Tuesday, April 3, 2018

Blueberry Stem Gall



Mark Bower sent me this picture of a "weird thing" he found while out photographing mushrooms.  He described it as being hard and growing off of a plant.  Now when a man whose passion is photographing fungi thinks something he found is "weird" I have to take it seriously.  To me it appeared to be a deformed part of a plant stem, likely a gall.

Gall in winter - Joshfecteau.com 
Using my years of Master Naturalist training and botanical skills, I Googled "kidney shaped gall" and came up with several pictures of blueberry stem galls.  That opened up a lot of references to the gall and the wasp that forms it, Hemadas nubilipennis.  I was intrigued by the name and searched for the origin of this name and only came up with "pennis" which is Latin for "feather" or an aluminum coin of Finland  eliminated with the introduction of the Euro.   But I digress.....

H. nubilipennis - John van der Linden
The wasps are tiny, measuring less than a tenth of an inch.  They emerge during the blooming season and most are females.  Following mating, a female wasp lands on a tender shoot, frequently under the leaf litter. Facing the tip, she plunges her ovipositor into the stem to deposit an egg, then moves slightly forward to repeat the action multiple times. There are typically 12 eggs in a gall.

Wasp larvae in the gall - Molly Jacobson
The eggs will hatch in 12-14 days and the damage of the larvae feeding causes the stem to swell,  producing the gall.  This provides nutrition and protection for the developing larvae.  The gall turns hard in the fall and the larvae will pupate in it in the spring.  Extension.org

Galls are a major problem among blueberry growers and the primary treatment is pruning of parasitized stems.  Elimination of the wasps takes persistence over years.  Although blueberries are self-pollinating, cross pollination by insects produces larger fruit.  Spraying insecticide as a secondary treatment against the wasp is tricky as the wasps emerge in blooming season at the same time as honeybee pollination.  Michigan State

Wasp emerging from a Blueberry Stem Gall - Nature Posts
There is a fantastic set of photographs of the tiny gall wasps emerging on the Nature Posts blog but they aren't all of the gall with H. nubilipennis.  It turns out that at least 6 other parasitic wasps call these galls home.  One is a chalcid wasp called Ormyrus vacciniicola.  I went straight to Charley Eiseman's Bugtracks where he describes two more eurytomid wasp species that are known to parasitize Hemadas nubilipennis exclusively,  Eurytoma solenozopheriae and Sycophila vacciniicola.  This is where it gets really weird.  He goes on to say:
"Eupelmus vesicularis (Eupelmidae) has also been reared from these galls, and by contrast is one of the least host-specific chalcidoids–its known hosts include six different orders of insects.  It is thought to have been introduced from Europe in straw, and the list of host species in North America is said to be “tiresomely long.”
So of my gall emergences so far this year, one in four has been the actual gall inducer, and the rest have been parasitoid wasps.  I think this is pretty representative of what’s going on out there–there seem to be more parasitoids than anything else.  Hemadas nubilipennis galls are multi-celled, so it will be interesting to see what else comes out of the ones I collected." - Charley Eiseman
We haven't identified many blueberry plants in our valley and I wonder how the wasp found a victim.  We will start looking more carefully for other plants.  Details on the lowbush blueberry are at llinoiswildflowers.info.  For an interesting history of the blueberry stem gall read pages 14-17 of this 1981 Ospry newsletter.
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Some of the findings from MN Wildflower Walk I are at this link.  WW II (not the war) is on April 14th at 1PM.

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