2 warm winters spurring spruce aphid outbreak

Posted: Thursday, February 22, 2001

The Cooperative Extension Service is warning homeowners that Sitka spruce trees are fighting for their lives against a pest called the spruce needle aphid.

"The mild winter weather is ideal for buildup of aphid populations. Even the recent cold weather has not been cold enough to eliminate the population, although more than likely numerous aphids have been either killed or slowed way down in their feeding," said cooperative extension agent Jim Douglas.

Because there have been two warm winters in a row, homeowners are concerned about their ornamental trees, "the ones that grandpa planted," said horticulturist Brian McWhorter, who works in the garden center at Don Abel Building Supplies.

Commercial stands of trees, however, are not expected to be severely impacted, according to the U.S. Forest Service.

Aphids, also called "ant cows" and "plant lice," are one of the most common pest groups for ornamental plants, trees and shrubs. There are many species, each targeting a specific plant or class of plants.

Aphids normally begin feeding in February, Douglas said. Freezing weather during March or April kills aphids, but five days of 18-degree temperatures or below is needed.

"Five days will kill off enough aphids to lessen the severity of attack and allow the trees to retain enough needles to restore good health," Douglas said.

He is concerned Juneau will not get those days this year. However, in microclimates such as Tee Harbor or the Mendenhall Valley, where temperatures can be 10 degrees colder than downtown, or at altitudes well above sea level, it may be cold enough.

Large brown patches, often lacking needles, alert homeowners that their trees are under attack.

There are two ways to bolster trees' health without relying entirely on Mother Nature, or being at the mercy of global warming, Douglas said. One is to treat them with ACE Caps, implant cartridges inserted into the base of the tree. The caps release pesticide that is carried by sap into the needles and kills feeding aphids.

"If you treat your trees and your neighbors don't, your trees are still protected for this season," Douglas said.

ACE Caps are carried by Western Auto Marine and Don Abel. Twenty-five cartridges cost $39.99, far below the several-thousand-dollar lvalue of a mature tree.

"Jim Douglas and I found that ACE Caps are the best products for these 100-foot trees; you can't spray them," McWhorter said. A hundred-footer is worth "tens of thousands" in landscape value, he said.

The other way to help trees is to fertilize them in the spring with half a pound of 20-20-20 fertilizer per inch of diameter. Spread the fertilizer in a circle extending to the tree's drip line, below the edge of branch growth.

Aphids are hard to eradicate because they are masters of reproduction. A complete cycle from live birth to sexual maturity requires only 10 to 14 days.

Although aphids are a danger to trees standing one by one along the waterfront or in yards, they are not a serious threat to commercial timber, said entomologist Mark Schultz with the U.S. Forest Service's Pacific Northwest Forestry Sciences Laboratory in Juneau.

"There is more sunlight on a crown that is not surrounded by another crown," Schultz said. In other words, the denseness of forest timber keeps sunlight away from spruce and they don't get sufficiently warmed up to attract aphids. Thus they have minimal impact on logging.

For more about aphids, consult the handout on how to use ACE Caps available at the Cooperative Extension Office, in Vintage Park across from Carrs. Or consult a Web site available at Hot Links at juneauempire.com.

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