A fungal pathogen of the cellar spider, Pholcus phalangioides

Pholcus phalangioides (cellar spider) is a cosmopolitan species commonly found in homes and other buildings occupied by humans.  It has been proposed that P. phalangioides, whose bite is harmless to humans, may be important in limiting numbers of Loxosceles reclusa, the brown recluse spider. Occasionally P. phalangioides is found dead and covered in a dense white fungus. We first determined what fungi were associated with cellar spiders by plating fungal covered spiders on potato dextrose agar. Eleven species of fungus, including four putative pathogens, were isolated and sent to the USDA for identification. We then tested to determine whether the identified fungi were pathogenic to P. phalangioides.   Live spiders were collected and exposed to each possible fungal pathogen (6 individuals per fungal species). Only one fungus, Engyodontium aranearum, caused mortality. In further tests exposure to E. aranearum resulted in 100 percent mortality between days 13-20 post-exposure in infected spiders (N = 13) compared to no mortality in uninfected spiders (N = 12). Dose dependent trials at 10³ (N = 15) and 10⁵ (N = 15) spores per mL resulted in a 48% and 67% mortality rate, respectively.  These results strongly suggest that P. phalangioides is susceptible to this fungal pathogen; the fungus may be common in many homes considering the spiders’ almost world-wide distribution.  Given that the presence of P. phalangioides may be effective in regulating populations of the potentially dangerous brown recluse, it is important to understand the factors affecting its survival.