My introduction to the wonderful world of plant galls began with observing vibrant red, wart‑like galls, known as "bladdergalls," adorning the upper leaf surfaces of a silver maple tree. The galls consist entirely of plant tissue and are produced under the plant gene-manipulating direction of the Maple Bladdergall Mite, Vasates quadripedes (family Eriophyidae). I'm probably not alone with this being the first gall ever encountered.
Of course, mine is a cautionary tale. I did not know that maple bladdergalls are a gateway-gall; they can lead to a serious hard-core gall-addiction. It took me years to muster the courage to say this: my name is Joe Boggs and I'm a gall-oholic.
The eriophyid responsible for maple bladdergalls only produces this type gall and no other. The equally common maple spindle galls, which are sometimes called nail galls, are produced by a different eriophyid mite, V. aceriscrumena. This eriophyid mite species never produces bladdergalls.
Maple bladdergalls have been great teachers of gallology. The first lesson I learned is that galls can change appearance as they age, or "mature." The maple bladdergalls change from bright green to deep red and eventually turn black.
The second lesson is that populations of plant gall-makers tend to rise and fall dramatically from year-to-year. I often re-visit the same trees year after year and I've found that while a tree may reward me with a huge gall display one year, it frequently disappoints the next. My maple bladdergall photos below demonstrate this; same tree, different years.
Finally, very few galls that are produced by insects and mites cause any real harm to the overall health of their host plants. I sometimes get reports of maple bladdergalls causing defoliation. However, I've never observed this first-hand. I suspect reported defoliation is perhaps connected to other issues including poor site conditions (e.g. poor drainage), nutrient deficiencies, maple anthracnose, maple petiole borer, etc. One of the most common diagnostics missteps is to blame the obvious.
Other Eriophyid Gall Oddities
The Black Tupelo Bladdergall Mite (Eriophyes nyssae) produces galls that look very similar to those found on maples. Black Tupelo (Nyssa sylvatica) (a.k.a. black gum, sour gum) provides a twofer with the eriophyid, Eriophyes dinus, producing crinkled leaf edges. The crinkling is sometimes referred to as Black Tupelo Leaf Roll Galls.
One of my favorite plant galls is produced by the Poison Ivy Bladdergall Mite (Aculops rhois (= A. toxicophagus)) on its namesake host. The galls vaguely resemble the itchy skin blisters we suffer when we contact the plant. I like to imagine that gall-infested poison ivy plants suffer the same agonizing itch, but they have no fingers!
I've found that boxelder (Acer negundo) can challenge the saying "leaves of three, leave it be," and the handiwork of the Boxelder Bladdergall Mite (Eriophyes negundi) doesn't help. The galls bear a striking resemblance to those on the three leaflets of poison ivy. However, the boxelder bladdergalls undergo a distinctive change in appearance as they "mature."
Early on, they appear as small bladdergalls. Later, they become much larger and produce velvet-like patches on the corresponding lower leaf surface. They are even given a different common name of "Boxelder Velvet Galls." I originally thought bladder and velvet galls were the work of two different gall-makers, but the literature attributes both types of galls to the same eriophyid mite.
Galls produced by the eriophyid, Eriophyes brachytarsus, are another type bladdergall that changes form as they mature. At first, the galls look like typical bladdergalls and are called "walnut bladdergalls" in some online references. However, as they mature, the galls become distinctively pouch-like and are referred to as "walnut pouch galls." Eventually, the galls break open like popcorn to reveal tufts of silvery-white hairs.
I first encountered the fuzzy, cauliflower-like galls produced by the eriophyid, Aceria cephalanthi, on common buttonbush (Cephalanthus occidentalis) last year in northeast Ohio. I could find no information in the literature about this gall-maker. However, given the rise of buttonbush as a prized bumble bee magnet in pollinator gardens, I predict this gall will gain greater notoriety.
As with the vast majority of plant galls produced by arthropods (e.g. wasps, midges, etc.), those that are induced by eriophyids cause little to no harm to the overall health of their plant hosts. Indeed, I contend that they add ornamental value to their tree and shrub hosts. Of course, I'm a gall-oholic.