Invaders on the horizon: Threats to Minnesota oak woodlands

By Lesley Tylczak

Emerald ash borer seems to be on everyone’s mind. Ash trees are a traditional component of Minnesota’s forest landscape, and losing them means more than just a few empty spaces in the canopy—it means an affecting change to the character of the state. But the emerald ash borer isn’t the only insect pest that stands to degrade or eliminate valuable native trees.

APHIS, the Animal and Plant Health Inspection Service (and part of the U.S. Department of Agriculture), created a list of the exotic species they believe are not yet found in the United States, but pose the greatest risk to the nation if they become established. Of that list, 15% were pests of oak trees—including the top two, the oak splendor beetle (Agrilus biguttatus) and the oak ambrosia beetle (Platypus quercivora). Though neither beetle is found in the North America yet, the threat posed by these insects is clear.

Oak splendour beetle. Photo by Steven Valley via Click for original.

The oak splendor beetle, for example, is a type of insect known as a “jewel beetle”, thanks to its long, iridescent wing covers. Does the sound of a long, shining wood-boring insect feel familiar? Yes, oak splendor beetle and emerald ash borer, as two members of genus Agrilus, are close cousins, and they attack trees the same way. Oak splendor beetle can lay dozens of eggs under the bark of oak trees (genus Quercus). When these eggs hatch into larvae, the larvae burrow through the phloem and sapwood of the tree. This girdles the tree from the inside, preventing the upper part of the tree from getting nutrients from below. If the larvae burrow through enough of the circumference of the tree, the tree dies. Just as EAB has exterminated countless stands of ash, oak splendor beetle stands to wipe out countless native oaks.

Oak ambrosia beetle is also similar to another well-publicized problem: oak wilt. Though oak ambrosia beetle is troublesome on its own, creating small holes in the wood of oaks and other species, it is the exotic fungus carried by the beetle which proves to be the greatest danger. This fungus, Raffaelea quercivora, is lethal to many oaks in its native Japan. If oaks in the United States were exposed, it could easily be even more detrimental than oak wilt, which in turn is spread in part by sap beetles, which carry the disease-bearing fungal pads on their feet.

Rosy gypsy moth. Photo from USDA APHIS archives, via Click for original.

These are not the only species on APHIS’ list. Defoliators take other high-ranking positions. Gypsy moth (Lymantria dispar), an example of an invasive moth, defoliated over 1.2 million acres of woodland in 2010. Its close relative, the pink gypsy moth (Lymantria Mathura) is thought capable of similar damage. Tortrix moths (“leafrollers”), such as light-brown apple moth, variegated golden tortrix, and summer fruit tortrix are also of particular concern. Though defoliation rarely kills a tree in a single year, repeat defoliation can be fatal.

The key to preventing the establishment of invasive species is vigilance. Oak splendor beetle and oak ambrosia beetle are both transported in untreated wood products. Moths can also be carried in commercial products. Awareness of proper treatment for firewood and transport wood (such as pallet wood) is essential in preventing the introduction of species from abroad. First-detector networks are also crucial for woodland health. The sooner that invasions are identified, the sooner they can be treated, and the higher the odds of eradicating the pest organism or preventing widespread establishment.

Oaks are long-lived, valuable, and part of our forest heritage. While the species discussed here are not known to be established in Minnesota, they have potential to be introduced and may threaten native oaks if that occurs. Dutch elm disease, chestnut blight, and emerald ash borer have already taken their toll on American forests. Family forest owners can help to ensure that oaks remain safe from the threats that linger on the horizon by remaining informed about insect and disease threats and carefully monitoring their woods for new arrivals.

Lesley Tylczak is a graduate student in the University of Minnesota Entomology Department.

The University of Minnesota Extension Forestry team.

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1 Comment

  1. Lesley,

    While it is useful to have a handle on potential future I & D problems
    facing the Lake States, it seems to me more productive to emphasize
    the positive ( rather than the negative). “Threats” is a strong word

    Perhaps rather than stress the gyspy moth’s defoliation history, you might
    mention that there are signs that it is “naturalizing” in our forests…and that
    the gloom and doom that was predicted twenty years ago for that pest has
    not come to pass….

    This is just one example…there are others….which I’m sure you are aware

    Landowners are best served by those who do not spread fear, but rather

    Peter Bundy CF
    Masconomo Forestry.