Lecture 29. Revenge of the Insects: Ecological Backlash

  1. Introduction
    1. Ecological Backlash
      1. counter-responses of pest populations or other biotic factors in the environment that diminish the effectiveness of management tactics
      2. resistance, resurgence, and replacement
    2. Ecology and Evolution
      1. ecological backlash is one expression of genetic responses of a population to a selection pressure (resistance) and of variable impact of a mortality factor on different species (resurgence and replacement)
      2. for resistance, contributing factors include the strength of the selection pressure, genetic diversity in target population, and preadaptive conditions (traits that enhance the likelihood of resistance developing
        1. need for insects to detoxify plant chemicals is preadaptive for insecticide resistance (because mechanisms for detoxifying chemicals exist in insects prior to exposure to insecticides)]
  2. Resistance
    1. Definition and Perspectives
      1. the ability of certain individuals to tolerate or avoid factors that would be lethal or reproductively degrading to most individuals in a normal population
        1. genetic/evolutionary phenomenon, evolution in progress
        2. preadaptive; works through selection not habituation (habituation - the property of nerves becoming less sensitive through repeated exposure, or of an organism increasing detoxifying enzymes through repeated, low dose exposure
        3. insecticides greater than 10 x the LD50 for a strain is sufficient to be called resistant
      2. medical vs. agricultural resistance (to illustrate phenomena)
        1. medical - microbes with very faster generation times, high genetic diversity (can easily produce resistance strains in the lab), however, drug resistance has not become a serious problem until recently (decades after first use of antibiotics).
          1. why? -
            1. can use wide variety of toxicants (many, many antibiotic modes of action
            2. can use mega doses (10X, 100X, 1000X) to kill resistant microbes with a dose-sensitive resistance (resistant at low dose but killed at high dose)
            3. even so, drug resistant has developed (e.g., high frequency of resistant strep in hospitals, tuberculosis strains now affected by only one antibiotic)
        2. agricultural - high resistance for insects (short generation times, high genetic diversity), only now seeing resistance in weeds to herbicides (generation times, genetics, and relatively slow dispersal of resistance germplasm)
          1. key points for insects: high genetic diversity for insects, good dispersal ability for insects with resistant genes, and relatively few (less than a dozen) modes of action
    2. Conventional Insecticides
      1. first reported instance 1908 San Jose scale (Quadraspidiotus perniciosus) to lime-sulfur treatment in Washington
      2. 1946 DDT resistant houseflies in Sweden
      3. 1997 over 500 arthropod species resistant to 1 or more insecticide groups (35% species in Diptera); also 200 plant pathogens, 273 weed species, 2 nematodes, 5 species of rodents
      4. mechanisms of insecticide resistance
        1. physiological resistance
          1. decreased penetration through cuticle
          2. altered site of action
            1. cyanide (HCN) resistant scale (Calif. red scale) has different cytochrome
            2. KDR - knockdown resistance with DDT and pyrethroids involves nerve insensitivity
          3. increased excretion - very common
          4. sequester in fat body (or encapsulate)
        2. biochemical resistance - detoxification
          1. degradation - enzymatic, many different pathways and compounds e.g.. mixed function oxydases (MFO's), reductases, etc.
