Hungry Invaders

hungry invaders

by guest author Zander Evans

Dutch elm disease changed the forests and cities across the country by removing most, if not all, elms from our wooded wetlands and shade streets. A small beetle from Asia (Scolytus multistriatus) helped spread the disease. The gypsy moth (Lymantria dispar) continues to spread across the country from Massachusetts where it was introduced, leaving major patches of defoliation in its wake. More recently, an even smaller insect, the hemlock woolly adelgid (Adelges tsugae) has killed millions of hemlock trees from Georgia to Maine. The emerald ash borer (Agrilus planipennis) found in 2002 near Detroit, Michigan, is an aggressive killer of ash trees.

What do all these insects have in common? They were all brought to North America from Asia or Europe. These exotic insects have caused havoc in our forests because the trees they attack have developed little resistance to them. Our forests are filled with native insects that attack and sometimes kill trees, but because these insects evolved along with their hosts, they don’t cause complete mortality that non-native species can.

Invasives have negative effects on forest hydrology, carbon storage, and nutrient cycling. The economic cost of invasive insects is staggering. In one study, just three insects cause approximately $1.7 billion dollars in damages in the United States annually. There are over 60 invasive forest insects in the U.S. and some researchers predict a new species will become established here every 5 to 15 years. Some insects, like the hemlock woolly adelgid, focus on just one kind of tree, while others, like the Asian long-horned beetle, can attack a wide range of trees. You can see which insects are likely to attack your forest with maps such as the Alien Forest Pest Explorer ( or the Pest Tracker (

So what does a forest steward do with all these threats out there? The first answer has to be concerted effort to limit new introductions through increased surveillance at ports and other introduction pathways. For an individual landowner, maintaining or enhancing species diversity a cornerstone of forest management in the face of the invasive insects (and many other threats). Intact, diverse forest ecosystems may be more resistant to invasion. Early detection is the next key step. Know the insects (and diseases) most likely to invade your forest and keep a look out for signs of their arrival. For example, the emerald ash borer leaves a ‘D’ shaped hole when it borers out of an ash tree (and branches may start to die from the top down). Many states have programs to help landowners recognize the signs of exotic forest pests or can send out experts to identify the cause of suspiciously unhealthy trees.

And don’t move firewood! One of the big ways invasive insects travel around the country is in firewood. People cut up a dead tree not knowing it harbors an invasive insect and then bring the wood to their camp or campsite where the insect can spread to the forest. Don’t let these invaders hitch a ride! Learn more at:

Invasives are not invincible. Much of their competitive advantage comes from escaping the predators, pests, and pathogens of their region of origin. When those predators, pests, and pathogens catch up with an invader in a new region, the invader is less able to cause unusual damage or disrupt ecosystems. For example, Entomophaga maimaiga, a fungus that attacks gypsy moth, appears to have begun to limit the extent and impact of outbreaks in the areas longest infested by gypsy moth. If we can keep forests healthy and diverse, we give them the best possible chance of resisting and recovering from invasive insects.


Pest Tracker: exotic pest reporting


Also by this author:

Invasive plants, Insects, and Diseases in the forests of the Anthropocene

The speed of invasion: rates of spread for thirteen exotic forest insects and diseases