was my first encounter with the most widespread recluse spider in the world: Loxosceles rufescens, commonly known as the Mediterranean recluse, since it is believed to have evolved in that region. For some reason, it has a greater propensity than any other species in its genus for surreptitiously traveling in human cargo and starting populations wherever it disembarks (which ecologists call “anthropochorous,” i.e., spread by man).

 In addition to southern Europe and the Middle East, it is now found across much of southeast Asia, as well as Japan, Madagascar, Australia, numerous Atlantic and Pacific islands, and the continental U.S. When collected in this country, it is typically misdiagnosed as the much better-known brown recluse, L. reclusa, by folks who get as far as the violin-shaped mark on its carapace and its three sets of two eyes, and assume that’s all they need to know. In fact, the two species are almost identical in size, form, color and venom chemistry. Differences in adult male and female genitalia are pronounced, however, and are described and illustrated in a forthcoming article in American Entomologist.

THE D.C. DISCOVERY. Although L. rufescens frequently is found outdoors in the warmer parts of its Old World range, all confirmed North American collections have so far been from inside structures, mostly institutional (e.g., governmental, academic, museums) or commercial. A typical state or city record involves one or a small number of specimens encountered in a single building, and invariably is regarded as representing isolated introductions due to a high traffic of incoming material from overseas, rather than as a predictable component of the local urban ecosystem.

In recent years, it has gradually dawned on entomologists in several East Coast cities, including the District of Columbia, that the occurrence of L. rufescens in their downtown core areas does not conform to the “single pinpoint” model but rather is an area-wide phenomenon. The recognition of its abundance had to await the broad acceptance of the humble, yet revolutionary, sticky trap as an urban pest management inspection tool. Despite strong advocacy by the early structural Integrated Pest Management community in the 1970s, they really only began to be widely used by the pest control industry in the 1990s. As soon as they did, discoveries of the Mediterranean recluse began to sharply increase. During the 10-year period from 1998 to 2007, my colleagues and I, together with our contractor technicians, stumbled on this species purely by chance in 14 federal government buildings in an area of Washington, D.C., roughly five miles across.

The structures varied widely in size, from about 13,000 to more than a million gross square feet. They tended to be relatively old, with an average age of 74 years. All but two collection sites were either on the ground floor or below. And like the majority of published collection records nationwide, initial detection of the spiders typically involved only one or two specimens, mostly on glue traps that had been placed to monitor for cockroaches. Superficially, it appeared as if L. rufescens was merely a curiosity in these buildings, represented by a small number of individuals that were barely eking out an existence. This was somewhat odd, since synanthropic (commonly living with humans) Loxosceles species, such as L. reclusa, often build up extremely dense populations in structural habitats.

THE RECLUSIVE RECLUSE. As it turns out, the bulk of our glue traps were in the wrong place. Subsequent investigation revealed an abundance of both adult and immature spiders in the deepest, darkest, most remote recesses of the urban environment, far beneath the greatest concentrations of human activity. These habitats, such as steam tunnels, crawlspaces and machine or utility rooms, tend to share a suite of distinctive characteristics: 1) non-air conditioned, rarely illuminated basement or sub-basement space that is physically secluded and inaccessible to all but a small number of personnel; 2) surrounded by a relatively old, deteriorating masonry foundation; 3) filled with stored or discarded items that serve as additional harborage; and 4) occupied by American cockroaches (Periplaneta americana) and/or subterranean termites (Reticulitermes spp.) that serve as convenient prey. Like an arachnid phantom of the opera, L. rufescens haunts these man-made caverns, and except for immediately adjacent areas, exhibits only low levels of dispersal into the rest of the structure. It is truly one of the most reclusive members of a genus famous for secretive and cryptic behavior.

 We believe this explains the typical rarity of encounters in many buildings. The densest concentration of spiders we observed was in a small, unlit pipe basement with a dirt floor strewn with concrete rubble and debris: During one visit, 46 specimens were captured by three people in one hour, with many additional individuals spotted (see Figures 2 and 3 on pages 76 and 80). But the only hint this population even existed was the capture, two years earlier, of a pair of adult males that had wandered onto a sticky trap in a ground floor room directly above the space. And when the basement was finally inspected for the first time, only a single spider was discovered because the investigators were preoccupied with searching for the source of an American cockroach infestation.

THE CAVERN CONNECTION. I became particularly interested in L. rufescens because the ecology of claustral (confined) or hypogean (underground) environments, such as caves and animal burrows, is the central, unifying paradigm of urban entomology as a basic scientific discipline. Buildings biologically mimic such habitats with their encompassing shelter, relatively stable and moderate microclimates, and simple arthropod food chains dependent upon a reliable, external source of organic material that is often directly provided by vertebrates — that would be us, filling in for bats and their guano. And if a building is like a cavern, what would a city be?

