Forest disturbance by invasive insects and diseases in the United States

Invasive insects and pathogens (diseases) are introduced to the United States through international trade, and spread through means of natural and human-dispersal.

[2] The importation of live plants has been the pathway attributed to over 60 percent of the most damaging non-native pests to the United States.

[2] Live plants are an especially effective pathway because they provide sustenance to the pest over long journeys on otherwise inhospitable cargo ships.

These low quality wood products, like crating and pallets, often retain the outer part of the tree (bark and phloem) where insect life can be harbored.

[1] Other less common pathways include "hitch-hiking" directly on cargo ships and containers and in air passenger baggage.

[1] Once introduced into forests of the United States, invasive insects and pathogens can spread in two ways: natural and human-assisted dispersal.

The rate of natural dispersal varies depending on the adaptations of the pest species but is more diffuse, spreading outward from the invaded area.

A typical dispersal of invasive insects and pathogens is characterized by both means: diffuse spread coupled with long-distance "jumps" to discrete areas.

For example, discrete populations of emerald ash borer have been traced to remote campsites where natural spread would not have been possible.

A pathogen may slowly weaken a tree (beech bark disease) or it may cause rapid decline (chestnut blight).

The decline of a more dominant tree (American chestnut) or one that fills a unique role (such as a nitrogen fixer) has a greater impact.

[1] Short-term disturbances are physical damages directly caused by the invasive pest and are measured on a time-scale of months to years.

Oaks—a favorite food source for spongy moth larva—produce a reduced acorn crop for years after a defoliating event.

[10] Recent and rapid outbreaks such as emerald ash borer, however, have invigorated interest in understanding the ecological consequences of invasive pests and how to restore affected forests.