Meet the Honeydew Gall-Wasp Gall
April 4, 2025
It’s Spring!
When the urge to nature journal is too hard to resist, but everything around you is still brown, crispy and covered in dust, I begin looking for any curious phenomena suitable for rabbit trailing (even rabbit tracks work!). The other day while hiking a steep hill, I was looking for sign the oaks were close to breaking bud. Spotting a string of little dark red beady objects lining many of the mostly leafless stems of am oak was certainly encouraging. From a distance they appeared to be swelling leaf buds; at least their color seemed right. But an up-close inspection revealed my hopeful find as last December’s vacated “homes” occupied by oak gall wasp larvae. Having never seen this species of stem gall before, naturally this would be a perfect most curious phenomenon to tackle. And that’s just what I did!
The Honeydew Gall-Wasp Gall
Arriving back home with a small collection of oak stems crowded with tightly packed galls (resembling miniature bread loaves), my work began. Assuming the host oak was a Gambel’s (Quercus gambelii), I began my search of stems galls on that species. Carefully scouring the literature the genus appeared to be Disholcaspis, but none of the species seemed a good match. So after many days in quandary, I consulted my favorite oak gall ID specialist, firing off a lot of questions, written descriptions and photos. Patiently awaiting his reply, I continued to find more resources to review. Gall wasps have a complex life cycle, and their galls are a challenge to identify.
After 2 weeks a welcome reply arrived from the specialist. Boy did I feel silly, knowing I should’ve known better! The oak host was not a Gambel’s but a cross between Gambel’s and Shrub Live Oak (Q. turbinella) which produces a hybrid called Wavyleaf oak (Quercus x undulata). Correcting my mistake was key to identifying the stem gall! Picky little wasps, huh? Happy the specialist agreed the genus is Disholcaspis, the obvious species responsible for the gall was turned out to be D. spissa; the Honeydew Gall-Wasp ….. making my discovery the Honeydew Gall-Wasp Gall!
Lesson Relearned
When nature journaling, despite how excited you may be to find answers, it’s always, always best to slow down, breathe, carefully observe, ponder, question, make connections, and enjoy the journey ahead of the destination!
Some Gall Descriptions and A Prize!
Outside appearance: Young galls of this wasp are yellowish and hairy. Mature galls have beige-brown to weathered grey sides, are black on top, and have a dull matte surface. They can have a round to ovoid to a rectangular ‘bread-loaf’ like shape. Texture appears mealy-granular. Galls sit snugly directly on the stems (sessile), and singly or in clusters of long compact chains that form all around the stems of host oak species.
Inside the gall and back outside again: Each gall has one thin walled cell or chamber (known as monothalamous) that sits above the bottom of the gall. This is the larval chamber and is imbedded in dense cellular tissue that becomes a pulpy flesh with age. While the larvae are actively feeding and growing, they produce a copious amount of sticky-sweet honeydew that accumulates on the top of the gall. All of this honeydew attracts hordes of hungry ants and yellowjackets. The ecological importance of these secretions is unmistakeable; it provides a high energy food source for the insects. And while the ants and yellowjackets feed, they inadvertently protect the gall, like little bodyguards, from parasites and predators intent on infecting or eating the growing larvae inside. And another interesting ecological thing ….. the reason the tops of the galls are black and not the same color as their sides, has to do with a fungus called black sooty mold. The sugary secretions produced by the larvae accumulate in quantities too irresistible to the sooty mold. Colonies of the fungus develop rapidly on the honeydew giving the gall tops a dusty or powdery black color. So honeydew is the perfect medium for black sooty mold to complete its life cycle.
What about that Prize? Read on to learn more! Close-up inspection of the galls I collected, and there were about 25, revealed all but one had a single exit hole in a side just below its crusty red-black top. I learned the adult wasps emerge from these stem galls by late December; sometimes waiting for warmish temps until mid January. What happened to the adult wasps in the one gall without an exit hole? Because the gall had a sooty top, implying the larvae had been eating, growing and secreting honeydew, maybe the larvae died at some point or failed to develop into an adult? Had to know! Cutting the gall to find out wasn’t easy. The outer crust fell away first, then using steady knife pressure on the punky innards, the gall popped open, like a box of ‘Cracker Jacks!’ And there, near the bottom of the package, was a single sealed chamber. Carefully I was able to tease out the contents with fine pointed tweezers until out blurped the Prize! An intact cream colored gooey looking larvae, followed by a small puddle of viscous liquid. Even though the larvae filled up most of the chamber, there wasn’t any sign it was alive. And after an hour, when there still wasn’t any movement …. well, darn, I felt he must’ve died sometime before morphing into adult form. But peeking inside the gall did answer the question about the absence of an exterior exit hole.
And now an interesting note about these types of cynipid oak gall wasps: On my diagram, I used the term ‘agamic’ which is a formal label included with the genus/species name (i.e. Disholcaspis spissa ‘agamic’). This means these wasps have an asexual all female population that emerges from galls in late fall/early winter to lay fertilized eggs without needing to mate with males. Then a bisexual generation follows where adults emerge from galls in late winter/spring/early summer. This is the typical 2 generation annual life cycle of cynipid gall wasps, where the galls developed from each generation usually look very different and even occur on different parts of the host plant (on stems vs leaves). It’s interesting that nothing is known about the bisexual generation of D. spissa, but the asexual generation is common, well documented and abundant.
Known host oak species for D. spissa: Look for these galls on Shrub live oak (Quercus turbinella), Wavyleaf oak (Quercus x undulata), Mexican blue oak (Q. oblongifolia), Arizona white oak (Q. arizonica), and Shinnery oak (Q. havardi).
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Hope you found my post interesting. Have you ever been curious enough about plant galls to look inside? Recalling the first time I saw a gall, I had no clue what it was and why it was precariously clinging to a plant leaf. It was round and hard, and reminded me of a tiny rusty red ping-pong ball. Was there something inside? Would it be squishy, alive, have teeth and bite, or perhaps whatever it was was dead and oh so smelly? Maybe it was some weird kind of flower bud, or a fruit lost under the leaf? Despite being a bit nervous, I needed to know what, if anything was inside. So I cut it in half, and in the center of the ball, suspended on hundreds of delicate threads, was a tiny, wormy creature twisting and turning and wriggling to music only it could hear! That was over 45 years ago, and to this day I still find galls irresistible!
As always, thanks for stopping by!
A Few References
Russo, Ronald A.: (2021) Plant Galls of the Western United States, section on tree galls; oak galls of the SW, page 173.
Weld, LH: (1957) New American Cynipid Wasps From Oak Galls https://www.gallformers.org/gall/948
a curious nature 