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Pest Control

  1. What is a pest?
    1. microbes that cause human or other animal or plant diseases (viruses through worms, etc)
    2. nuisance insects
    3. insect disease vectors
    4. insects and other animals (eg rodents) that destroy crops and/or food
    5. predators that prey on domestic animals
    6. macro (eg termites) and micro (eg mold) decomposers that decompose wood and other building materials, fabrics etc
    7. plants that hinder desirable land uses (ie weeds)
  2. For food production, the important control measures in the last 100 or so years have been chemical pesticides (used against animal pests) and herbicides (used against plant pests)
    1. food production is reduced by more than one third by pests; without pest control agents the damage would be much worse
    2. the chemical agents have undesirable side effects; an alternative approach is to alter the environment in some way so that damage to crops from crops is minimized
      • biological agents or chemical agents with specific properties are used to alter some aspect of the ecosystem
  3. The Chemical Biocides
    1. going back 60 years or more, inorganic toxins like lead arsenate were used to kill just about everything (including children and farm workers)
    2. in the late 30s and 40s organic compounds began to be used to kill insects and other pests
      • dichlorodiphenyltrichloroethane (DDT) was introduced as an insecticide in 1938
        • this compound is effective against a broad spectrum of insect pests but is relatively nontoxic to mammals
        • its cheap
        • it has been very effective in reducing the incidence of insect-spread malaria, sleeping sickness and typhus (and therefore has saved and continues to save millions of human lives)
        • changed the face of agriculture
        • Mueller was awarded the Nobel Prize for the discovery that DDT is an insecticide
      • DDT was soon joined by a host of other chlorinated hydrocarbons
        • these are absorbed by the fatty tissue around bug nerves then cause convulsions and death
    3. in the 50s, it became apparent to scientists that these chemicals were having a destructive effect on birds
      • the effects are due in large part to the bioaccumulation of persistent pesticides
        • DDT is not readily decomposed or digested in the food chain, though it does break down to DDE
        • DDE disrupts Ca+2 metabolism in birds, causing eggs to be thin shelled and easily broken
        • although it is not known to be toxic, DDT also accumulates in the fatty tissues of humans and other animals
      • in 1962, Rachel Carson summarized these scientific findings in the bestseller Silent Spring
        • rallied public sentiment against DDT and other chemical pesticides
        • fired up the ecology movement of the 60s and 70s that led to the creation of the EPA
      • the EPA banned DDT along with a number of other chlorinated pesticides (chlordane is still used against termites)
    4. the chemical pesticides in use today are not persistent like the chlorinated hydrocarbons; these compounds are degraded within a few weeks or months
      • organophosphates and carbamates have mostly replaced organochlorine compounds for killing bugs in the US
        • organophosphates are essentially nerve gas; they inhibit the insects’ acetylcholinesterase; this enzyme is critical in shutting off messages from nerves to muscles; again, convulsions and death
        • carbamates are relatively new as pesticides; they work by a similar mechanism to that of organophosphates; they degrade quickly
      • on the other hand, the compounds (organophosphates, carbamates) are quite toxic to mammals and other animals
        • the manufacture of these compounds can also be quite dangerous as the citizens of Bhopal can attest
      • these substances probably do not pose a hazard to the general public but do cause acute poisoning of 1000s of farm workers every year
      • these compounds are quite wide spectrum, and thus kill beneficial insects (including bees) along with pests
    5. This effect on birds (including the near extinction of the bald eagle and other fish eating birds) is one example of the negative side effects of chemical pesticides
        • for humans, the effects of pesticide residues is widely disputed; some blame pesticide residues for neurological and other health effects, including cancer; others claim that the real world effects are negligible in comparison with background
      • there are natural (botanical) products used as biocides, including nicotine, pyrethrin and rotenone
        • these are biodegradeable and tend to have less ecological impact and human toxicity (except for nicotine)
    6. two other effects include the breeding of resistant pests and the outbreak of secondary pests
      • a chemical pesticide puts a powerful selection pressure and the rapidly reproducing insect community; the individuals with the physiological or morphological abilities to withstand an applied chemical continue to reproduce; their offspring do quite well because the competition has all been killed off (as have their predators)
