1. Mountain Pine Beetle Dendroctonus ponderosae lives across western North America. It's the insect responsible for destroying enough spruce, pine, and fir in British Columbia to cover an area as big as North Carolina or New York, and now it's threatening Alberta. It also has ruined huge stands of Colorado pine in recent years. Once it infects a tree, there's no cure. Aside from killing it over time, the beetle also carries a fungus that turns the sapwood blue. A lack of killing frosts in recent years is suspected to have extended the bug's lifespan and let it to be more ruinous than normal. Fire prevention programs also are blamed because the beetle prefers to attack the kind of old, weak trees that often get eliminated by natural or manmade fires. Source: Colorado State University Extension Service.
The wasp carries a fungus that can weaken trees and make them susceptible to feeding by larvae. Sirex woodwasps like to begin building in stressed, injured trees before moving to healthier ones. That makes it different than native woodwasps, which only go after dead and dying trees.
It particularly likes the kind of stressed trees that are used to make wood pallets and packing materials. Those pallets often get shipped around the world, giving the wasp a free ride to new territories. Source: Pennsylvania departments of Agriculture and of Conservation and Natural Resources, U.S. Forest Service Pest Alert.
3. Emerald Ash Borer Agrilus planipennis 'Fairmaire' is best known around Detroit, where it's responsible for the bulk of 30 million ash trees destroyed since it arrived in summer 2002. Its territory now extends from western Pennsylvania to northern Illinois.
Ash borer larvae feed on the inner bark of the ash tree (and that tree alone), disrupting the tree's ability to carry water and nutrients. It could do to ash trees what blight did to the chestnut tree. Source: www.emeraldashborer.info (U.S. Forest Service and various states and universities.)
4. Fusiform Rust Formally, it's Cronartium quericum f. sp. fusiforme, but it's better known as fusiform rust, canker rust, or fusiform canker. It causes millions of dollars of damage each year to slash and loblolly pines as well as to some longleaf pines. Trees with stem cankers can't be used for poles or pilings.
Fusiform rust spreads via fungus spores that resemble a fine orange powder. These lead to brown, hairlike bristles on the underside of oak leaves, containing spores that travel back to new pine needles and branch tips. The fungus then infects seedlings, causing them to swell and produce the canker. Source: University of Georgia Cooperative Extension Service.
5. Ips Engraver Beetles Look for the scooped-out posterior to identify these nasties, which kill more Southern pine timber than any other forest insect outside the Southern pine beetle. Ips avulses, grandicollis, and calligraphus usually attack injured, dying, or recently felled trees, particularly those affected by drought or lightning strikes. Source: U.S. Forest Service