Sagebrush Galls

The image of a solemn cowboy silhouetted against a setting sun with sagebrush as a stalwart companion dominates our conception of the West. The landscape is dominated by sagebrush, or Artemisia spp. A member of the sunflower family, this genus has astonishing adaptations that allow it to thrive in torrid summers and frigid winters. Initial encounters with this genus reveal its conspicuous, rather intense, fragrance. Physiological attributes aside, Artemisia is a vital player in range ecology: it is a main component of the desert climax communities, is a food source and protective cover for wildlife, contributes to the structural diversity of the vegetative landscape, and has the potential to provide microhabitat to support invertebrate life.

Abnormal growths along branches and leaves became apparent to me as I struggled to comprehend the numerous species and subspecies of this shrub. Some galls looked like an earthen red and green versions of a cheerleader’s pom-pom. Others had similar coloration, but were tumorous in shape with a smooth epidermis, their interior the color of brown sugar and cottony-fibrous texture. The center of these galls revealed a small larva and confirmed my assumption that they were insect-induced.

Although Fronk et al. (1964) identified four different dipterous species in Wyoming that produce Artemisia galls, there are seven in total. These galls are magnets for of other insects including beetles, Hemiptera species (not true insects) and Lepidoptera species; all were observed over-wintering in the galls. Of course, there are also parasitic wasps drawn to the galls deposit their eggs in the galling larvae.

In 2000, Sanver and Bradford compiled information from published journals on inquiline (organism that derives benefit from gall tissue and not necessarily the larva it contains) communities: Unlike the concepts of diversity or abundance that tend to positively correlate with proximity to the equator, the highest gall frequencies occur in temperature regions with harsh climates. Southern Idaho matches that description perfectly. In Arizona, Ferdinand and Price (1988), observed increased galling occurring in hot dry areas compared to riparian areas of the same latitude and altitude

None of the information I perused mentioned Artemisia species and sub-species hosting specific insects or galls that would make identification easier, but Floate et al (1996) did show three Chrysothamnus nauseosus sub-species were able to be identified with gall characters more effectively than by plant morphology. If only sagebrush species had such a diagnostic tool, seasonal range/bio. techs like myself, would have a much easier time collecting data in the field. The host specificity of insects on Chrysothamnus nauseosus (recently renamed Ericameria nauseosa) may be evidence of a diverging population right  before our eyes. Possible reproductive and physiological changes in the host plant caused by these galls and their insects could act as a selection pressure resulting in further speciation.

That is cool to think about. So, with all the sagebrush species and their sub-species, it makes me wonder if they are converging or diverging. My boss is confident they are diverging. However, could climate change cause previously diverging groups to overlap in range, hybridize, outbreed and reconnect gene pools? Sounds like a nice research project.

 

 

Fernanded, G. Wilson, and Price, Peter W. 1988. Biogeographical Gradients in Galling Species Richness. Tests of Hypotheses Oecologia, Vol. 76, No. 2 (1988), pp. 161-167Published

Floate, K.D., Fernandes, Wilson G., Nilsson, Jan A.. 1996. Distinguishing Intrapopulational Categories of Plants by Their Insect Faunas: Galls on Rabbitbrush Oecologia, Vol. 105, No. 2 (1996), pp. 221-229

Fronk, W.D., Beete, A.A., and Fullerton, D.G. 1964. Dipterous galls on Artemisia tridentate complex and Insects associated with them. Annals of the Entomological Society of America 57: 575-577.

Sanver, Dilek and Hawkins, Bradford A. 2000. Galls as habitats: the inquiline communities of insect galls. Basic Appl. Ecol. 1, 3–11.