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Tracking the spread of ecological pests

Photo by Kerry Wixted

Cary Institute ecologist Dr. Shannon L. LaDeau is working to understand how environmental conditions influence the spread of undesirable pests and pathogens such as West Nile virus and the hemlock wooly adelgid. This work includes understanding the dynamics of pest communities and taking into account things like land use, climate change, and combined stressors.

As a post-doctoral fellow with the National Science Foundation, LaDeau analyzed bird population data, with the goal of revealing new information about West Nile virus in North America. She demonstrated large declines in common bird species and increased virus transmission in disturbed landscapes.

LaDeau is adding a field research component to her West Nile virus investigations. She is also embarking on a new research program looking at hemlock tree mortality in the Northeast. Recently, we discussed her work.

West Nile virus was detected in New York in 1999; within a decade, it has spread throughout most of the U.S. and Canada, infecting countless birds and thousands of humans. How did it move so rapidly?

When the virus arrived, competent vectors and hosts were already present across North America. For West Nile virus to persist, it needs both mosquito and bird species that can concentrate the virus in their bloodstream at levels high enough to be infective to other organisms. Crows, jays, and robins are among the most effective hosts. We suspect that mosquito movement and migrating birds played a role in facilitating the spread.

To identify how bird populations have changed since the virus emerged, you assessed North American Breeding Bird Survey data. What did that reveal?

American crow populations have had the most drastic and widespread reductions; American robins, blue jays, chickadees, eastern bluebirds, and tufted titmice have also suffered declines. unfortunately, the virus has probably infected a broader range of birds than we’ve been able to detect. Most surveys are not designed to estimate populations for things like birds of prey, water birds, or nocturnal species.

How do humans become infected?

Humans only become infected when the virus has built up to high levels in the mosquito-bird cycle. Culex mosquitoes, which are the primary vectors, tend to feed on birds. They move to mammals when birds are scarce. In the Northeast there is evidence that mosquitoes may shift their feeding to favor humans in the late summer, when many birds disperse. This is generally when we see peaks in human epidemics. Virally speaking, humans are a dead end; we don’t build up enough of the virus to re-infect mosquitoes.

So understanding mosquito population dynamics is critical?

Very much so. I am currently assessing mosquito abundance and diversity along an urban-to-rural gradient in Baltimore. West Nile virus infections in humans and birds are more prevalent in urban areas; we are trying to understand why. This spring and summer, I will be looking at how land use and climate influence mosquito communities. Vector species may be more adept at colonizing degraded sites. Intact ecosystems have a higher diversity of mosquitoes, many of which are less competent at spreading West Nile virus.

You are also beginning a research project on forest pests and pathogens.

In collaboration with Dr. Gary M. Lovett, I am researching two insect pests in hemlock-dominated forests in the Northeast. We are interested in ascertaining how things like drought and pollution influence mortality in infested trees. Like the West Nile virus research, we are trying to read ecological cues to predict what the future environment will look like.

What drew you to this type of research?

It’s fascinating to try and unravel how environmental stressors like climate and human development interact to influence pests and pathogens; this type of information is essential to managing ecosystems in a way that supports biological health.

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