As North Carolina State University asks rhetorically: “Who was the first farmer on Earth, over 60,000,000 years ago? Which insect group includes the most invasive and deadly vectors of tree diseases? And which fungi have foregone sex and independence only to become a garden crop that smells like ripe fruit?” That’s the bizarre world of ambrosia symbiosis between wood boring beetles, nutritious fungi, and bacteria — all right here in Georgia.
But first, a little background. Ambrosia beetles belong to the weevil family and live in symbiosis with ambrosia fungi. Their habit is to excavate tunnels wherein they cultivate gardens of the fungi they import to feed their larvae. At first no one was particularly concerned: primarily found in tropical settings, the beetles colonize dying or recently dead trees. The sawdust that results from their boring tunnels is pushed out as fragile toothpick-like strands, easily broken off, leaving pencil-lead sized holes. The beetles are tiny, blackish-brown, somewhat like the Southern Pine Beetle, and do not in themselves cause the decline of the host tree.
However, concern mounted rapidly as ambrosia beetles — and there are upwards of 3,500 species — began migrating out of the tropics into temperate climates and attacking live trees. Opinions vary as to why this change of behavior should be. Biologist Jiri Hulcr of North Carolina State University suspects “it’s simply an unfortunate coincidence, an evolutionary mismatch.” Confused by the odor of unfamiliar trees, they think living trees are actually dead. The trees have either no immune defense or an exaggerated one and overreact, destroying themselves.
Whatever the reason for migration, the potential damage is extraordinary, since there are many kinds. In 2005 the redbay beetle, an east Asian native, was identified in Georgia. Its preferred host is the laurel family, to which the avocado belongs. By 2009 it was spreading rapidly throughout the southeastern United States. In Georgia we can add tulip poplars, oaks, ornamental cherry, crepe myrtle, redbud, hickory and Japanese maple. In fact, Asian ambrosia beetles will attack almost any broadleaf tree or shrub that is a suitable size — healthy or not.
Ornamental nursery stock seems to be particularly susceptible, as are orchard fruit trees: granulate ambrosia beetles have been reported in the damage of over 100 species of trees. In North Carolina, styrax, dogwood, redbud, maple, ornamental cherry, Japanese maple, and crepe myrtle have all felt the effects. Other reported hosts there include pecan, peach, plum, persimmon, golden rain tree, sweet gum, Shumard oak, Chinese elm, magnolia, fig, and azalea.
According to pest surveys recorded at ports-of-entry, 58 percent of all intercepted insects are bark beetles. There is no way to control them, as a single beetle could be enough to create an invasion. Beyond that, females often are capable of self-fertilization and may produce only females, which then are ready to reproduce again in as little as two weeks.
Still, it is not the beetles themselves that kill the tree; it is the fungi, their food source, which block xylem vessels and interfere with vascular transport. Hosts may produce an immune response, cutting off their water supply to try and kill the fungus, but that also kills the tree. Little is known about ambrosia fungi. Asexual, they are thought to be totally dependent on the beetles for their transport to appropriate hosts, thus the symbiosis: fungi and beetles completely dependent on each other.
Control is obviously difficult: Once the beetles have bored into tree trunks and branches, insecticides can’t touch them and fungicides are ineffective. It is crucial to monitor with traps perhaps as early as February, when beetles emerge either to reinfect the same tree or move to a new one. When beetles are first detected, spray trees with permethrin or bifenthrin, found in such products as Spectrum, Ortho, and Bayer Advanced systems. Systemic products such as imidacloprid are ineffective, since the beetles do not ingest vascular plant tissue. If trees are heavily infested standard advice is to remove and destroy them.
Nowhere in my research have I seen weeping willows mentioned, but those little toothpicks are worrisome. Apparently ambrosia beetles are everywhere. I will be on the lookout in February.
I am indebted to publications by the Georgia Center for Urban Agriculture and the North Carolina Cooperative Extension Service.
Information about Extension Solutions for Homes and Gardens can be found on the University of Georgia Cooperative Extension website, www.caes.uga.edu/extension/cherokee ; or contact the Cherokee County Extension Office, 1130 Bluffs Parkway, Suite G49, Canton, GA, 30114, 770-721-7803. The Georgia Master Gardener Extension Volunteer Program is a volunteer training program offered through county offices of the University of Georgia Cooperative Extension.