Michael Davis and Roland Dute

A damaging but interesting insect - the Asian ambrosia beetle - has been introduced in recent years to orchards and field nurseries in the Southeast. AAES studies are helping provide information about this potentially devastating insect and may eventually lead to new control measures.

The Asian ambrosia beetle (Xylosandrus crassiusculus) is a tiny insect - females are only 1.5 millimeters long (Figure 1), and the males are
even smaller. The beetle was first reported in the United States in a peach orchard in South Carolina in 1974. However, it did not become a serious problem until the last five to seven years. Nurseries from Texas to Maryland to Florida have reported moderate to heavy losses as a result of beetle attacks, and the number of attacks has been growing. The list of hosts includes pecan, peach, maple, oak, and several ornamental trees.

Figure 1. Female ambrosia beetle in her gallery in a Yoshino cherry trunk (magnified 16x).

An AAES study was initiated in 1994 to learn more about this insect and the damage that it causes and to provide information that might help control this damage in the future.

Careful examination of ornamental cherry, one of the beetle's many hosts, has shown that the insects do not attack a tree randomly. More than 80% of the bore holes were made at the sites of lenticels, areas on the tree's surface where the cells are loosely packed and presumably easier to penetrate.

Trees that are attacked do not initially show many symptoms. Since the insects are so tiny, their entry holes into a host's trunk are inconspicuous. One interesting sign of attack is toothpick-like cylinders of sawdust pushed out of the holes as the insects bore into the tree (Figure 2).

Figure 2. Redbud trunk with frass protruding from beetle entry holes.

Internally, the sapwood of attacked trees becomes discolored around the tunnels excavated by the beetles (Figure 3). Within a month of attack, the trees wilt rapidly from the tops, and often the plants die. The amount of dieback depends on the age and health of the attacked tree.

Figure 3. Larvae and eggs in beetle gallery inside cherry wood. Note the white fungal hyphae (ambrosia fungus) lining the tunnel (magnified 130x).

How can an insect so small do so much damage? Though the insects attack in large numbers, their tunnels should only cause a general decline in health rather than rapid death. The answer lies with the beetles' life cycles. Various investigators have shown that these insects bore into the sapwood of young trees in early spring and carry with them a fungus. The fungus grows inside the tree and lines the interior of the tunnels. The females then lay their eggs in these locations and the larvae feed on this fungus, or “ambrosia,” as they develop (Figure 3). At maturity, the beetles mate inside the tunnels, after which the females fly away in search of a new host.

The ambrosia fungus is a member of the genus Ambrosiella, which is often not pathogenic. However, ambrosia beetles have been known to introduce other fungi that are pathogenic along with the ambrosia fungus. The pathogenic fungi include Fusarium and Ceratocystis species.

AAES studies of the interaction between pathogenic fungi and wood were done on eastern redbud and Japanese cherry. These studies indicated that the damage done to trees occurs because of blockage in the wood's vessel elements. That blockage occurs through several means. The galleries of the beetles can cause some blockage. Another means is by-production of threadlike strands called hyphae by the pathogenic fungi. These hyphae can grow into the vessel elements and can appear in large enough numbers to impede almost completely the flow of water (Figure 4). The energy needed for hyphal growth is provided by a tree's own food storage cells. Also trees can inadvertently plug their own vessels by producing gums. Both cherries and redbuds produce gums as a defense mechanism when wounded or attacked by a pathogen (Figure 5). These gums inhibit the spread of the pathogen but can also block vessels. So in trying to protect itself the tree can actually compound the problem.

	Figure 4. Fungal hyphae occluding cherry vessel members and ray cells (magnified 520x).
	Figure 5. Cherry vessel member with a fungal hypha and gum (magnified 410x).

Unfortunately, this insect/disease complex is relatively new in the Southeast and an effective control has not been devised. Since the fungus establishes itself in the sapwood, traditional fungicides are not effective. Current control recommendations are aimed at the beetles. Traditional borer sprays, such as lindane or chlorpyrifos products, are recommended but have proven inconsistent.

Researchers elsewhere are experimenting with traps. The use of traps shows some promise, but results are still preliminary. If the traps prove to be effective, they can be used to monitor populations so growers can make more timely spray applications. Even if the traps prove useful, removal and immediate destruction of infected trees are crucial to prevent further spread of the insect and its fungal inoculum.

Davis is a Graduate Student and Dute is a Professor of Botany and Microbiology.