Freeze repercussions

There is an unseen world out there that we rarely intersect with.  Even us gardeners, who like to think that we’re more connected to the natural world than, say, the average computer geek, do not very often run into the great web of life with its cascading waves of cause and effect.

Deadly signals: Telltale tubes announce the presence of granulated ambrosia beetles, which are apt to kill the host tree. photo by Edmund Taylor

Sometimes an event such as this year’s Easter-weekend freeze opens a door into the hidden intricacies of the natural world. A few days ago, I was checking some of my beloved Japanese maples and other small trees that were damaged by the freeze to see if there were any signs of life. Instead of new leaves emerging from the branches, I saw tiny pale tubes emanating from them.

My hopeful mood turned sour, for I’d seen these signs before—the calling cards of the granulated ambrosia beetle. This minute beetle was introduced into Charleston, S.C., 30 years ago and has since spread all over the South. I was first introduced to it in 1981 when it put a scare into the South Carolina peach industry.

When its favorite host trees are stressed or damaged—as mine certainly were over Easter—the ambrosia beetle keys in to a stress-related chemical they emit. Once it lands on a tree, the female beetle bores a perfectly round hole into the sapwood and excavates a brood gallery for its young. The excavation yields the little sawdust tubes I saw protruding from the trunk and larger branches of my maples. Granted, these signature strands of sawdust are not always evident, since they are very fragile and can easily be destroyed by wind or rain.

Not that it matters to most gardeners, but the damage to the tree is not caused by the beetle, per se. The intricacies mentioned earlier continue to unfold since the beetle carries with it a fungus that it inoculates the tree with. The beetle larvae feed on the growing fungus, not on the tree itself. In fact, the beetle is doing nothing more than farming a fungus just as we might farm wheat or corn. In addition to providing food for the growing larvae, the ambrosia fungus plugs a tree’s water conducting vessels, or xylem, and this helps bring about its demise. Established trees have a better chance of surviving than nursery stock or newly planted ones.

Unfortunately, ambrosia beetles have very expensive tastes. In my garden they have attacked Japanese maples, Styrax, Stewartia and dove tree—basically my most treasured small trees. In addition, they’re particularly fond of cherry, dogwood and magnolia as well as many other ornamental and fruit trees.

What can be done if you discover you have infested trees? The short answer is, not much.

Once the beetles are in the tree, they can’t be killed. If you have the beetle in some trees but not others and the unaffected trees are potential hosts and are of high value, you may want to spray the trunks of uninfested trees with an insecticide containing pyrethroid. It needs to be reapplied every couple of weeks while the beetles are most active through the spring. The larvae take 55 days to develop into adults, and it may be useful to remove dead trees before the life cycle is completed. The removed trees need to be burned or chipped, or the beetles will hatch out and perhaps attack another one of your favorites that they didn’t get the first time around. Removing infested trees can help to reduce the population and pest pressure on your remaining trees.

If you do have a badly infested tree, it’s actually beneficial to leave it for a while (three to four weeks) before taking it out, since the infested trees are magnets for more beetles. This also gives you time to ensure the tree will not recover from the attack. Late-arriving beetles may be drawn to already infested trees and ignore other potentially threatened trees.

The good news is that these beetles are not normally a problem for the average gardener unless we have some event that damages or badly stresses our trees. For trees that are infested but seem to be recovering, it is important to limit additional stress—especially drought. If your tree leafs out and the new shoots start wilting, you will know the fungus has clogged the xylem and that the tree will probably not make it.

Another thing to watch out for is infested nursery stock. The nursery may not be aware that the trees have been attacked. Be particularly suspicious of any trees with wilted shoots and examine their trunks for the diagnostic sawdust strands or the tiny round shot-holes that are evident even if the sawdust has been knocked off. If you find such trees at your nursery, be sure to let the owner know.

I have seen some newly planted trees and shrubs killed outright from the April freeze, but the ambrosia-beetle infestation is the worst result I’ve encountered from it. Many plants hit by the freeze, especially the herbaceous ones, are recovering nicely; and before too long the Easter damage will not be so glaring. Some roses and suckering shrubs had their tops killed back and are just starting to sprout back from the crown. Before yanking something out that appears dead, give it plenty of time to push out new growth. This is the time to give your garden a little extra TLC to help it recover from one of the worst episodes it has undergone in quite a while.

Be patient and give those unsightly shrubs a chance to return to their former glory.

[Garden designer Edmund R. Taylor lives and gardens in the wilds of Madison County. He can be reached at swallowtailgardendesigns@hughes.net]