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BIOLOGICAL PE|ST CONTROL PRECEPTS
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Table 11.1(Examples showing host densities) |
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Homopterous Insects as
Biological Control Candidates Scale
insects, mealy bugs, whiteflies and aphids have been targets of numerous
biological control projects. The first example of a scale insect being
controlled biological was, of course, the cottony-cushion scale, Icerya purchasi Maskell, in California around 1880. Novius (Rodolia) cardinalis (Mulsant) gave complete
control in 24 additional countries. Cryptochaetum
iceryae (Williston) was also
variously involved. Other examples include the citrophilus mealybug, Pseudococcus fragilis Brain, in
California; the coconut scale, Aspidiotus
destructor Signoret, in Fiji, Mauritius and Principe; Green's mealybug, Pseudococcus citricolus Green, in Israel; the red wax scale, Ceroplastes rubens Maskell, in Japan;
the coffee mealybug, Planococcus kenyae
(LePelley), in Kenya; and the citrus snow scale, Chienaspis citri (Comstock), in Florida and Peru, the woolly
whitefly, Aleurothrixus floccosus
(Maskell); the walnut aphid, Chromaphis
juglandicola (Kaltenbach), and the blue aphid, Acrythosiphon kondoi
Shinji, in California Many additional homopterous insects were controlled either
completely or substantially, and some partially. Such terms to describe control
levels, although imperfect, still are used widely today. Scale insects alone
account for nearly one-half of all projects against insects where some degree
of success was evident. By 1973 about 67% of all complete successes, 31% of
all substantial successes, and 43% of all partial successes involved scale
insects. Homoptera as a whole contain about two-thirds of all successes. The unusual biological control success rate
with Homoptera may reflect a greater amount of effort; but it also indicates
that these insects are more amenable to biological control in that about 78%
of all attempts against them were successful. They are frequent invaders, and
therefore deserve more attention than other insects. Certain biological characteristics make
Homoptera especially good candidates for biological control. Most are
sedentary in habit and distributed in colonies. There is chronological
continuity of all life stages in a population throughout the year in most
species; and there is a certain degree of population stability conferred by
perennial host plants upon which they occur. Parasitoids and predators are
more likely to reach full effectiveness on this type of host population. It
is thought that the 78% success figure could be much higher if efforts were
continued on previous candidates, as was the case with the red scale work in
California, which involved the work of two generations of scientists. Examples
That Demonstrate Precepts Cottony-cushion scale--Icerya purchasi Maskell-- This species was accidentally introduced into California
around 1868 and became extremely serious 19 years later (1887). Australia
happened to be the native home as determined by the scarcity of this pest
there. Introductions in 1888-89 were made of Novius cardinalis
and Cryptochaetum iceryae. Cryptochaetum was first regarded as the most promising species; however, Novius outperformed it in the
commercial, drier areas of citrus. Complete control was achieved by the end
of 1889. More recent studies by Jose Quezada (1973)
showed that both natural enemies are effective. Novius is dominant in desert areas and displaces Cryptochaetum in competition. Cryptochaetum is dominant on
the coast and tends to displace Novius.
The two species co-exist in the intermediate zones. This example lends support to the precept of
multiple introductions of natural enemies: as many potentially effective
natural enemies of a pest as possible should be introduced. The most
efficient in a given habitat will tend to displace the others and produce
better overall control. Competition is not generally regarded as an
incumbrance to the overall effectiveness of natural enemies in biological
control Florida red scale--Chrysomphalus ficus Ashmead.-- This species invaded Israel around 1910. There it was
attacked by an indigenous ectoparasitoid, Aphytis
chrysomphali (Mercet) whose
effect was negligible. Pteroptrix
(Casca) smithi (Compere) and Aphytis
holoxanthus DeBach
(--originally thought to be A.
lingnanensis Compere) were
introduced from Hong Kong in 1956-57. Complete control was achieved in 2-3
years with A. holoxanthus on the coast, and
partial control in the hot Jordan Valley. The importance of biosystematics to
biological control is illustrated in the history of these introductions. The original
Hong Kong material consisted of several parasitoid species, none of which
were identified prior to establishment in the field. A mixture of parasitoids
was later thought to have been in the initial imported material (even
including some phytophagous thrips, because they had the appearance of Aphytis spp. to the
investigators). More than one species of Aphytis
entered this way undetected because of difficulties of separating the various
species. It was thought that the California red scale parasitoid, Aphytis cohnei DeBach, and the purple scale, Lepidosaphes beckii
(Newm.), parasitoid Aphytis lepidosaphes Compere entered
Israel in this way. Pteroptrix smithi had
no apparent effect on initial biological control of Florida red scale in
Israel. it dispersed through the range of its host, but there is no evidence
that it detracted from the effectiveness of A. holoxanthus.
