Training for invading: Did Europe's Ice Age give exotic pests the edge?

When Bill Mattson, research entomologist with the Forest Insects unit in East Lansing, refers to invading aliens, he's not talking about little green men. "In the last 500 years, nearly 2,000 insects and 2,000 weedy plants have invaded North America," Mattson wrote in a November 1996 BioScience article. "When the outrageous economic and ecological costs of the wanton spread...become common knowledge...there will be a public outcry for action."

It didn't take long for his prediction to come true. On June 17th of this year, Vice President Gore published an official response to a letter of concern about exotic pests signed by more than 500 leading scientists. A week later, Mattson was summoned to testify at a Congressional Committee on Resources subcommittee hearing on exotic pests threatening the health of the National Forests.

Of the 400 insect pests established on woody plants, wrote Mattson, five percent have caused severe ecological impacts. The European elm bark beetle, for instance, has contributed to the spread of Dutch elm disease and the death of most of the American elms in eastern North America. What about the remaining 95 percent of these biological pollutants? "Very little is known about their influence on natural processes," Mattson warned. "However, evidence suggests that all may irrevocably change their respective invaded ecosystems." To counter this bad news, Mattson offered some solutions:

1. Plug the "leaky dikes" to minimize further introduction and spread of exotics. 2. Limit pest population growth by, for instance, importing natural enemies, genetically bolstering plant defenses, and creating environmental conditions that give plants a fighting chance.

More and better research is the foundation for what Mattson calls a trench war. "One of the first things we have to ask ourselves," Mattson said "is 'Why are European exotics doing so well on our continent?' " (They do better here than our insects do over there.)

His answer became the "crucible theory" developed along with Pekka Niemela, of the University of Joensuu, Finland. The theory posits that the European invaders do well for two reasons. One, North America offers them ideal conditions, and two, evolutionary forces back home have made them colonization experts.

To the European invader, North America is a candy store of feeding opportunities. There are more host plants here, including many species that have become extinct in Europe, but that evidently still can be recognized by the insects. On each of these plants, there are fewer competing insects per leaf than the invaders are used to encountering in Europe. European insects may also be better "take-over artists" than North American insects because of their unique evolutionary gauntlets. The greatest gauntlet was the Pleistocene glaciations, which banished biota to small, ice-free refugia. As Mattson explained, "Europe's east-west mountain range supported its own glaciers and ice sheets, further shrinking the area that biota could use for retreat. Different species were forced together in far-flung pockets along the midslope of the range.

"Many species went extinct in those close quarters, but others capitalized on the situation, developing new reproductive techniques such as uniparental reproduction (e.g., selfing, as in mother-son and sibling matings and parthenogenesis). This genetic flexibility allowed even small populations to reinvade the land and react nimbly to environmental hardships."

North America's glaciation, in contrast, pushed species southward, but not into such constricted areas. The evolutionary crucible was not as extreme, and American species never had to be as aggressive as their European counterparts.

The result is that many European insects, tested first by ice and later by human-caused disturbance, can outcompete our native species. This "hostile takeover" phenomenon seems to hold for European plants as well. Understanding why your competitor succeeds is a time-honored tactic in American commerce. By researching pests in the same way, Mattson hopes we can learn to shift the balance of power back towards our native species.

This article first appeared in the July 1997 edition of the North Central News, a newsletter written by the USFS NC Forest Expt. Station and is based on the paper, "Invasion of North American forests by European phytophagous insects" , by Pekka Niemela and William Mattson and can be found in BioScience. 46(10): 741-753 from 1996