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                                     AND THE ENVIRONMENT



I.  Manipulation refers to those procedures that help the establishment and activity of natural enemies.


II.  Manipulation of a natural enemy or its environment may be justified if a definite need exists and a reasonable

      assurance of success is  possible.


III.  Certain factors associated with the habitat, the host, or the natural enemy itself may render an entomophagous

        organism ineffective as a biologicalcontrol agent, but still be subject to manipulation.


                    A.  Habitat.


  1.  certain adverse climatic factors, such as heat, cold, low humidity or wind.


  2.  the presence of unattractive or otherwise unsuitable host plants.


  3.  scarcity of food or water for adult natural enemies.


  4.  interspecific competition among natural enemies.


  5.  pesticides.


                                  6.  cultural practices.


                    B.  Host.


  1.  lack of synchronization of host-parasitoid generations.


  2.  host plant resistance.


  3.  resistant host strains.


  4.  periodic scarcities of suitable host stages.


                     C.  Natural Enemy.


                                  1.  may exhibit an annual ovarian diapause and migrate away from its hosts at certain times of the year

                                       (e.g., Coccinellidae).


                                  2.  reproductive rate may be too low.


                                  3.  may exhibit an adverse tendency to disperse, coupled with an inability to find mates at the resulting

                                       low densities.



IV.  Generally, manipulation of a natural enemy should only be attempted if it involves:


                      A.  Some periodically occurring, unfavorable environmental factor.


                      B.  A lack of some easily supplied requisite.


                     C.  Some simple or minor, but correctable, intrinsic shortcoming.



V.  Methods Employed.


                      A.  Periodic Colonization.


  1.  involves periodic releases of mass-produced or field-collected natural enemies.


  2.  two types.


                                   a.  inundative releases.



      (1).  have largely been employed against the egg stage of univoltine pests.  Control is largely the work

              of the insects released, not their progeny.


      (2).  has been called a biotic insecticide since host mortality is more or less immediate, and there is no

              prolonged interaction  between host and natural enemy populations.


      (3).  this method is best employed against pests of high value crops, against univoltine pests, or against

              multivoltine pests that  reach injurious levels during but one generation annually.


    b.  inoculative releases.


      (1).  where the interaction between host and natural enemy populations persists through more than one

              generation of the natural enemy, and control is largely effected by the progeny of the beneficial

              forms released.


      (2).  inoculative releases may take the form of accretive releases where small numbers of natural enemies

              are periodically released against low density pest populations.


      (3).  entomophagous insects and their pest hosts may also be colonized concurrently in areas with a known

              history of pest invasions or where hosts are too scarce to support natural enemies, this in anticipation

             of pest invasions (e.g., Cryptolaemus on citrus mealybugs in California).


                      B.  Selective Breeding.


                                1.  not a proven method to date.



                                2.  challenging field for research.


                      C.  Environmental Manipulation.


  1.  supplying artificial structures which serve as shelters or as nesting sites for natural enemies.


  2.  supplying supplemental food for adult natural enemies.


  3.  providing alternate hosts for beneficial insects or providing their phytophagous hosts with alternate host



  4.  artificially supplying suitable host stages when these are unavailable in the field.


                                  5.  controlling honeydew-feeding ants.


                                  6.  modifying the habitat to eliminate or reduce the adverse effects of cultural practices, pesticides, dust

                                       deposits, etc.


VI.  Further Details.


        A.  Various text examples and Rabb (1962) describe how the construction of nesting shelters encouraged high local

              populations of Polistes wasps in cotton fields in the West Indies and in tobacco fields in North Carolina, increasing

              the total predation of  injurious  lepidopterous larvae. 


Nesting boxes provided for insectivorous birds in some intensively managed European forests also resulted in

  increased predator  densities and protection from defoliating insects.


        B.  Many adult natural enemies utilize exudates from floral or extra-floral nectaries, as well as pollen, as sources of

               nutrients and water.  The culture or conservation of plant food sources in the proximity of cropland and orchards

               has been found to enhance  the  effectiveness of various natural enemies.  Pollen is known to be an important

               supplementary food for adult, aphid-feeding  Syrphidae  and Coccinellidae as well as certain predacious mites.


The long-practiced method of clean cultivation for weed control may be undesirable from the standpoint of

 removing wild plants infested with honeydew-producing insects or containing nectaries.


        C.  Colonization of alternative insect hosts may improve synchronization between a pest and its natural enemies. 

                Several benefits that may be derived from this technique are:


                                1.  the damping of extreme oscillations in natural enemy and host population densities.


                                2.  maintaining functional natural enemy populations by providing a continuous food supply during periods

                                     of low pest densities.


                                3.  providing suitable overwinter hosts.


                                4.  promoting maximum distribution of the natural enemy.


                                5.  reducing intra- and interspecific competition among natural enemies (cannibalism and combat).



                        D.  Modifications of adverse cultural practices.



1.  cultivation may kill soil-inhabiting beneficial insects or pupating, non-subterranean natural enemies.  Reduced

      or delayed cultivation may reduce this mortality and also dust.  Dust is especially known to harm parasitoids

      and predators; it can be minimized by sprinkling, by planting cover crops, by paving access roads or by holding

      cultivation to a minimum.


               2.  properly timed irrigation may promote epidemics of fungal pathogens of insect pests by providing the proper

                    conditions of humidity in the microenvironment.  Improperly timed irrigation, on the other hand, may drown or

                    drive away beneficial insects. 





Altieri, M. A. & D. K. Letourneau.  1999.  Environmental management to enhance biological control in agroecosystems.  In:  Principles and

       Application of Biological Control.  Academic Press, San Diego CA.  1046 p.


Rabb, R. L.  1962.  Integration of biological and chemical control.  Manipulation of the environment.  Bull. Ent. Soc. Amer. 8:  193-95.