Allergies: Information and Much More from Answers.com
- ️Wed Jul 01 2015
Unwanted plants or plants whose negative values outweigh the positive values in a given situation. Weeds impact growers each year in reduced yield and quality of agricultural products. Especially in tropical areas, irrigation systems have become unusable because of clogging with weeds. Weeds can harbor deleterious disease organisms and insects that harm crops and livestock. In addition, weeds can cause allergic reactions and serious skin problems (poison ivy), break up pavement, slow or stop water flow in municipal water supplies, interfere with power lines, cause fire hazards around buildings and along railroad tracks, and produce poisonous plant parts. See also Allergy.
The most serious weeds are those that succeed in invading new areas and surviving at the expense of other plants by monopolizing light, nutrients, and water or by releasing chemicals detrimental to the growth of surrounding vegetation (allelopathy). In the plant kingdom, dozens of species have been shown to release allelopathic chemicals from roots, leaves, and stems. See also Allelopathy.
Classification
Weeds can be classified as summer annuals, which germinate in the spring, set seed, and die in the fall (crabgrass); winter annuals, which germinate in the fall, set seed, and die in the spring (common chickweek); biennials, which germinate one year, overwinter, set seed, and die the following summer (wild carrot); simple perennials, which live for several years but spread only by seed (dandelion); and creeping perennials, which live for several years and can spread both by seed and by underground roots or rhizomes (field bindweed).
Control methods
Hand pulling, fire, flooding, and tillage are useful for controlling weeds. Insects and pathogens have also been introduced to control certain weed species. Techniques such as herbicides, computerization of spray and tillage technology, remote sensing for weed mapping and identification, and laser treatment have also been explored. Herbicides have resulted in large improvements in the availability and quality of food. They have increased the feasibility of no-till agriculture, leading to significant reductions in soil erosion. They commonly kill weeds by disrupting a physiological process that is not present in animals. Some, however, are moderately high in toxicity and must be used carefully. Rare individual weeds that are genetically resistant to the herbicide have flourished and reproduced, leading to populations of weeds resistant to that herbicide. See also Herbicide.
Biological control
Biological control involves the use of natural enemies (parasites, pathogens, and predators) to control pest populations. In the case of weed pests, the primary natural enemy groups utilized are arthropods, fungal pathogens, and vertebrates. The two major approaches are classical and inundative biological control. Biocontrol can be a highly effective and cost-efficient means of controlling weeds without the use of chemical herbicides.
Classical biocontrol (also termed the inoculative or importation method) is based on the principle of population regulation by natural enemies. Most naturalized weeds leave behind their natural enemies when they colonize new areas, and so can increase to significant densities. Classical biocontrol involves the importation of natural enemies, usually from the area of origin of the weed (and preferably from a part of its native range that is a good climatic match with the intended control area), and their field release. Imported biocontrol agents must be host specific to the target weed.
Inundative control is the mass production and periodic release of large numbers of biocontrol agents to achieve controlling densities. It can be used where existing populations of agents are lacking or where existing populations that are not self-sustaining at high, controlling densities can be augmented. A chief advantage of this method is that it can be integrated with conventional fanning practices on cultivated croplands.
Arthropods, especially insects, are heavily utilized as imported biocontrol agents in uncultivated environments such as grasslands and aquatic systems.
Rusts (Uredinales) are the fungal group most frequently employed as imported agents in classical biocontrol programs. The rust Puccinia chondrillina, imported from Italy, controlled the narrow-leaf form of skeletonweed (Chondrilla juncea) in Australia. Formulations of spores of foreign and endemic pathogens can be used as mycoherbicides in inundative applications.
Vertebrate animal agents, such as goats, typically do not possess a high degree of host-plant specificity, and their feeding has to be carefully managed to focus it on the target weeds. See also Arthropoda; Fungi.