Not to stretch the point too much, but the dense concentration of large structures and associated underground infrastructure characteristic of urban core areas is essentially a manmade version of karst topography, a region riddled with subterranean hollows of all shapes and sizes. From this perspective, to consider L. rufescens in Washington, D.C., as the sum of discontinuous populations established in numerous buildings seems ecologically naive. It may be far more accurate to consider a given basement as just a convenient portal for us to sample what, in the spiders’ world, is a more or less continuous system of habitats below the surface. This is certainly true for what appears to be its major prey.

In fact, the reduction in number of eyes, long, skinny legs relative to the body, starvation resistance, and reduced intraspecific aggression of recluse spiders in general are reminiscent of arthropods that are adapted to life in caves, and the genus Loxosceles contains numerous species that are commonly found in subterranean habitats and several that live nowhere else. The Mediterranean recluse has successfully invaded a few natural caves around the world, including on the Hawaiian island of Kauai. And its colonization of underground infrastructure in Washington is not unique — a South American recluse species, L. laeta, has done the same thing in an even more extensive area of old concrete and brick commercial buildings in a Los Angeles suburb. But the poster child for a troglophilic (cave-loving but not cave-dependent) species pre-adapted to the structural environment is an extremely long-legged and presumably hypogean recluse spider from South Africa, L. parrami. I say “presumably” since it closely resembles known cave-dwelling Loxosceles spp. from that area. However, although large numbers of L. parrami have been found in buildings since it was first discovered 30 years ago in a Johannesburg bathroom, its primary natural habitat is still unknown.

PUBLIC HEALTH vs. PR. At this point, the Mediterranean recluse has been recorded in at least 25 of the continental United States, widely scattered from California to Florida and north to Washington state, as well as Toronto. Many of these records undoubtedly represent isolated introductions, but numerous anecdotal accounts suggest that, like in Washington, D.C., it has become endemic in the urban core ecosystems of Philadelphia and New York. Because of the remoteness of its preferred habitat there is no way of readily knowing its true distribution or abundance in most places. I can’t imagine that it doesn’t occur — somewhere — in almost every state. On an international basis, it likely began its globetrotting adventures on the same European sailing ships that so efficiently disseminated Periplaneta americana, establishing itself principally where native Loxosceles were absent. A few old specimens in the collection of the Smithsonian Institution indicate that it has resided in Washington for at least 84 years. From a purely academic perspective, I hope that drawing attention to its existence leads to increased vigilance for its presence among pest managers and, eventually, more information on its biology.

That being said, it’s clear that drawing attention to its existence is not going to make life any easier for those of us in the pest management business. The biological and medical bases for strongly doubting claims of recluse bites when an offending spider is not observed and collected have been exhaustively presented by Rick Vetter of the University of California at Riverside. One of the logical centerpieces of the argument that “loxoscelism misdiagnosis syndrome” is epidemic on a nationwide basis is that despite being strongly synanthropic, L. reclusa is not particularly anthropochorous — contrary to the prevailing urban mythology, it hardly ever is found outside of its native range. This does not seem to matter to the legions of clueless medical professionals who continue to identify all mysterious necrotic skin lesions as brown recluse bites in areas where there are no brown recluse populations.

But in areas where L. rufescens has been collected, convincing your average office building tenant (not to mention your average physician) that they are not in any significant risk from this nearly identical species is likely to be a tough sell, indeed. With venom that has demonstrated essentially the same range of effects as that of L. reclusa, the Mediterranean recluse may prove to be a convenient phantom to support people’s phobias and preconceived biological superstitions.

In Washington, we have never encountered any service personnel who work in or close to the areas of greatest L. rufescens abundance who had even been aware that the spiders were present, much less had any claims or knowledge of any bites. But we have also found that, although the species is very susceptible to thorough treatments with sprays and dusts, control lasts only as long as it takes for an infestation to re-establish itself by leaking back in from population reservoirs deep within the surrounding, inaccessible building foundations — usually only a matter of several months. Therefore, our policy at present is that — as with their Periplaneta prey — permanent eradication of L. rufescens is infeasible in most locations and that monitoring and treatment of known habitats must continue indefinitely. As an entomologist, I see this as a regrettable loss of valuable study material, but unlike my first encounter with this spider 20 years ago, I’ve since reconciled myself to a separate reality.

All photos are © Al Greene/Nancy L. Breisch.

The author gratefully acknowledges Dr. Nancy Breisch for unwavering encouragement to rise above his considerable arachnophobia. I would also like to thank Dr. Breisch, Tom Boardman, Ben Pagac, Ed Kunickis, Randy Howes and Paul Brown for their invaluable assistance in gathering data on L. rufescens, and Rick Vetter for fruitful arachnological discussions. The opinions expressed herein are the views of the author and do not necessarily reflect the official policy or position of the U.S. GSA.

The author is entomologist and National IPM Coordinator for the U.S. General Services Administration in Washington, D.C. He can be reached via e-mail at agreene@giemedia.com.