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                              COLONIZATION, RECOVERY & EVALUATION

                                         OF NATURAL ENEMIES


I.  Colonization


A.  Refers to the field release and manipulation of imported natural enemies for their establishment, and to

      favor their spread and increase in a new environment.


B.  The natural enemy must be permanently established in at least one locality for success to be claimed. 

      Then this serves as a locus for its natural spread, or as a field colony and source of material for redistribution



II.  Insectary propagation of imported natural enemies has been circumvented on occasion by repeated introductions

     of the insects from abroad, followed by their direct and periodic release in the field.


A.  Direct releases may be necessitated by economic considerations, difficulties of culture, or by lack of

      adequate insectary facilities.


B.  Direct releases are not encouraged by some biological control workers who maintain that insectary

      propagation offers several advantages:


  1.  provides adequate numbers to insure the greatest latitude in the timing and geographical coverage of



  2.  insectary culture insures vigorous stocks of natural enemies for release.


  3.  insectary propagation affords an excellent opportunity for detailed study of the biologies and host



III.  Usually a few specimens from initial insectary stocks of an imported natural enemy are released in the field on the

       chance that these limited numbers may be adequate to achieve establishment.


A.  Such attempts usually fail to attain establishment.


B.  Worth a try, however, especially as it might preserve some genetic variability that could be lost in culture.


IV.  Ecological Factors Influencing Success or Failure


A.  Failure of natural enemies to adapt to the climate of the release area has accounted for the largest number

      of unsuccessful colonizations.


  1.  result of direct natural enemy mortality.


  2.  sometimes lack of synchronization between host and natural enemy, in temperature climates especially.


B.  Initial releases of a new species should cover as diverse a climatic area as possible to insure that climatic

      conditions most suited to that particular species are encountered.


C.  A series of strains of the species of natural enemy ought to be tried, since some strains will be better

      adapted to different climates.


D.  Alternate hosts can be important in carrying the natural enemy over unfavorable principal host seasons.


  1.  oligophagous parasitoids may exploit a number of host species to maintain their populations during times

      of principal host scarcity.


  2.  initial releases made under varied conditions can insure that environments frequented by suitable hosts

       are encountered.


E.  Already-established entomophagous species, although less effective as natural enemies, may compete for

      hosts and prevent the limited numbers of individuals of a newly-liberated species from establishing a permanent



  1.  can be minimized by releasing large numbers of a species at each colonization site.


  2.  release sites can be chosen where competitors are rare.


  3.  host insects may be protected with field cages until they multiply sufficiently to hold their own.


F.  Predatory arthropods or insect pathogens may decimate and prevent the establishment of a newly-colonized



       e.g., The scorpion fly, Harpobittacus nigriceps, caused very high mortality among larvae of the cinnabar moth

at colonization sites and thus prevented establishment of this moth for the biological control of the toxic weed,

 tansy ragwort, in Australia.  This was despite a mass rearing program where 500,000 larvae were liberated

during the 1960-61 period.


G.  Other factors of lesser concern:


  1.  unsuitability of certain host plants as shelter for the colonized natural enemy.


  2.  host species may be physiologically unsuited to parasitoid development.


  3.  a highly developed dispersal habit may retard or prevent establishment.


V.  Release Numbers


A.  There are no reliable means of estimating the minimum number of individuals necessary to establish imported

       natural enemies.


B.  Theoretically, this number may be as few as a single mated female, yet sometimes tens of thousands were required.


VI.  Excessive difficulty in the initial establishment of a species indicates its lack of adaptability to the new environment and

 its limited promise as a biological control agent in the area released.


A.  Clausen (1951) after careful analyses of the most successful cases of biological control achieved to the 1950's,

      formulated what has become known as his three-generation, three-year theory. 


  1.  an effective parasitoid or predator can be expected to show evidence of control at the point of release within

      a period of three host generations or three years' time.


