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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
plants.
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.
REFERENCES:
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.