        • hundreds of insect species have become resistant to common pesticides (I guess this is good business for the chemical companies)
        • mosquitoes worldwide have become resistant enough to DDT so that malaria is again on the rise
      • remember predator-prey relationships? Reduce the number of prey and the predator declines in numbers as well (even more severely it turns out)
        • with the predators decimated, the original pest or some other species is left unchecked; a population explosion may occur (either a resurgence of the original pest or a secondary infestation by some other)
        • the predator species may be cute little songbirds (or any ecologically important species)
      • these problems of resistance and resurgence (and acute toxicity) occur even with nonpersistent pesticides
  4. Alternative Methods of Pest Control
      • if we understand the life cycle of a pest and its ecological connections, we may be able to protect crops, etc, from damage without risking poisoning farm workers; a natural pest control measure might also be genuinely effective, without resurgence and resistance
      • Nebel gives four general types of natural controls:
        1. cultural controls = nonchemical manipulations of the environment that hinder pest proliferation or exclude pests
          • the classic example is window screens
          • other sanitary engineering processes (protecting watersheds, etc) serve as cultural controls against microbial and other parasitic pests
          • draining swamps is a cultural control against mosquitoes
          • crop rotation serves as a cultural control, since many pests need a specific host who is rotated out; the timing of plantings can also have an effect
          • quarantine of possible carrier materials
            • agricultural inspection stations
            • zebra mussel inspections and quarantines
            • chemical agents might be effective in this situation
        2. natural enemies = predation maintains the "balance of nature"; the presence of a predator may maintain the population of a pest around some lower level; as long as this level is below that at which crops are damaged to the point of non-profitability (the so-called economic threshold) then the natural control can be considered effective
          • ladybugs are good to have in your garden (and spiders are good to have in the house)
          • some of the worst pests are those that have been introduced to a foriegn habitat => introduce a species the feeds on the first invader and a population balance may be reestablished
          • this can be the ideal pest control technique because it reproduces itself
          • as mentioned above, the use of chemical pesticides may be counterproductive in this respect since the broad spectrum toxins may kill the control species as well as the pest (or may induce a sudden famine for the control species)
          • the potential for biological controls has been barely tapped
        3. genetic controls = pests species have evolved to exploit some host or other food source; if that target species can be genetically altered in some way, it may no longer suit the pest
          • a large goal of plant breeding efforts has been the generation of parasite (eg fungus) resistant strains
          • breeding may also be effective against insect pests
            • many plants produce chemicals that are noxious or toxic to insects (eg, nicotine is a powerful toxin to insects and humans alike)
            • other plants have physical characteristics that repel or destroy insects (eg, turpentine glues down bugs that would seek to feed on injured pines)
            • breeders may be able to select for some natural chemical or physical attribute to increase the resistance of a plant against some pest
            • genetic engineering has the potential to introduce a myriad of traits not already present in a plant (or other organism)
          • over the course of evolution, insects evolved around many defensive measures that plants came up with; likewise, insects can become resistant to the human-bred anti-insect traits
        4. natural chemical controls = the use of an insects own hormones or pheremones to control its population
          • a hormone can sidetrack the life cycle of a pest
          • a pheremone can be used as bait to trap or poison males or simply as a diversion to reduce the number of successful matings
          • another biological control is the use of sterile males = large numbers of these are raised in the lab, irradiated and released; these outcompete the wild males and can greatly reduce the number of pests born in the next generation
  5. Integrated Pest Management is the use of the entire spectrum of pest control measures, natural and artificial, based on the application of scientific knowledge in conjunction with thorough surveillance of pests (see p 423)
      • this approach can greatly reduce the need for chemical pesticides and the associated dollar, health, and environmental costs

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