A biological invasion is an enormous increase in population of some kind of living organism. It happens when an organism — like an insect — arrives somewhere beyond its previous range, when it breaks out past its natural barrier, unbalancing the biological order. More than 450 non-native insects have invaded our forests and urban trees since European settlement. In this series, we'll explore four of these insects, and the scientists studying and combating these pests. In 1957, a British ecologist, Charles S. Elton, gave three radio presentations entitled “Balance and Barrier.” Within a year, he had expanded these ideas into what was to become a bible for practitioners of a burgeoning new science: invasion biology. In a tribute to those broadcasts, this six-part series will explore biological invasions — and their repercussions — in the Midwest and the Northeast. Related Research: “Ecology of Forest Insect Invasions” (2017) “A Highly Aggregated Geographical Distribution of Forest Pest Invasions in the USA” (2013) “Historical Accumulation of Nonindigenous Forest Pests in the Continental United States” (2011) “Economic Impacts of Non-Native Forest Insects in the Continental United States” (2011) “The Ecology of Invasions by Animals and Plants” (1958) Scientists: Therese Poland, Project Leader/Research Entomologist, Northern Research Station, Lansing, Michigan Sandy Liebhold, Research Entomologist, Northern Research Station, Morgantown, West Virginia Robert Haight, Research Forester, Northern Research Station, St. Paul, Minnesota Produced by the USDA Forest Service Northern Research Station. Want more information? Visit us at: www.nrs.fs.fed.us/podcast/1/1/ Any questions? Connect with us on Twitter!