Ixodes scapularis, the black-legged tick (formerly the deer tick), is prevalent in deciduous forests and edge habitat near homes in the Northeastern US. Humans have tried to control populations of this tick because it vectors Borrelia burgdorferi, the bacterium that causes Lyme disease. There has been little success, however, possibly due to poor understanding of the ecology of the natural enemies of the tick. Several entomopathogenic fungal species are known to affect ticks, but most are not viable under elevated temperatures or low humidity, and each species is thought to be most pathogenic at specific temperatures and levels of relative humidity (RH). We predicted that fungi that are most virulent as pure isolates under laboratory conditions may not function the same under field conditions. To test the environmental range at which Beauveria bassiana and Metarhizium anisopliae were most virulent, we exposed field-collected nymphal ticks to each fungal species, and responses were compared to a control with no fungus. Ten replicate ticks per fungal treatment in each temperature x humidity combination were incubated at relative humidities of 100%, 95%, 85% or 75% in temperatures of 15 oC, 25 oC , 30 oC and 40 oC for two weeks.
Results/Conclusions
Both fungi increased tick mortality above the control, particularly at high humidity. In another assay conducted at room temperature, local isolates were compared to commercial products containing these same two fungi, alone or in combination. At 100% RH, the commercial product containing B. bassiana was slightly more virulent to the black-legged tick than the product containing M. anisopliae. In contrast, the local isolate of M. anisopliae was more virulent than B. bassiana at higher humidities, while B. bassiana was more virulent at lower humidities. In addition, the combined local isolates killed more ticks than other treatments at all levels of RH tested. Based on these results, it is likely that RH affects the virulence of native B. bassiana and M. anisopliae to the black-legged nymphs in situ and that there may be differences between these entomopathogens may produce different effects when alone versus together. When this approach was tested in the field, fewer ticks were recovered from cages treated with both commercial products than in cages treated with either product alone. Products containing local isolates of several species of fungi applied at moderate to high humidity levels might be the most effective biological control product for black-legged ticks.
