Blonding and Blushing in Ash and Hemlock Trees in Upstate New York

April 21, 2023


Photographer: Roger A. Hopkins  
Summary Author: Roger A. Hopkins  

Photos 1 and 2 show the effects of “blonding” on Ash trees (Fraxinus sp.). Photos 3 and 4 show the effects of “blushing” on Hemlock trees (Tsuga canadensis). Both effects are caused by Downy Woodpeckers and Hairy Woodpeckers (Picoides pubescens). The birds are searching for and feeding on insect larvae that are tunnelling underneath the bark and feeding on the cambium tissue. These effects are particularly evident on dead or dying trees. The woodpeckers can hear the insects and rip off piles of shredded outer bark, seen in photos 1 and 4, to get to them.

The blonding of the Ash in photo 1 probably happened last spring and the color has been bleached to gray. Photo 2 shows the color of the recently exposed inner bark and was probably “blonded” recently. On photo 3 the "blushing" red color of the inner bark of the Hemlock can be seen. The yellow color is the inner wood of the tree where the bark has completely fallen off.

The damage to the Ash trees is caused by the Emerald Ash Borer (EAB) (Agrilus planipennis) while the primary cause of Hemlock damage is the Hemlock Wolly Adelgid (HWA) (Adelges tsugae). In both cases, the insects, as a result of their feeding, unintentionally cut off the circulation of water and nutrients in the tree.

But there is an interesting difference between the two. The woodpeckers on the Ash trees are feeding on EAB larvae, the insects that are killing the tree. The larvae (inset 1) are nearly an inch-long (2.5 cm), juicy morsels that the peckers love. On the Hemlocks, the HWA beetles (inset 4) are tiny, about 1 millimeter long, and would not make much of a meal for a woodpecker. The larvae the “blushing” woodpeckers are after are other insects unrelated to the HWA, namely the Hemlock Borer (HB) (Melanophila fulvoguttata). These insects have been able to infest the tree because the HWA has weakened the tree’s natural defenses. The HB will finish off anything left undone by the HWA.

The loss of these wonderful and important trees may be another result of climate change in the northeast: Winters aren’t always cold enough to kill the insect crop; drought during growing season robs the trees of essential water. 

Ash - Bock-Harvey Forest Preserve, New York Coordinates: 42.4066986, -76.6234481

Hemlocks - Stevenson Forest Preserve, New York Coordinates: 42.4093840, -76.6382324

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