Biological Control of Ticks
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.
While investigating how forests responded to defoliation stress, Institute ecologists discovered that white-footed mouse populations played a large role in regulating the moths. Key predators on gypsy moth pupae, research showed that moth populations declined when mice were abundant.
More interestingly, scientists discovered a connection among acorn production, mouse population size and the number of blacklegged ticks infected with Borrelia burgdorferi, the bacterium that causes Lyme disease.
Ecology of Lyme disease
Lyme disease is caused by a spirochete bacterium, Borrelia burgdorferi. This bacterium is transmitted to humans during blood meals taken by infected ixodid ticks on human hosts. Several ixodid tick species can transmit the disease; in eastern and central North America the primary vector is the blacklegged tick, Ixodes scapularis.
Larval ticks hatch uninfected and are not initially dangerous to humans. If they feed on an infected host during their larval blood meal, they can become infected and later transmit Lyme bacteria to people. Whether a larval tick will acquire an infection and thus molt into an infected nymph depends largely on the species of host on which it feeds.
Different species of tick hosts tend to have different probabilities of transmitting an infection to a feeding tick. In eastern and central North America, the host most likely to transmit an infection to a feeding tick is the white-footed mouse (Peromyscus leucopus), which infects between 40% and 90% of feeding larvae. Eastern chipmunks (Tamias striatus) and shrews (Blarina brevicauda and Sorex spp.) tend to be moderately competent reservoirs for B. burgdorferi. Most other mammalian, avian, and reptilian hosts have a considerably lower reservoir competence.
Lyme disease prevalence & mice
White-footed mice are the principal natural reservoirs for Lyme disease bacteria. Ticks that feed on mice are highly likely to become infected, making them capable of transmitting Lyme disease to people during their next blood meal. When they feed on mice during their larval and nymphal stages, ticks are more likely to survive and molt. Ticks that feed other vertebrate host species have a comparatively lower survivorship. Consequently, we have hypothesized that the greater the abundance of mice during the midsummer peak in larval tick feeding activity, the greater the probability that questing larval ticks will encounter a mouse, and the higher the probability that they will molt into an infected nymph capable of transmitting Lyme bacteria one year later.
Our long-term monitoring of mouse abundance, tick abundance and infection prevalence in southeastern New York State supports these hypotheses. In addition, because we have demonstrated a correlation between acorn production and mouse abundance the following year, we have hypothesized that acorn abundance is a good predictor of abundance and infections prevalence of ticks almost two years in advance. This hypothesis also is supported by our monitoring data.