Pest Control II:

Tumbleweeds

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Kingdom – Plantae

Division – Angiospermae (flowering plants)

Class – Eudicotidae

Order – Caryophyllales

Family – Amaranthaceae

Genus – Salsola

Species – tragus

    More properly known as Russian thistle, Salsola tragus originated in the Eurasian steppes east of the Ural Mountains. The plant reached the United States sometime in the early 1870's and quickly became an icon of the Old West, the tumbleweed.

    In late spring or early summer, when the first rains or snowmelt arrive in the growing area, blue-green stars emerge on the ground. Consisting of stems and leaves with a rubbery texture, this annual herb grows rapidly, reaching the size of a bowling ball in a week. Over the summer and into the autumn the plant grows into a roughly spherical, cage-like structure from one to several meters wide. Then the plant dies and dries out. Late in autumn a layer of cells at the base of the plant weakens and breaks when the wind pushes the plant. As it rolls and bounces over the landscape the plant skeleton scatters as many as a quarter of a million seeds, dropping them onto a path that may extend for kilometers.

    Tumbleweeds significantly affect the erosion of the soil by the wind in open country, especially in areas in which agriculture consists of dry-land farming. First, they remove excessive amounts of moisture from the soil, thereby depriving the soil of the glue that holds it together. In competition with a wheat crop, one Russian thistle removed forty-four gallons of water from the soil. On fallow land the thistle would take even more water. And second, the motion of the tumbleweeds degrades the thin crust that typically protects soil from wind erosion. The thin, spiny branches of the tumbleweed gouge the soil as the weed moves over the landscape, thereby exposing small areas of the soil to the wind: as those areas erode, they grow wider and more dust goes airborne. Indeed, we may assert that the presence of tumbleweeds in North America contributed to the magnitude of the vast dust storms that accompanied the great drought of the 1930's, commonly called the Dust Bowl.

    We don’t want to see a repetition of that experience, so we want to eliminate as many factors in its creation as we reasonably can. Tumbleweeds are one of those factors. Because the tumbleweed is an invasive species, we can exterminate it and restore the ecosystem that preceded its arrival with a clear conscience.

    To deal with this menace and obliterate it we need something that will attack it before it begins spreading its seeds. Because ranchers use it as a minor supplement to livestock feed, allowing their animals to forage on it, we know that it does have some chemical-energy value. Thus we might want to develop a robot that feeds on the plant and simply eats it into extinction.

    A good design would give us a robot that might resemble a crab the size of a Shetland pony. It would have the typical arthropod mouth parts, beginning with mandibles that can cut plants to pieces that the other mouth parts move to the robot’s stomach. The pincers end in claws that resemble the jaws of pliers rather than possessing the cutting edges that we find on natural crab claws: with those pincers the robot can grip a plant and pull it up with its roots. Sidling crabwise across the landscape, the robots will devour the Russian thistles and also the dried out tumbleweeds, the latter in order to clean up the mess that they represent and to eliminate any residual seeds.

    Each robot’s carapace will be covered with solar cells. We know that the robots will be wandering over the deserts of the Southwest, so they will need an auxiliary power supply to augment their main source of energy. Their main source of energy, of course, will be the chemical energy that they obtain from ingesting Russian thistle, tumbleweeds, and/or other organic detritus.

    Inside its body the robot carries a stomach that consists of several flattish chambers stacked one atop another. An Omniphage plate lines the inside of each chamber on the sides and bottom and it disassembles the organic matter fed into the chamber, taking it apart molecule by molecule and atom by atom. The Omniphage will combine the free carbon and hydrogen atoms that it obtains with oxygen from the atmosphere to produce carbon dioxide, water, and electrical power to run the robot: in essence, the Omniphage operates as a rather elaborate fuel cell. The remaining material from the robot’s feed, along with the carbon dioxide and water, gets excreted.

    At first the robots, in their platoons, will patrol the areas where the Russian thistle and its consequent tumbleweeds are abundant. Clearing out the densest concentrations of the weeds will most rapidly reduce the number of seeds produced and set. Given the widespread distribution of the weeds, we can see clearly that rapidly reducing their rate of reproduction is a necessary first step toward their eradication. As the weeds become sparser on the landscape the robots will spread out and adopt new tactics to find and exterminate the plants. Using models copied from the science of epidemiology, they will treat the Russian thistle as a disease outbreak and adapt techniques that doctors have devised for dealing with those. And, like smallpox, this disease will eventually be eradicated.

    The robot crabs can also exterminate other invasive plant species, such as dodder and purple loosestrife. But the real challenge for the robots grows in the South: it’s kudzu.

    Related to peas, kudzu was brought to the United States from Japan in 1876 and since then it has spread across twenty states. It can grow as much as two meters a week and it covers thousands of square kilometers of field and forest. Being a vine, it climbs: it covers trees, bushes, fences, lampposts, and buildings, all in a thick green blanket that kills any plant life underneath it. It has so far been effectively unstoppable. It is certainly something that we want to exterminate (and there’s a hint in there for a joke about Dalek Pest Control).

    In this case our robot crabs will need help. Because the crabs can’t climb, we will need smaller robots that might resemble beetles to assist them. Those smaller robots will climb onto the kudzu and ascend to the high places that the vine has colonized. There the beetles will cut the vines, eating what they need to keep their power up and dropping the rest to the ground, where the crabs will deal with it.

    But even the crabs won’t be able to cope with all of the material being harvested in the effort to eradicate the plant. A third robot will have to come into play. Those will be large machines, capable of processing several tonnes of material every hour. They may simply resemble boxes moving on multiple legs and they will ingest all of the cut kudzu and the detritus taken from underneath it. The boxes will use Omniphage/Omnifex combinations to transform the ingested material into some useful form, such as, for example, cellulose expressed as artificial wood.

    Because kudzu can regenerate itself from even a tiny piece of root, the robots will have to revisit the site frequently to tear up any new growth. But eventually the kudzu will die out completely and North America will be free of another pest.

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