In late October 2009, the managers of the Back River Wastewater Treatment Plant in Baltimore, MD sought assistance in mitigating what they described as an “extreme spider situation” in their sand filtration facility. The building, consisting of almost 4 acres (16,099 m2) under a single roof but with no side walls, had been prone to extensive colonization by orb-weaving spiders since its construction in 1993. However, the present infestation was considered to be worse than normal, and the facility’s maintenance and operations personnel had voiced concerns over the potential risk of bites.
As an interagency team with expertise in arachnology, urban entomology, and structural pest management, we were unprepared for the sheer scale of the spider population and the extraordinary masses of both 3-dimensional and sheet-like webbing that blanketed much of the facility’s cavernous interior. Far greater in magnitude than any previously recorded aggregation of orb-weavers, the visual impact of the spectacle was nothing less than astonishing. In places where the plant workers had swept aside the webbing to access equipment, the silk lay piled on the floor in rope-like clumps as thick as a fire hose.
This report has 3 objectives: (1) to document the phenomenon, providing photographs, species determinations, and estimates of the total extent of web construction and numbers of spiders involved; 2. to compare this remarkable concentration of normally solitary orb-weaving spiders with similar megawebs reported from both anthropogenic and non-anthropogenic habitats, as well as to review the range of antecedents for this behavior; and (3) to emphasize the potential research utility of aquacentric structures such as sewage treatment plants as readily accessible “culturing facilities” for predictable, dense aggregations of these spiders.
…Webbing: The most distinctive manifestation of the spider population inside the sand filtration facility was the enormous mat of webbing that obscured about 70% of the ceiling on either side of the central corridor, a mostly unbroken expanse of relatively dense, uniformly thick (about 0.5 to 1.0 cm) silk that covered a total area of about 8,731.42m2…Together with the main horizontal expanses under the side roofs, the total laminar webbing in the building was about 8,922.42m2.
…Spider: …Although 9 genera in 6 families were represented (Table 2), the webbing was almost entirely the product of two hyperabundant species, Tetragnatha guatemalensis O. Pickard-Cambridge and Larinioides sclopetarius (Clerck), which respectively comprised 63.4% and 21.6% of the non-hatchling sample of 7,023 individuals.