By GARLAND KENNEDY
Sentinel Staff Writer
Alder trees in and around Sitka have recently suffered defoliation at the hands of green alder sawflies, but entomologist Elizabeth Graham says never fear, the bugs don’t pose a serious long-term threat to the trees.
The invasive insects consume the leaves of alder trees, sometimes causing a great deal of visible damage and raising alarm among people concerned about the health of the local trees.
“These trees are pretty resilient and able to recover,” said Graham, an insect expert with the U.S. Forest Service in Juneau. She noted that the feeding damage coincides with the life cycles of the sawflies.
“Most of the feeding damage occurs pretty late in the season,” she said.
This means that the trees have, in most cases, already done their growing for the year. Graham said that “even though it can look pretty bad, the overall tree health is usually sustained because they (the alders) are at the end of their growing season.”
A new leaf grows from a bug-eaten alder tree today. (Sentinel Photo)
Because of this, tree mortality because of green alder sawfly feeding is quite uncommon.
Graham noted that in cases where tree mortality does occur, it’s usually in combination with other factors. She said that drought stress or alder cankers, a fungus, can combine with sawflies to kill already weakened trees.
But for the most part, the damage done by the sawflies is aesthetic. Graham said most of the damage is done by sawfly pupation chambers in dead, rotting wood.
She said that one local family reported that “sawflies were creating those little pupation chambers in the rotting wood of their windowsill.” However, Graham believed that this said less about the destructive potential of sawflies and more about the need to replace a rotting windowsill.
Graham concluded “as far as invasive species go, this is not the worst one. We have several things that feed on our alders and they (alders) seem to be doing just fine.”
There were reports of extensive alder defoliation by sawflies in 2010 and 2013, but as Graham noted, the trees recovered. Heavy feeding can reduce growth rates, she said, but beyond that she didn’t see a reason to be overly concerned about the green insects.
