Isolation of Metarhizium guizhouense and Metarhizium robertsii Strains from Soil-Exposed Amblyomma americanum (Acarina: Ixodidae) from Northwest Arkansas, USA

Abstract

The lone star tick, Amblyomma americanum (L.) (Acarina: Ixodidae), is the most abundant tick species found in Arkansas and is involved in the transmission of pathogens of medical and veterinary importance. When not feeding, most non-nidicolous tick species shelter in the soil and leaf litter where they may be exposed to and potentially infected with entomopathogenic fungi that reside naturally in the soil. Entomopathogenic fungi in the genus Metarhizium Sorokīn (Hypocreales: Clavicipitaceae) have shown promise as biological control agents of ticks. Here, the first study to isolate and identify Arkansas-derived isolates of Metarhizium from A. americanum ticks is presented. We exposed 320 ticks artificially to native soil from Savoy. Of these soil exposed ticks, 2.5% of adults and 1.5% of nymphs displayed signs of infection with Metarhizium. Of the infected Savoy adults, 3.3% were females and 1.7% were males. Similarly exposed ticks from West Fork resulted in only 2.4% of nymphal ticks being infected with this fungus. Eight isolates of Metarhizium were cultured from infected ticks exposed to soil from these locations. Four of these Metarhizium isolates (3 from Savoy and 1 from West Fork) were identified to species by sequencing of the ITS locus and the EF1-α genes. Three Savoy strains (P10N1, P10AF1, and P2AM1) had identical sequences and were identified as Metarhizium robertsii (Bischoff, Rehner & Humber) (Hypocreales: Clavicipitaceae). The strain from West Fork (P9N2) was identified as Metarhizium guizhouense (Chen & Guo) (Hypocreales: Clavicipitaceae). The ITS and the EF1-α sequences of the Savoy strains showed 100% similarity to M. robertsii strains ARSEF 2575 and ART 500, respectively. The ITS and EF1-α sequences of the West Fork strain showed 99% similarity to M. guizhouense strains ARSEF 977 and CBS 258.90, respectively. This study demonstrates that entomopathogenic fungi M. guizhouense and M. robertsii are pathogenic to and can be isolated from A. americanum. Furthermore, the EF1-α genetic marker was shown to be a very effective tool for distinguishing different species of Metarhizium from ticks when used in conjunction with ITS sequence data. Standardizing the use of ticks in soil exposure methods for isolating entomopathogenic fungi could be useful for obtaining isolates that are highly virulent to A. americanum. The isolation and identification of Metarhizium spp. from A. americanum in Arkansas indicates that further exploration of entomopathogenic fungi as biological agents to control A. americanum is warranted.