Invasive Species Hopscotch

Years ago, the Oregon Department of Agriculture (ODA) responded to a call from a railroad company that reported a worker had been stung by ants. I went was sent to investigate. The train was stopped on a siding in Salem. One of the boxcars contained cotton seed destined for cattle feed. It took me only a few minutes to find red imported fire ants (RIFA), Solenopsis invicta, in the cargo. There were a lot of them. The boxcar was sealed up and fumigated.

RIFA is one of the old-guard invasive species in this country. Native to South America, these ants were introduced to Mobile, Alabama in the 1930s. They have since spread throughout the entire southeastern United States, and in the 1990s, they made the hop to southern California. As their name suggests, fire ants have a powerful sting -- it is painful to humans and fatal to baby birds and newborn animals. The mounds they build interfere with machinery. RIFA is old news in the Southeast, but it is back in the scientific spotlight.

Scott Bauer, a U.S. Department of Agriculture researcher, has been studying the genetics of RIFA. He and his collaborators collected ants from 2,144 RIFA colonies in 11 countries, including newly invaded sites in the Caribbean, Australia, New Zealand, Taiwan, and China. Genetic analysis indicates these new invasions are coming from the United States, not RIFA’s homeland in South America.*

Nik Grunwald at Oregon State University is uncovering a similar hopscotch pattern of spread by sudden oak death (SOD), Phytophthora ramorum. There are three clonal lineages of SOD in North America. One was introduced into California. Another first appeared in British Columbia or Washington, and a third hopscotched here from Europe. Amazingly, scientists haven’t yet found this pathogen’s native range!  It must be pretty inconspicuous at home.

Another invasive species playing hopscotch is the granulate ambrosia beetle, Xylosandrus crassiusculus. Home for this wood borer is Africa and Asia, but it invaded the southeastern United States in the 1970s. In 2004, we caught a bunch of them in The Dalles. They weren’t coming from Africa or Asia; they were hitchhiking in green railroad ties imported from the Southeast. It took us a couple of years to eradicate them.

Secondary invasions may be more important than we realize. Populations of invasive species are often much higher in invaded territories outside their native range and those populations may be pre-adapted for playing invasive species hopscotch.

*Science News. March 26, 2011. Vol. 179(7) pg. 15.

Dan Hilburn