Those tree pests won't be around forever

Posted: Wednesday, August 09, 2000

The forest that surrounds us stretches hundreds of miles, rolling over mountains and along the shore so thickly that it seems eternal. We consider it environment, background, setting for other lives and acts. The living elements of this milieu are rarely considered as individuals until something changes, something jars our complacency.

The thoughts of it being vulnerable to acts of human disregard or being overharvested are incorporated into news stories or press releases. Wildfires or pollution are seen as dangers, but the natural world has hazards too. The predators and diseases of the forest are cyclical, and when their numbers are up or conditions are ripe their effect is tremendous.

The last two weeks the spruces have been changing, clusters of trees have been turning brown, some are going golden-yellow and worried observers have been searching for answers. The outbreaks are not universal; clusters of trees show the symptoms, but others right nearby show none. People call the extension service and ask for the pest scout, local tree fanciers ask each other and some are convinced that the forest is dying. Luckily there is another answer, and last weekend two of the informed sources were able to give out the information on our radio show.

Elisabeth Quadra, the University Extension Service Pest Scout, and Paul Hennon, a Forest Sciences Laboratory Pathologist, reassuringly gave some perspective to the matter. Two causes are acting together: an insect population peaking and the yearly cycle of a fungus that lives on the needles of the trees. The combined effect of the two is dramatic, and in some cases will be fatal to the individual trees, but thoughts of epidemic death can be put aside.

One of the agents is the Spruce Needle Aphid, the other Spruce Needle Rust. They are always with us, their numbers rising and falling according to some rhythm yet to be understood. Some years the effect is barely noticeable, others like this one, they are everywhere.

The rust is the most interesting; it's a parasitic fungus that alternates between two species. One generation lives on the Sitka Spruce, but the spores from that cycle must find Labrador Tea or some other Ericacous species to live on. After they grow and have spores from the tea, they can then pass back to the spruce and do it again. What a complicated life cycle.

The effect of the rust is seen on the needles from the last two years, not on the new growth. The current year's needles will become vulnerable at the next spore-making phase of the tea. What we see on the rust infected spruces is the result of the ripe spore packages bursting open and releasing the yellowish spores, waiting for wind and rain to spread them to new hosts.

Both species, spruce and Labrador Tea, have to be nearby for the rust to have a complete life cycle, so the outbreaks are clustered in areas that have both, particularly wet places, muskegs and boggy sites. The Back Loop near Goat Hill, North Douglas Highway, Eaglecrest Road and out by the Herbert River are hard hit, but trees all over the Mendenhall Valley are showing the symptoms too.

The aphids are much less discriminating. They seem to be in population clusters that swell locally and are nearly absent in other places. All this means is that we don't have any idea what drives them. Affected trees can be seen in broad areas like along Thane Road at the end of town or across the channel near the tour bus turnout, and in smaller clusters in neighborhoods all through the valley.

The damage pattern from the aphids is less clear-cut than the rust. There are no demarcations between the old and new needles, and aphids like them all. Whole branches, then whole interiors, then the whole tree turns brown as the feeding insects turn the juicy interiors of the needles into more aphids. Aphid populations can grow tremendously during the spring and early summer as egg laying gives way to live birth.

Early in the year aphids lay eggs; babies hatch, mate and lay more eggs. Longer warmer days and plenty of food cause the aphid life cycle to change and by mid-summer the aphids are giving birth to live females that are already pregnant. If all the aphids that were born in a year were still alive at the end of the season the earth would be covered with them a foot deep.

Luckily they have predators and this plague, like the rust, too shall pass.

David Lendrum is a master gardener and owner of Landscape Alaska. Any responses or questions can be sent to

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