In areas where Lyme disease is endemic, it is desirable to control populations of native ticks, which transmit several pathogens to humans causing Lyme and other diseases. We are seeking environmentally safe and effective means of controlling tick populations using the fungus Metarhizium anisopliae, a native species known to attack ticks under some circumstances. Our approach is considered 'augmentative biocontrol' because it consists of increasing the probability of contact between ticks and natural enemies, thus augmenting the effects of potential control agents.
We have been working with an entomopathogenic fungus, Metarhizium anisopliae, because it is native, widespread in soil and leaf litter in North American forests, and virulent against ticks. Our approach is to assess the sensitivities of different tick life stages (eggs, larvae, nymphs, adults), and conditions (unfed or replete) to the fungus, with the purpose of designing systems of fungus delivery that will maximize impact on ticks while minimizing nontarget effects (the fungus is pathogenic to many insects as well as ticks). We have found that adult ticks are more susceptible than are larvae and nymphs, and that the potential exists for aerial spraying of forest understory vegetation with fungus solution in the fall, when adults are most active (Benjamin et al. 2002). More striking is the enhanced effect of fungus on engorged, compared with flat unfed ticks. This result suggests that applying fungus to vertebrate hosts for ticks, on which the ticks engorge, might be a safe and effective means of reducing tick numbers. Field trials have partially supported this suggestion, but these efforts are hindered by the large number of hosts on which the ticks feed and the difficulty in delivering fungus directly to hosts.
We have also found that the fungus can compromise the health, body condition, and reproductive output of ticks even when it doesn't kill them ("sublethal" effects), and that the effects of fungus might be enhanced when delivered in combination with low doses of a relatively safe chemical pesticide, permethrin (Hornbostel et al., 2004, J. Med. Ent. 41:922-929 and Hornbostel et al.). Finally, we have found moderately strong effectiveness of M. anisopliae when delivered to the nesting materials inside experimentally deployed mouse nest boxes. By targeting the fungus directly at hosts, rather than broadcasting it into the environment, efforts to control ticks can minimize effects on nontarget organisms.