  2.  a fully effective parasitoid or predator is always easily and quickly established.


  3.  failure of a parasitoid or predator to become established easily and quickly indicates that it will not be

     fully effective after its establishment is achieved.


  4.  colonization of an imported parasitoid may well be discontinued after three years if there is no evidence of



  5.  Clausen admitted that establishment might be attained by further effort, but that a species that requires such

      efforts will be of little real value, and its mere establishment will not compensate for the additional costs and labor



  6.  Clausen's views have been criticized for neglecting those importations that result in a partial degree of biological

     control, which at least reduces the number and amounts of chemical treatments required.


VII.  After establishment in one locality, natural spread of a natural enemy species is usually aided by distributing field-

      collected adults or parasitized hosts to new locations.


VIII.  Recovery


A.  Field observations of the natural enemy (especially in the case of predators.).

B.  Parasitoid rearing from field-collected hosts.

C.  Dissection of parasitized, field-collected hosts.

D.  Sweep-net or suction machine sampling for adult parasitoids and predators.


IX.  Prediction of Success


A.  The colonization of entomophagous and phytophagous natural enemies largely remains a matter of empirical

      trial and error.


B.  Data from past efforts suggest that the probability of a newly colonized entomophagous species becoming

       permanently established averages one in three.


C.  Predictive data gathered at the point of origin of the natural enemy may require a decade or labor intensive,

      costly effort.  Most projects do not have adequate funds to support such studies, nor may control be delayed

      for that long a time.  Nevertheless, in certain cases, such as in the biological control of weeds and medically

       important arthropods, lengthy pre-introduction studies are required to preclude the introduction of harmful species.


X.  Evaluation


A.  There is continued effort being made in biological control to devise techniques for quantitatively evaluating

      the effect of natural enemies on pest populations in the field.


B.  Evidence for the occurrence of biological control is of three major types:


  1.  data showing a reduction in the pest population density invariably followed the introduction of the natural

      enemy, time after time, in place after place.


  2.  data showing that following the establishment of a natural enemy, the pest population remained at a much

       lower average density than before the establishment of the natural enemy.


  3.  data showing a decidedly higher survivorship of the pest when it was protected from attack by the natural



C.  Some newer approaches that have resulted in variable success are:


  1.  attempts to correlated host and natural enemy population dynamics.


  2.  analyses of life table data.


D.  Experimental Methods


  1.  mechanical exclusion.


  2.  chemical exclusion.


  3.  trap-method.


  4.  hand removal exclusion method.


  5.  biological check method (= use of ants to interfere with natural enemies).


  6.  naturally-occurring exclusion.




Gonzalez, D., W. White, C. Pickett, V. Cervenka, M. Moratorio & L. T. Wilson.  1988.  Biological control of variegated leafhopper

      in grape IPM program.  Calif. Agric. 42:  23-5.


Legner, E. F. & H. W. Brydon.  1966. Suppression of dung inhabiting fly populations by pupal parasites.  Ann. Entomol. Soc.

     Amer. 59(4):  638-651.


Legner, E. F. & R. A. Medved.  1979.  Influence of parasitic Hymenoptera on the regulation of pink bollworm, Pectinophora gossypiella,

      on cotton in the lower Colorado Desert.  Environ. Entomol. 8(5):  922-930.


Legner, E. F. & A. Silveira-Guido.  1983.  Establishment of Goniozus emigratus and Goniozus legneri [Hym: Bethylidae] on navel

     orangeworm, Amyelois transitella [Lep: Phycitidae] in California and biological control potential.  Entomophaga 28(2):  97-106.


Legner, E. F., W. D. McKeen & R. W. Warkentin.  1990.  Inoculation of three pteromalid wasp species (Hymenoptera: Pteromalidae)

      increases parasitism and mortality of Musca domestica L. pupae in poultry manure.  Bull. Soc. Vector Ecol. 15(2):  149-155