The landscape (flora and fauna) of North America has been greatly changed over the last 500 years due to introductions of non-native species. Prominent introduced species of insects include the common house cricket, Hessian fly, Gypsy moth and a recent crop pest, the soybean aphid. I have previously posted about soybean aphid here and here.
The soybean aphid feeds on soybeans during the summer, sometimes developing large enough populations to require that soybeans be treated with pesticides. This is an additional cost that farmers did not previously have. Once the soybeans are harvested, the soybean aphids must find a suitable host for the winter. The soybean aphid evolved in Asia and its overwintering plant in Asia is the buckthorn, Rhamnus cathartica. Without buckthorn and without soybeans, both non-native species, the soybean aphid could not survive in North America.
Soybeans were introduced to the United States near Savannah, Georgia in 1765 by Samuel Bowen a sailor who imported the plants from China. Bowen manufactured and marketed soy sauce. Soybeans were a minor crop until the Dust Bowl. Farmers found that soybeans would fix nitrogen and improve the soil. Soybeans were suitable for large scale mechanical planting and harvesting using tractors. Henry Ford, of Ford Motor Company, promoted the use of soybeans in part as a way to promote sales of his Ford tractors for large scale agriculture. Soybeans have increased in importance as a crop, especially in the Midwest where rotations of corn and soybeans (farmers switch planting these crops every other year) are used to manage soil nutrition and insect pests such as corn rootworm.
Buckthorn was brought to the US from Europe in the mid-1800s. Buckthorn will form dense hedges and was commonly used in landscaping. However, buckthorn produces berries that are eaten and spread by birds. With few insects and diseases in its new North America home, buckthorn has spread across the landscape of the Midwest of the United States. The nursery industry stopped selling buckthorn in the 1930s, but by then it was too late. Today, buckthorn is so widespread it would be nearly impossible to eliminate.
The introduction and spread of buckthorn changed the landscape to allow the soybean aphid to establish, thrive and become an invasive pest. In addition to allowing soybean aphids to survive the winter, buckthorn also serves as an alternate host of crown rust fungus, a pest of oats. Oats planted near infested buckthorns suffer damage from the rust.
Several instances of creating expensive new problems by importing species have led to restrictions on moving plants and insects. These rules and regulations are often not understood or fully appreciated by the public. Importing ornamentals from other lands seems harmless enough. However, the buckthorn – soybean aphid story is a cautionary tale about the potential danger of importing seemingly innocuous species.