Bug o’the Week – Galls VI

Salutations, BugFans,

As veteran BugFans can attest, the BugLady is intrigued by galls.  How many kinds are there?  To quote from the first BOTW on galls (October, 2009) “Lawlor, in Discovering Nature Close to Home, states that North American plants support more than 2,000 kinds of galls – 800 different kinds form on oaks alone, about 125 kinds on roses, and more than 50 kinds on goldenrods (genus Solidago).”

So many galls, so little time.

Why are galls?  Here’s the quick and dirty explanation offered in a Wisconsin Extension vineyard report for Door County.  “Gall formation in many instances is initiated by egg laying (oviposition) by the adult form of an insect or by feeding of early larval stages. Feeding by certain gall-making insects results in the release of salivary fluids that may contain plant growth regulating substances (Auxins, IAA) and plant digesting enzymes, pectinases, proteases, and cellulases. The growth regulating substances released by feeding insects work in concert with the grapevines’ response to insect attack. The grapevines’ response to mechanical or chemical irritation is to isolate the toxins or invasion, resulting in a tumorous mass of tissue or gall.”  In addition to galls caused by insects and mites (and nematodes), various fungi, bacteria, and viruses may cause galls on plants.

Many gall makers have complicated life cycles that, like the waterlily/reddish-brown plum aphids of recent BOTW fame, may include alternate hosts, and both sexual and asexual generations; in many cases, great chunks of their life histories are unknown.  A gall maker tends to be named after the gall it makes.

The GRAPE FILBERT GALL is described in “The Ohio Naturalist” journal (December 1914) as a “Bud gall, being a spherical mass 15-50 mm. diameter, of small, lozenge-shaped galls, each about 5 x 15 mm. Leaf-green, covered with a felty yellow or orange pubescence. Infrequent.”

It has been on our radar for some time – according to the 1916 Bulletin of the University of the State of New York, “Apparently the same gall is found on wild frost grape in Illinois and was described and figured by Messrs. Walsh and Riley in 1868.  They state that the gall develops from a common center at a point where a [leaf] bud would ordinarily occur….. Large specimens of this gall bear a general resemblance to a bunch of filbert or hazelnuts as they grow on a bush, which led to the designation vitus-coryloides” (Vitus is the genus name for grapes, and Corylus is the genus name for filbert/hazelnut).  Its cause is the Filbert gall maker midge now named Schizomyia coryloides, in the fly family Cecidomyiidae.  For many galls, there’s not much information that is more recent than these century-old sources; here’s a drawing of it from 1883 http://www.spiderpic.com/stock-photos/istockphoto/11378771-grape-vine-filbert-gall.

One of the big questions about galls is whether (other than cosmetically) they harm a plant.  Sometimes.  The larger, woodier, heavier galls that inhabit branch tips and persist for a year or more may weigh a branch down and make it more susceptible to wind and rain damage, but many of the smaller galls that grow on leafy tissue are nothing to worry about.  Where would they be, after all, if they habitually killed their host plants?  The Wisconsin Extension report concludes, “Galls may look destructive, but galls seldom injure the plant.  Grapevines can support a large number of galls and still grow and reproduce normally ……. most galls that infect the soft tissue (leaves, tendrils, shoots) of grapevine are of little economic importance.

ROUGH BULLET GALLS are woody, slightly elliptical galls that occur singly or in groups on twigs of bur oaks https://bugguide.net/node/view/699180/bgimage.  As in many other oak galls, the gall maker is a tiny wasp – this one named Disholcaspis quercusmamma in the family Cynipidae (as Matthew Wills cheerfully informs us in his blog “Backyard and Beyond, “Why, yes, a translation of that would be “oak breasts.”).  Here’s the gall maker: http://www.wildlifenorthamerica.com/Insect/Oak-rough-bulletgall-wasp/Disholcaspis/quercusmamma.html.

A female-only generation exits the galls (one individual per) in late fall https://bugguide.net/node/view/265686/bgimage) and almost immediately oviposits.  The all-female (asexual) generation of this tiny wasp alternates with an even tinier sexual generation that hadn’t even been identified as recently as 2009 when researchers managed to rear some.

When the BugLady started photographing this gall, it was green and was attended by ants.  Turns out that rough bullet galls are one of a number of species of galls that produce a sweet substance that, like aphid “honeydew,” attracts insects like bees and wasps.  And ants https://bygl.osu.edu/node/907.  The ants may be deterring both the insects that graze on oak leaves and the parasitic wasps that seek to lay their eggs in the gall.  Honeydew-making galls are induced by just a few genera of Cynipid wasps.  In her search, the BugLady came across some papers about hairstreak butterflies that feed on honeydew from galls and from scale insects.  For BugFans who want to wander down that very interesting side road, see this article in the American Entomologist https://academic.oup.com/ae/article/61/3/160/2194543 and this report on the first report: https://entomologytoday.org/2016/07/07/rare-butterfly-feeds-on-oak-galls-and-other-non-nectar-sources/.

A different Cynipid wasp, Andricus quercuspetiolicola (Quercus is the genus name of the oaks), makes the OAK PETIOLE GALL on various species of white oak.  Unlike the bullet gall, this gall is produced in the softer tissue of the petiole (leaf stem), around the base of the leaf, and so it falls off when the tree loses its leaves.  The gall starts growing in early spring, as the leaves start, and the gall makers exit by mid-summer.  “BugTracks,” the website of Charley Eiseman, co-author of the excellent field guide Tracks and Signs of Insects and Other Invertebrates, has some amazing pictures in his blog: https://bugtracks.wordpress.com/2013/11/25/two-year-gall/.

And – just when you thought you’d seen everything, an ant colony within a bullet gall http://www.gkochert.com/ants-in-gall/.

Kate Redmond, The BugLady

Bug of the Week archives:

Become a Member

Take advantage of all the benefits of a Riveredge membership year round!

Learn More