Vehicles are astoundingly effective at picking up and transporting invasive species. Put another way, invasive species are amazingly good at hitchhiking. Last week I wrote about research on seeds recovered from an Australian car wash. A reader passed on a tip about similar research going on in Montana1. Those experiments have shown that an SUV or truck driving on non-paved roads would, on average, pick up 176 seeds per 50-mile trip. ATVs were capable of picking up as many as 200,000 seeds over 48 off-road miles (4200 seeds per mile)! Not all of the seeds were from noxious weeds, of course, but hundreds of them were, and that is a problem.
Vehicles aren’t the only vectors for hitchhiking invasive species. This time of year, Oregon Department of Agriculture horticulturists are inspecting and certifying Christmas trees destined for out-of-state markets. Trees on the loading docks now are destined for Mexico, Pacific Rim countries, and Hawaii. The inspectors’ job is to make sure hitchhikers aren’t riding along with the trees. It is a Herculean task. The volume of trees is mind-blowing (7.8 million trees harvested annually in Oregon), and the potential hitchhikers are tiny and well-hidden.
Yellowjacket queens like to take shelter in Christmas trees during the fall. Hawaiian regulatory officials are concerned that German yellowjacket, Vespula germanica, an invasive species now found throughout most of the United States, including Oregon, will find its way to Hawaii on the Christmas tree express. Our most common native yellowjacket, V. pennsylvanica, invaded the islands years ago and forms enormous colonies there.
Mexico has a list of potential invaders they are concerned about, including Douglas fir twig weevil and Douglas fir tussock moth (both native to Oregon), and German yellowjacket and European pine shoot moth (invasive species in Oregon). Conscientious growers keep the hitchhikers to a minimum by storing their cut trees off the ground and mechanically shaking trees before baling and loading them.
So far the first 70 truckloads of Oregon Christmas trees to reach the Mexican border have passed through without incident. However, two containers of Christmas greens were rejected recently in Japan due to another hitchhiker, strawberry root weevil (invasive species). Good job, Japanese inspectors--I’m sorry we missed them on this end.
Government inspectors do catch a lot of potential invaders, but they are only part of the solution. There is a role for us. We can help reduce the spread of invasive species by not transporting hitchhikers. Rinse off your vehicle, boat, ATV, and boots ASAP after leaving natural areas, and this year give your Christmas tree a good shake before bringing it inside. You might be surprised at who was on track to crash your Christmas party!
1Rew, L. and F. Pollnac. 2010. Seed Dispersal by Vehicles. News from the Center for Invasive Plant Management, MT St. Univ.: http://www.weedcenter.org/newsletter/docs/2010-04-seed-dispersal.pdf