          2. conjugation - usually add H20 solubility factor to molecule so it can be excreted, made less toxic e.g. glucon is primary conjugate in insects (glucuronic acid for mammals)
        3. behavioral resistance-insects avoid chemical, especially important of chemical if irritating
          1. e.g., some mosquito rectors of malaria shifted from endophagic to exophagic (feeding indoors to outdoors, because of insecticide treatments in homes)
        4. other terms or types
          1. cross resistance - resistance to one insecticide provides resistance to different insecticides
            1. freq. mechanism through common detoxification scheme or active site insensitivity; e.g., KDR
            2. cross resistance within class (common mode of action) or across insecticide classes
        5. pesticide treadmill - increasing freq. and dosages which result in more rapid buildup of resistance; based on the notion of coping with resistant individuals by increasing dose (does not work in the long term, sometimes not even in the short term)
    3. Resistance to Insect Growth Regulators (IGRs)
      1. currently 13 species Diptera, Coleoptera, Homoptera and Lepidoptera- mechanism uncertain but reduced penetration through cuticle and increased metabolism of IGR seem likely
    4. Resistance to Microbial Insecticides
        1. documented cases of resistance to Bacillus thuringiensis (Bt) in Indian meal moth,, almond moth, tobacco budworm, Colorado potato beetle, diamondback moth, and house fly
        2. also resistance w/resistance to viruses; e.g., spruce budworm and silkworm (beneficial in this instance)
    5. Resistance to Parasitoids
      1. very uncommon but can occur
      2. examples:
        1. larch sawfly resistant to introduced ichnenmonid
        2. house flies resistant to pteromalid (in lab)
        3. anopheline mosquito species to nematode parasite
    6. Virulence to Resistant Plants
      1. biotypes, strains of pests able to circumvent resistance in a plant
      2. examples: Hession fly, green bug, and many, many others
    7. Resistance to Crop Rotations
      1. extended diapause in northern corn rootworms, 2 yr. life cycle rather than 1 yr.
      2. change in oviposition pattern - western corn rootworms in small region of IL and IN now ovipositing in soybean (not a host)
    8. Resistance to sterile-male Releases
      1. some evidence for differences in screwworm mating and release flies become less successful
    9. Resistance to Pheromones
      1. not yet demonstrated (but pheromones not used on a large scale)
    10. Managing Resistance
      1. promoting resistance:
        1. operational: prolonged exposure, each generation selected, of pressure, no refugia, large geographical area, selection before mating
        2. biological: little migration between pop, monophagous species, short gen. time, large # offspring /gen., mobile species, increasing potential for exposure
      2. slowing resistance
        1. combined tactics
        2. passive tactics where possible - technologies that don't select the pest population
        3. other approaches
          1. moderation - decreased selection pressure maintain susceptible gives
          2. saturation - overwhelm defenses (e.g., synergists)
          3. multiple attack - like combined tactics but use several insecticides or several resistance genes
  3. Resurgence and Replacement
    1. Definitions
      1. resurgence - increase in pest #'s to levels equal or greater than before tactic employed
      2. replacement - management of one pest species results in a hither to unimportant species becoming a pest ( a secondary pest)
      3. hypothesizes:
        1. reduction in natural enemies
        2. favorable effects of tactic on pest
        3. removal of competitive species
    2. Reduction in Natural Enemies
      1. resurgence
        1. many natural enemies susceptible to insecticides (more susceptible)
          1. e.g., alfalfa springtail in Australia: DDT depresses pest population 12 days, at end of season treated population has 5x more springtails than untreated; insecticide killed predaceous mites
        2. favorable effects
          1. hormoligosis - reproductive stimulation by sublethal doses of insecticide
            1. e.g., twospotted spider mite, granary weevil, western corn rootworm
      2. replacement
        1. mechanisms behind replacement, essentially the same as for resurgence
        2. examples:
          1. mid 1960s, action against pink bollworm led to outbreaks of cotton leafperforator and spider mites; similarly, action against boll weevil has contributed to problems with bollworm
          2. treatment for European corn borer in corn can led to outbreaks by spider mites (apparently not hormoligosis or action on predators or parasitoids)
    3. Managing Resurgence and Replacement
      1. avoid hormoligosis
      2. avoid destroying natural enemies
      3. physiological selectivity - use selective insecticides
      4. ecological selectivity - use ET's, low rates, avoid treatment broad areas, timing
  4. Other Forms of Ecological Backlash
    1. Enhanced Microbial Degradation
      1. insecticides and herbicides rapidly degraded by soil microbes; problem w/ some soil insecticides (carbofuran-Furadan and isofenphos - Amaze)
      2. upsets in community balance
        1. tactics changes balance between species
        2. e.g., screwworm readication caused in white-tail deer pop, deer host for southern cattle tick, tick pop. increased ( & on cattle)


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