The Tangled Tale of the Ash Tree Bolete

By Rachel Sargent Mirus

If the enemy of my enemy is my friend, surely the friend of my enemy is my enemy. This inverted cliché is a way to characterize the tangled relationship between ash trees and the ash tree boletus.

Ash tree bolete (Boletinellus merulioides) is a fan-shaped brown mushroom with an off-center stem. It grows with ash trees in eastern North America; but this combination is strange. Typically, when fungi appear consistently near a particular tree species, the fungus that produces these fungi has a symbiotic or parasitic relationship with that tree species. When the researchers took a closer look at the ash tree bolete fungus, they didn’t find either type of connection. Instead, they discovered a secondary, symbiotic relationship between the fungus and its ash enemy: an aphid that feeds exclusively on ash trees.

In 1987, mycologists Mark Brundrett and Bryce Kendrick at the University of Waterloo in Ontario, Canada, examined ash roots during a field study and found abundant ash tree boletus sclerotia. Sclerotia are the hardened form of mycelia (root-like structures that allow fungi to absorb nutrients). Brundrett and Kendrick found the sclerotia in the ash tree boletes to be small hollow balls. They partially covered the associated roots and also spread in the nearby soil. Although hollow, these sclerotia were not all hollow. Attached to the roots were many waxy leaf curl ash aphids (Prociphilus fraxinifolii).

Leaf curl ash aphids are small, translucent green insects found only on ash trees. Like many other aphids, they modify the host’s growth form to create safe feeding and breeding platforms. As the name suggests, leaf curl ash aphids cause leaf curl on new leaves. Crushed leaves provide both food and shelter for a growing aphid colony. While the damage caused by borers can be a nuisance—curled leaves drop prematurely, honey and wax secreted by insects make a mess under infected trees—they rarely pose a serious threat to a healthy ash tree.

But why did these aphids take shelter under the trees, with boletes? Female aphids drop from trees to the soil each summer, where they can live and form their own subterranean aphid colonies parthenogenetically—in other words, without the need for fertilization. These clonal colonies absorb nutrients from the roots from mid-summer to late fall, often enclosed in the sclerotium of the ash tree boletus.

What Brundrett and Kendrick discovered is that aphids use sclerotia for shelter and protection. Sources pay rent in the form of honey they secrete, which contains the sugars, amino acids and minerals they get from the tree. The researchers believe that ash tree bolete derives a nutritional benefit because this mushroom is more prolific than other mushroom-forming fungi in the same habitat.

Ever since I learned about the tangled story of ash, aphids, and fungi, I’ve been looking for ash tree boletes. When I finally found the mushrooms, they were right in front of my house. They ranged from two to four inches in width and had spongy sepia lids. Excavating their bases, I found small, hollow spheres that corresponded to Brundrett’s pictures of sclerotia, but were empty of aphids. We’ve never seen evidence of aphids on our ash trees – it seems that fungi in our soil must survive without insect partners.
As with any symbiotic relationship, a threat to one partner can be a threat to the other. The ash-aphid-bolete relationship demonstrates the ecological losses—certainly some of them unknown and overlooked—that come with the spread of an invasive insect, the emerald ash borer (EAB), and the subsequent decline of native ash species. “I’m worried about the future of the ash boletus,” Brundrett said. “The last time I was in Canada, the ash trees were severely reduced and the fungus is likely to follow.”

Brundrett’s point is a valuable reminder that threats to ash trees also threaten all species that have an ecological relationship with these trees. In other words, the friend of my enemy can be my enemy, but in the face of a great environmental threat, all fates are intertwined.

Rachel Sargent Mirus lives in Duxbury. Illustration by Adelaide Murphy Tyrol. The Outside Story is commissioned and edited by Northern Woodlands magazine and sponsored by the Wellborn Environmental Foundation of New Hampshire Charitable Foundation:

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