Dave Shetlar (Professor Emeritus, OSU Entomology) and I have recently observed newly developing oak rough bulletgalls in central and southwest Ohio, respectively. The galls are produced under the direction of the gall wasp Disholcaspis quercusmamma (family Cynipidae). We're also starting to see the gall's security detail, but more about that later.
This gall wasp has a complicated life cycle involving two different types of oak galls that give rise to asexual and sexual wasps at different times of the year. The alternation of two different reproductive modes and life-styles between generations of a species is known as "heterogamy."
Heterogamy may be a familiar term to alert BYGL readers. The gall wasp Callirhytis cornigera which produces horned oak stem galls, as well as tiny leaf galls, is another heterogamous gall-maker. I described its unusual life cycle in a BYGL Alert posted earlier this season (see "Celebrate Arbor Day with Horned Oak Galls," by clicking this hotlink: https://bygl.osu.edu/node/1530 ).
A Complex Life Cycle
The life cycle of the oak rough bulletgall wasps involves the current bulletgalls as well as leaf galls that will develop in the spring. The bulletgalls arise from the meristematic stem cambium and will give rise to female wasps around the end of September or early October; no males are produced.
Wasp emergence is heralded by a small hole in the gall. Once the wasps have emerged, the "spent" galls shrivel and darken in color. Most will remain attached to continue providing gall-interest next season.
The self-fertile females that emerge from this season's stem galls crawl to a dormant leaf bud where they lay a single egg per bud. These eggs along with the resulting wasp larvae stimulate the tree to produce small, inconspicuous leaf galls in the spring. Both male and female wasps develop inside these leaf galls and adults emerge later in the season.
The mated females fly or crawl to the most recent twigs where they insert their eggs through the phloem to be in contact with the cambium. The wasp requires the services of undifferentiated (meristematic) cells to grow their bulletgalls.
Wondrous Gall Formation and Function
Gall formation is a wondrous but poorly understood process. As with many insect gall-makers, the oak rough bulletgall wasp uses various chemicals to turn plant genes on and off at just the right time to induce and direct gall formation. Thus far, no researcher has ever duplicated this interaction without the aid of an insect gall-maker.
Oak rough bulletgalls provide everything that's needed to protect and nourish the single developing wasp larva residing in a chamber located at the center of the gall. The wasp larva has chewing mouthparts, but rather than devouring its gall-home, the larva grazes on a continuously recharged supply of food called nutrient tissue that lines its chamber. It would be like living in a home with pizzas continually emerging from the walls! Well, you get the picture.
Paying for Protection
One of the most fascinating features of many plant galls, including oak rough bulletgalls, is the inclusion of extrafloral nectaries (a plant organ) in the gall structure. The nectar oozes across the surface of the galls and can become colonized by black sooty molds.
The first time I observed blackened bulletgalls, I thought the sooty molds had colonized sugary honeydew dripping onto the galls from the backend of phloem-sucking insects such as aphids or soft scales. Of course, there was not such infestation. The sugar source was from the galls themselves.
The nectar exuded from the extrafloral nectaries attracts a variety of stinging insects including bald-faced hornets (Dolichovespula maculata) and yellowjackets (Vespula spp.) as well as biting insects such as carpenter ants (Camponotus spp.).
Yellowjackets and bald-faced hornets are Jekyll and Hyde wasps. They are predaceous beneficial insects during most of the growing season chowing-down on soft-bodied insects such as caterpillars and sawfly larvae. However, in the fall, they switch from high-protein diets to high carbohydrate diets. They become a nuisance as they compete with our own high-carb consumption in the form of soda, donuts, and certain adult beverages.
The wasp's high-carb hankering draws them to the nectar (a.k.a. wasp candy) oozing from the extrafloral nectaries incorporated in the oak rough bulletgall structure. Of course, ants sporting powerful biting mandibles will also show-up to the sugar party along with flies.
Presumably, the close attention of stinging and biting insects prevents the immature gall-making wasp larvae located within the galls from receiving the unwanted attention of predators and parasitoids. In other words, a little sugar bribe pays for the protection of the gall makers helpless offspring as they lounge about in their tiny chambers feasting on nutrient tissue "pizza."