New York and heading north?

Look at a penny and zero in closely at Lincoln’s nose. That’s the size of the redbay ambrosia beetle (RAB), a tiny insect and key player in laurel wilt, an aggressive fungal disease of trees in the Lauraceae family transmitted by the RAB.

Although important landscape and forest trees are in this family, mountain laurel, rhododendron and sweetbay magnolia are not.

Cornell University plant pathologist Dr. Margery Daughtrey said there have been several reports over the past four years regarding strange deaths of Long Island sassafras trees, which are in the Lauraceae family.

“In the cases where we looked at tree samples, it seemed to be a canker disease,” said Daughtrey. “Our investigation took us to Northport, in the central northern part of Long Island, where a homeowner had reported the problem.”

Cuts into the tree revealed streaking under the bark and xylem discoloration, which turned out to be characteristic of the disease. There was speculation about what was wrong, but no one knew for sure.

“We knew laurel wilt was a possibility,” said Daughtrey, “but it would have had to fly in a jet to get to Long Island.” Harringtonia lauricola, transmitted by the RAB, is the fungal pathogen that causes laurel wilt.

Dr. Jason Smith, forest pathologist, University of Mount Union in Ohio, has been studying RAB and its role in laurel wilt. Smith said there wasn’t much initial concern about laurel wilt or the beetle because one of the first hard-hit species was redbay, a broadleaf evergreen species in the laurel family that grows in the southeastern coastal plains. Despite its importance in local ecology, redbay wasn’t well-known. However, when avocado trees, also in the Lauraceae family, began succumbing to laurel wilt, research became a priority.

Known Lauraceae hosts in the U.S. include redbay, swamp bay, silkbay, avocado, camphor tree, sassafras, European bay laurel, Northern spicebush, lancewood and pondberry – a federally endangered species. Other Lauraceae species will likely be added to the list.

When dissected, very little fungal tissue is found in host species. Trees can be killed experimentally with fewer than 1,000 spores. Smith said this is related to how the tree responds to the fungus rather than the fungus killing the tree.

“The tree is basically killing itself because it’s overreacting to the fungus,” he said.

Ambrosia beetles are typically harmless and gravitate toward trees that have started to die or are dead, perhaps the result of freezing, flooding or lightning. Beetles bore into the tree and make horizonal tunnels that go deep into the sapwood. Smith said the fact that RAB are going beyond the bark is an indication they’re not feeding on the tree. The most alarming aspect of RAB and its role in laurel wilt is the beetle’s unique feeding and reproductive habits.

The fungal pathogen has unique morphology. It’s carried by RAB in specialized pouches near the mandible. It’s in the same pathogen group that causes Dutch elm disease, and the first to cause a complete systemic vascular wilt.

Bud Mayfield, research entomologist for the USDA Forest Service’s Southern Research Station in Asheville, NC, has been studying the habits of RAB and the spread of laurel wilt.

“We have been dealing with this insect in the southeastern U.S. for several decades,” said Mayfield. “It was a matter of time before it spread to the Northeast.”

Mayfield described ambrosia beetles as “fungus farmers” that carry fungi spores on their bodies or in specialized body structures. The insects carry spores into galleries they create in dead and dying trees.

“They typically inoculate the walls of galleries with the spores,” said Mayfield. “They tend the fungus as it grows on gallery walls, and the developing brood feeds on the fungus. The problem is that as a non-native species in North America, these redbay ambrosia beetles aren’t initiating attacks on healthy trees. One of those fungi the beetle cultivates for food is acting as a vascular pathogen on native laurel species like sassafras and spicebush.”

It’s difficult to see the 2 mm long, slender RAB, which is native to southeast Asia. RAB reproduces by arrhenotokic parthenogenesis: unmated females lay eggs, and those eggs hatch as flightless males that don’t leave the tree. When a female is fertilized by the flightless males, she lays eggs that hatch as females. New populations can start with just a few females colonizing a host. RAB doesn’t have to navigate the environment to mate – everything happens inside the tree. It’s an effective strategy for colonizing new areas.

Mayfield explained what’s known about the arrival of RAB. “In 2002, redbay ambrosia beetles were detected in a survey trap in Georgia in an early detection rapid response program of the U.S. Forest Service,” he said. “There were only a few beetles in the trap, and they were new to North America. There was no known connection with any tree mortality at the time. Over the next several years, there was unusual redbay mortality on the coast of South Carolina and Georgia, but no one made the connection to the beetle.”

In 2004, a researcher investigated redbay mortality in Hilton Head, SC. Since 2005, causal agents of the beetles and pathogens have continued to spread and colonize new hosts throughout the region. Identification of infections outside the region are likely the result of human assistance in moving infested wood. The insect is now in several states including North Carolina, Kentucky, Tennessee, Virginia and New York.

“Insects are landing on and entering far enough into trees to deposit fungal spores into the sapwood during initial attempts to colonize a host,” said Mayfield. “These initial attempts may not be successful, but beetles are getting in far enough that the fungus can germinate in the wood. Spores move through the tree, the tree begins to wilt, the tree has a reaction to the infection and closes off water conducting cells. As the tree is reacting, the sapwood darkens.”

After the tree wilts, weakens and begins to die, more beetles colonize the tree, and not just RAB. Other beetle species are attracted to the visibly weak tree. As beetles cultivate the fungi, new broods emerge and seek hosts. Initial signs of visible damage include wilted crowns and foliage, and early autumn color in summer.

Smith explained that part of the problem is the clonal nature of the RAB. “The beetles spreading this disease in North America are all identical,” he said. “That suggests there was a single introduction, which may have started from one single beetle that emerged from a piece of wood – a pallet or dunnage on a ship near the port of Savannah.

“We know this beetle carries a fungus. The beetles don’t eat wood or the tree – they farm their own food. The tree is a substrate in which they farm the fungus … The fungus it carries is also clonal and there’s very little genetic diversity in the fungus. The clonal symbiosis that has led to remarkable damage – billions of trees – is unprecedented. There is no single genetic lineage causing such devastation.”

by Sally Colby