You might have seen them. Pink or dry grey spheres hanging on a branch of a live oak tree. Curious, you might have picked at the thing, thought it was just some strange tree growth, then tossed it aside. But this little sphere is the larval stage home to an insect that has an amazing and complex life cycle: the Oak Gall Wasp, or Disholcaspis cinerosa.
Oak Gall Wasps are ¼” inch long cynipid wasps that are not commonly seen. In fact, the two types of galls these creatures cause trees to generate are more conspicuous than the insects themselves.
These galls are abnormal plant tissue swellings that the wasp induces. It’s thought that the plant begins to create the gall when either eggs are deposited or the eggs hatch and the larvae begin to feed. The gall grows around the larva for a short period, then stops, also ceasing to use the host plant’s nutrients. The little insect lives inside this gall which provides food and shelter to the larval wasp. These galls cause no harm to trees.
D. cinerosa produces two different generations each year, one being asexual and one being sexual. Each generation produces a different type of gall. The late summer/early fall galls can get up to an inch in diameter, and are larger than the spring gall which is like a kernel of wheat. The autumn season galls start out as light pink or pinkish brown and the internal tissue is soft. As the season progresses, the tissue turns brown and begins to dry. Pupation, the third stage of metamorphosis, occurs during November, and then follows the final transformation into the adult insect. Through December into January, the adult will emerge by chewing its way through the gall.
The insects that emerge from this larger gall are all females and are asexual, meaning, they do not require a mate to produce eggs. This female wasp will immediately find swollen leaf buds on live oak trees where she will deposit one to two eggs per bud. Her life is very short, however, as she only lives two to six weeks, with this time spent mostly laying up to 20 or so viable eggs. The fact this happens during winter has no adverse effect on the eggs, which remain dormant for these cooler months.
These eggs will hatch when the weather warms and leaf buds on the live oaks begin to open. The presence of new larvae will cause the tree to develop a new type of gall. This one is beige and much smaller than the winter gall. The wasp undergoes the complete metamorphosis from larva to adult in just a few weeks inside these smaller galls. The adults that emerge from them are both male and female. Mating occurs immediately, after which the males die. But the females don’t have much longer to live! They have about a week in which they lay about 15 eggs, and then the yearly lifecycle of the wasp starts over again.
These larger galls also help benefit insects other than this little wasp. Newly-formed galls attract parasitic wasps that lay eggs in the mantle of the gall. When these eggs hatch, these larvae feed on the outer tissues of the gall, but cause no harm to D. cinerosa. Additionally, the gall also secretes a sugary substance which attracts small flies, butterflies, predatory wasps, and honeybees. They feed on this until the gall dries up in late autumn.
When the adult D. cinerosa vacates the gall, other insects will inhabit the empty gall. These insects include scavenger ants, small spiders, lacewing larvae, and other small predatory wasps.
So as the weather continues to cool into November, take a look at some of the live oaks you might pass on campus. You could see several galls housing this fascinating insect.
The Campus Biodiversity series explores the urban wildlife and plant life of UT Austin.
INTERESTING TRIVIA
Sexuality researcher, Alfred Kinsey, did his doctoral thesis on gall wasps while studying at Harvard University under entomologist William Morton Wheeler. The American Museum of Natural History in NYC contains approximately 5 million wasps collected by Kinsey.
In Europe, related oak galls were used with iron (II) sulfate to make a purple-black or brown-black ink. These oak galls provide gallotannic acid for this process that has been in use from the 5th to the 19th centuries, and this ink is still sold today. (Photo: California State Archives)
SOURCES
Benskin, James G.; Clark, Wayne E.; Gaylor, Mike J.; Gordon, Frankie W.; Hamman, Philip J.; Morgan, David L.; and Reed, Hal C. “The Mealy Oak Gall on Ornamental Live Oak in Texas” Texas Agricultural Extension Service, February, 3, 2004. (accessed online: https://nueces.agrilife.org/files/2011/08/mealy-oak-gall.pdf)
Yudell, Michael (July 1, 1999). "Kinsey's Other Report". Natural History.
Iron Gall Ink, Wikipedia. (accessed online: https://en.wikipedia.org/wiki/Iron_gall_ink)
