FILE: <ent129.9.htm Comprehensive
Account #1 #2 #3 <Please
refer also to REVIEW>. <Navigate to MAIN MENU>
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
efforts.
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
releases.
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
relationships.
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
colony.
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
species.
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
establishment.
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
involved.
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
enemy.
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.
REFERENCES:
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