Distribution of entomopathogenic fungi and field tests of their potential as biocontrol agents for Ixodes scapularis

Background/Question/Methods Ixodes scapularis, the black-legged tick, formerly known as the deer tick, vectors Borrelia burgdorferi, the bacterium that causes Lyme disease. I. scapularis populations are expanding and Lyme disease is an important vector-borne disease in the US, with at least 20,000 reported cases each year since 2000. Efforts to reduce I. scapularis populations have focused on biological control methods including nematodes and parasitic wasps, though efficacy appears limited. Entomopathogenic fungi isolated from forested habitats have been shown to be virulent to ticks in the laboratory by releasing extracellular enzymes that allow penetration of tick cuticles. Because the virulence of entomopathogenic fungi varies with changes in relative humidity and temperature in the lab, seasonal climatic changes may also affect fungal pathogenicity in the field. To track natural spatio-temporal patterns of fungal abundance and match dominant fungal species to tick life stages, soil samples and host-seeking ticks were collected from twenty-five 1 m² forest plots from June 2007 to June 2008 in Westchester County, NY. Soils and tick exteriors were plated on ½-strength PDA and fungi were sub-cultured until pure isolates were obtained. Using molecular tools, fungal DNA was extracted from pure cultures, amplified with PCR using primers ITS-1F and ITS-4, and sequenced.

Results/Conclusions

Taxa identified from soils were Absidia glauca,  Cladosporium sphaerospermum, Cladosporium sp., Epicoccum nigrum, Mortierella alipina, Mortierella gamsii, Mortierella sp., Mucor recurvus, Mucor sp., Neonectria radicicola, Penicillium biourgeianum, Penicillium expansum, Penicillium herquei, Penicillium sclerotiorum, Penicillium soppii, Penicillium spinulosum, Penicillium sp.,  Trichoderma atroviride, Trichoderma hamatum, Trichoderma koningii, Trichoderma lixii, Trichoderma pachybasioides, Trichoderma sp., Trichoderma tomentosum, Trichoderma velutinum, Trichoderma virens, Trichoderma viridescens, Umbelopsis ramanniana, and Umbelopsis sp. Taxa identified from ticks were Alternaria sp., Cladosporium sp., Colletotrichum sp., Mucor circinelloides, Mucor sp., Penicillium expansum, Penicillium sp., Pestalotiopsis sp., and Sphaeropsis sapinea. Bioassays tested Mucor sp., Penicillium expansum, Cladosporium sp., Colletotrichum sp., and Umbelopsis ramanniana for pathogenicity to I. scapularis. Percent of ticks that died in each fungal treatment were 40, 30, 30, 10, and 10, respectively. Mucor sp., Cladosporium sp., and Penicillium expansum were significantly more virulent than the control (P< 0.01, P< 0.05, P<0.05, respectively). Additional bioassays are being conducted. The most pathogenic species identified from bioassays will be used in a field trial to assess their virulence in nature. From these data, I hope to identify seasonal patterns in entomopathogen abundance and to identify local fungi that contribute in situ to control of black-legged tick populations.