New research conducted by Michigan State University has confirmed what some previous studies were finding – that some of our common ash species may actually be showing tolerance to Emerald Ash Borer (Agrilus planipennis, EAB).
Sara Tanis of Michigan State University spoke at the 39th Annual Forest Health Review in Orillia this week and brought a little good news about this devastating pest. Sara works with well-known Forest Entomologist Dr. Deb McCullough and their team has been monitoring ash tree populations since EAB was first identified in the state. They noticed that 71% of the blue ash (Fraxinus quadrangulata) in their urban tree surveys were still alive, while other ash species (Fraxinus pennsylvanica) were completely killed.
The research team decided to create a mixed-species ash plantation and inoculated the trees with Emerald Ash borer as a controlled experiment to evaluate host resistance. They found that 100% of the black ash (Fraxinus nigra) and Green ash (Fraxinus pennsylvanica) were killed within 2 years.
The same study found that the blue ash (Fraxinus quadrangulata) and Manchurian ash (Fraxinus mandshurica) had a high level of tolerance to EAB. And indeed blue ash seedling trees have been shown to have great tolerance to EAB in the Great Lakes region. Unfortunately, both blue and Manchurian ash both have low seed germination which makes them difficult to propagate for nursery production and forest regeneration. Traditionally, growers would accelerate propagation of blue ash by budding it onto green ash understock (root system). The blue ash bud would grow and eventually become the top of the tree but the green ash understock imparted some susceptibility to EAB as can be seen in our urban specimens. Blue ash is no longer grown that way and this shade tree is now grown from a true seedling (on their own roots).
Limited seed source and germination percentages restrict blue ash availability in the trade. Despite that, Ontario nursery growers started to propagate blue ash a few years ago in hopes of reintroducing it into the landscape, but our growers report that actual blue ash sales have been very few and far between.
At the same time, the researchers noticed that the white ash (Fraxinus americana) were infested but that the trees were surviving the attack. Tree dissection revealed that although larval galleries could be found in the phloem, the tree was successfully walling off (CODIT) the larvae, leaving enough phloem untouched that it was able to support the tree. Scientists don’t yet understand the mechanism of tolerance but theorize that it is at least partially due, to female EAB host preference.
So when laying out planting plans, consider planting some of these “tolerant” species of ash as a means of encouraging biodiversity and succession for our ash species. When planning ash removals, consider leaving healthier white (and blue, if you can find it) ash behind to maintain some tree canopy. Woodlot owners are encouraged to leave some healthy specimens of various ash species as a means of increasing the chances of developing natural host tolerance to EAB.
On a related note, Ontario researchers continue to release 3 species of parasitoid wasps in an effort to help suppress EAB populations in Ontario. The wasps were discovered in China and have been released in North American ash forests for over 5 years. The good news is, these beneficial wasps have survived the last two Polar Vortex winters we’ve had. And even better, researchers have discovered additional native insects that feed on EAB. For more information, click HERE.