Monday, October 20, 2014

Goldenrod Galls

New research has shown that bison feeding on the prairie increases the diversity of species even as it reduces the biomass of the grasses and forbs.  As Dr. Matthew Moran explained to Entomology Today, arthropod abundance and diversity was greater in bison grazed prairie than in control fields.
"Even though the bison reduced the amount of grass by 50% or more, the overall abundance and diversity of arthropod herbivores and carnivores increased significantly, especially among sap-feeders. In addition, Dr. Moran suggests that this may also be beneficial to birds and and other animals that eat insects."
Goldenrod stem gall - REK
We recently explored the Rotary Club Nature Park in Mountain View with our friends Jack and Marty Toll.  Marty showed their beautiful Butterfly Garden, a wooly field of native nectar and host plants sure to warm the heart of any leipdoptera.  Their goldenrod scattered through the field also demonstrated another way that species diversity is maintained by providing a home for insects in the form of galls. 

Stem gall with larva - REK
Larva closeup - REK

The round goldenrod stem gall above contains the maggot larva of Eurosta solidaginis .  These in turn can feed the predaceous beetle larva of Mordellistena (below).  According to Bugguide, "Apparently smaller galls are parasitized more often than larger ones; but the largest ones are eaten more often by chickadees and wood peckers."  Once again this illustrates the importance of goldenrod in the food web.

Beetle larva is gall - Beatriz Moisset
There are many species of goldenrod including the genera Solidago and Euthamia.  Goldenrods in general support a wide range of gall-forming insects.  The plants survive the stress while contributing to insect diversity and the food web.  There are over 50 species of gall forming insects, two thirds of which are midges in the family Cecidomyiidae.  The genus Rhopalomyia alone has 16 species which produce galls in either the bud, leaf, stem, rhizome, or flower-head of their specific goldenrod host species.
Flower head or rosette gall - REK
There are other species that produce a wide variety of goldenrod galls.  While the round or elliptical galls on the stem are easily recognizable, the flower head, bunch or rosette galls are stranger.  They produce a mass of leaves in a cluster, somewhat similar to the mass of twigs of a witch's broom growth on a hackberry.
"In addition to the gall makers we should consider all the parasitoids, predators and inquilines. The total numbers must be quite substantial. In some cases more than fifty percent of galls are parasitized; furthermore, it seems that for each species of gall maker there are at least three or more different species of parasitoids, inquilines, etc."  (
"Inquiline" refers to an organism that lives commensally in the nest or structure of another organism, usually for the purpose of sharing food or resources. This could be a small spider on a larger spider's web or caterpillars living in ant nests while sharing their honeydew, in the common parlance, a type of "friends with benefits."  Many inquiline species live in the structures produced by gall making insects.

A wide variety of moths feed on goldenrod, and wasps, spiders, birds and other predators use their larva.  A patch of goldenrod can contain a whole microcosm of life.  An incredible food web awaits the patient naturalist with curiosity and a hand lens or a good camera.  We will look at some goldenrod inhabitants and their predators in a followup blog.

Many of the gall forming species and their predators are found on the site.

No comments:

Post a Comment