Prairie dogs in the western U.S. can carry the bacterium that causes bubonic plague, and their fleas play a primary role in spreading it among prairie dogs and to other animals. A group of scientists say insect-pathogenic fungi that target those fleas could be a useful tool in interrupting that transmission route. (Photo by USFWS Mountain-Prairie via Flickr, public domain)
By Stefan T. Jaronski, Ph.D.
Stefan T. Jaronski, Ph.D.
While peri-domestic rodents are famously responsible for bringing the bubonic plague bacterium into contact with humans, the prairie dog of western lore and tourist attraction is subject to the bacterium, vectored by its flea. The prairie dog, a keystone species on western rangelands and primary prey for the endangered black-footed ferret, can also be a victim of the bacterium, with entire colonies eliminated from bubonic plague outbreaks. Aside from increased potential for conservation dilemmas caused by plague's invasion of western prairies, plague in the western U.S. has the potential to spread by the drier conditions associated with climate change.
The prairie dog flea, as vector of the pathogen, is a potential weak link in disease transmission, and considerable effort is currently made to manage the flea population. Several chemical insecticides are used with varying efficacy, while fleas are evolving resistance to some compounds.
Meanwhile, insect-pathogenic fungi, namely strains within the genera Beauveria and Metarhizium, are being increasingly developed to manage a wide variety of insect pests. Why not for prairie dog fleas? In a new article in the open-access Journal of Integrated Pest Management, my colleagues at the U.S. Geological Survey and West Virginia University and I review existing research and explore the possibility of applying fungi as a biocontrol tool for prairie dog fleas that vector plague.
A microscope view shows a flea afflicted with the insect-pathogenic fungi Metarhizium, one of the varieties of potential use for targeting fleas on prairie dogs in the western U.S., to interrupt transmission of the bacterium that causes bubonic plague. (Photo courtesy of Stefan T. Jaronski, Ph.D.)
Well over 200 strains of insect-pathogenic fungi have been commercialized worldwide, with several "mycoinsecticides" registered in the U.S. and Canada. These fungi have proven safe for vertebrates, including humans. A limited body of literature indicates fleas such as cat, dog, and rat fleas are susceptible to these fungi. In the case of prairie dogs, the animal and flea habitat of underground burrows can be ideal for the survival and even reproduction and spread of these fungi. There is little ultraviolet light exposure, and abiotic factors seem favorable for fungal survival based on existing literature. Furthermore, targeting prairie dogs and their fleas would reduce exposure of the fungi to most terrestrial, non-target insects.
Application to the fleas on the animals, however, is the challenge. One cannot readily apply an aqueous spray to individual wild animals as one would domestic animals or crops. Colonies are large and the animals are preferentially cryptic except for the famous sentinels poised at the mouths of burrows, quickly retreating into their burrows at the approach of apparent danger.
Applying insect-pathogenic fungi to prairie dogs to target their fleas, which transmit the the bacterium that causes bubonic plague, can be a challenge. One application method under evaluation is blowing a dust formulation of the fungi into the first few meters of burrows, where the prairie dogs would contact the dust and track it deeper into the burrows. (Photo courtesy of Stefan T. Jaronski, Ph.D.)
Prairie dogs in the western U.S. can carry the bacterium that causes bubonic plague, and their fleas play a primary role in spreading it among prairie dogs and to other animals. A group of scientists say insect-pathogenic fungi that target those fleas could be a useful tool in interrupting that transmission route. (Photo courtesy of Stefan T. Jaronski, Ph.D.)
Fungi in the genus Beauveria, shown here in a lab culture, are among the varieties of potential use for targeting fleas on prairie dogs in the western U.S., to interrupt transmission of the bacterium that causes bubonic plague. (Photo courtesy of Stefan T. Jaronski, Ph.D.)
A dust formulation may be the most efficient carrier. Dusts of fungus spores and fine talc, diatomaceous earth, clay, or carnuba wax exist. Such formulations can be blown into burrows using hand- or machine-powered dusters. One approach would be to blow dust into the initial 1-2 meters of each burrow, so that dust-laden spores are tracked more deeply into burrow systems on rodent feet and fur. Spores could then transfer from the animals to their fleas and perhaps among fleas. There is a possibility that the dust formulation could be transported by the rodents into nesting areas to affect adult and larval flea populations. Beauveria conidia with talc has been experimentally used for dry spore applications.
In our view, fungi could serve as a valuable component in managing prairie dog fleas and the plague bacterium, and these application strategies are worth exploring. We need studies in which these entomopathogenic fungi are applied to rodent burrows, with evaluation of their potential impacts on flea populations and of potential costs and benefits to wildlife conservation and public health.
Stefan T. Jaronski, Ph.D., aka "The Bug Doc," is an insect pathologist retired (but "still kicking") from the U.S. Department of Agriculture's Agricultural Research Service. Today he is an adjunct professor of entomology at Virginia Tech University and owner of Jaronski Mycological Consulting LLC, based in Blacksburg, Virginia. Email: thebugdoc01@gmail.com
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