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Biological Control and Arthropod Associations of Range Weeds

BIOLOGICAL CONTROL OF BROOM SNAKEWEED

A DIVERSE NATIVE ARTHROPOD FAUNA


AN INFESTATION OF BROOMSNAKEWEED IN SOCORRO COUNTY, NEW MEXICO. LOOKING TOWARD THE NORTH.

The snakeweed plants in this picture are several years old.


THE SAME GENERAL AREA, AFTER HIGH MORTALITY CAUSED BY ROOT BORES, LOOKING TOWARD THE SOUTH


BLOOMING SNAKEWEED ALONG A ROAD ON THE JORNADA EXPERIMENTAL RANGE, DONA ANA COUNTY, NEW MEXICO.


A GENERALIZED VIEW OF A WOODY SNAKEWEED PLANT
- (Diagram by David Thompson)

Snakeweeds, genus Gutierrezia, apparently originated in North America. Only two North American species are woody, and these are the most important in regard to economic impact. One species, Gutierrezia sarothrae, has the base chromosome number for the genus and apparently all other species, including the herbaceous species, were derived from it by a process known as polyploidy (or duplication of chromosome sets). While this process is common in plants, it is rare in animals. Any duplication of chromosomes in these usually is lethal. At some point, probably about 10,000 bp (before present), somehow woody snakeweeds were transfered to South America, where they have speciated. None of the herbaceous snakeweeds are known from South America. Around the same time North America was gifted with mesquite, creosotebush and tarbush, all of which apparently evolved in South America. Because the woody snakeweeds evolved in North America, they have a very large arthropod fauna, including some important natural enemies. Because snakeweed produces around 10,000 seeds per mature plant and because woody snakeweeds are able to start growth early if they get winter rains, they have in some areas outstripped their enemies and caused undesirable infestations. In addition to being agressive growers and colonizers that are helped by some drought and overgrazing by cattle, the plants are also poisonous and can cause abortions in pregnant cows that eat the folliage when grasses are not available. Still, native insects and abiotic factors, account for a mortality of over 99% per year starting with the seed stage. Over the last seven years native snakeweed insects have been sampled at 36 sites in New Mexico, Arizona and Texas. As these samples are sorted we are starting to understand the fauna well enough to propose a model for the snakeweed system.


SNAKEWEED ARTHROPOD COLLECTION SITES 1989-1996

- All photographs were taken by David B. Richman, unless otherwise noted.