Rather, it is regarded as a complement to overall biological control of the
scale. Aphytis chrysomphali was entirely
displaced by A. holoxanthus, and another
parasitoid Habrolepis fanari DeLucci & Traboulsi,
which entered the scene later. This example illustrates the beneficial
aspects of multiple introduction. Competition did not obviously deter from
success in biological control. Aphytis
holoxanthus evoked
successful control in surrounding Arab countries and in other countries where
it was subsequently introduced (e.g., Florida, Mexico, South Africa, Brazil
and Peru). In Israel, the California red scale has in
recent years become a more serious pest. Previously, the Florida red scale
was an effective competitor with California red scale. However, the
suppression of the competitor by A.
holoxanthus has allowed the
California scale to increase. This illustrates the need for a multiple
project approach in biological control. California red scale--Aonidiella aurantii (Maskell)-- The red scale invaded California around 1868-75, and
attempts to introduce natural enemies were begun in 1889. However, the most
important natural enemies were introduced over 50 years later in 1948-47!
Biological control of California red scale was considered a complete failure
for those 50 years. During this "black out" period, Aphytis lingnanensis was not introduced from China because
taxonomists thought it already occurred in California. When it was finally
introduced in 1948, it was very effective in control and far superior to Aphytis chrysomphali with which it had been confused. A long series of failures to establish
imported natural enemies resulted because of inadequate taxonomic knowledge
of the host scales. Several parasitoids were repeatedly obtained from the
Orient from scales that were misidentified as the California red scale. Some
failures were also the result of cryptic effects of host plant on certain
endoparasitoids. The host plant imparted an intrinsic immunity to the scale.
All these and more errors led to the conclusion that no effective parasitoids
were present in the Orient. After clearing the confusion, two
endoparasitoids were introduced from China and established in California.
These were Comperiella bifasciata Howard (1941), and Prospaltella perniciosi Tower (1947). Pteroptrix (Casca) chinensis (Howard) was not successfully established due to
insufficient knowledge of its biology. This species might still be the final
best bet. Climate-related restrictions on Aphytis lingnanensis resulted in the importation of a better
adapted species from India and Pakistan in 1956-57. Aphytis melinus
DeBach, Aphytis fisheri DeBAch (a sibling
species of A. melinus) was also introduced
from Burma, but competition with the other Aphytis is thought to have precluded its establishment. The percent parasitization in areas where the red scale is now
held at low population densities by these parasitoids is only 15-20% on a
year-round average. This gives an example of the uselessness of a percent
parasitization figure, especially when it is known that the parasitoids kill
a lot of the scales by probing and host-feeding actions. When a particular
parasitoid population begins its activity on red scale in a citrus grove,
parasitization is low and the proportion of living scales is high. As the
percent parasitization approaches the "equilibrium" average of
15-20%, the proportion of live scales becomes low. Therefore, relatively
small increases in parasitization are reflected by relatively great increases
in red scale mortality. Olive scale--Parlatoria oleae (Colvee).-- The olive scale became established near Fresno, California in
1934, where it was a major pest of many deciduous fruit crops and ornamental
trees and shrubs. It spread over the entire Central Valley and down into
portions of southern California. There are two generations per year, one each
in the spring and autumn. On olive the autumn generation scales are direct
pests of the fruit. Aphytis maculicornis
(Masi) was introduced from Egypt in 1949, followed by continued searching for
natural enemies in Europe and Asia. Among the various parasitoids introduced
there were some distinct strains of A.
maculicornis. The Persian
strain alone was effective, and it was colonized by the millions. The percent
parasitization averaged about 90% at low scale densities (also about 90% of
the original population density). However, this drastic reduction was not
sufficient because even one scale per fruit was an economic loss. Aphytis maculicornis could not perform better because it was
unable to tolerate the heat of summer, and winter was equally severe on its
survival. In 1957 two more parasitoids were introduced
from Pakistan, namely Coccophagoides
utilis Doutt and Anthemus inconspicuus Doutt. Coccophagoides
was artificially spread by causing outbreaks of the host scales in orchards
with DDT, in order to temporarily reduce the effects of A. maculicornis.
Coccophagoides is endoparasitic
with primary and hyperparasitic habits, where the males are produced
hyperparasitically on females of the same species. It averages 40%
parasitization and occupies the niche left open by A. maculicornis
during summer, thereby contributing additional mortality to the autumn
generation. Coccophagoides
complements A. maculicornis, the latter being
the superior parasitoid when weather conditions are right. This example
illustrates another score for multiple introductions. Rhodesgrass scale--Antonina graminis (Maskell).-- A biological control project was begun in 1962 in portions
of the southeastern United States to control Rhodesgrass scale. Five species
of parasitoids were introduced as follows: Anagyrus antoninae
Timberlake from Hawaii; Xanthoencyrtus
phragmitis Ferr. from
France; Boucekiella antoninae (Ferr.) from France; Timberlakia europaea (Mercet) from France
and Anagrus diversicornis Mercet from
France. None of these species are known to have become established. A final
introduction of Neodusmetia sangwani (Rao) from India did
become established and finally controlled the scale (Schuster et al. 1971). The females of N. sangwani
cannot fly; therefore, the parasitoids were spread by airplane over the
scale-infested terrain. Rhodesgrass yield comparisons between treatments was
the most reliable measure of effectiveness, because percent parasitization by
the parasitoid was not often dramatic. This successful biological control effort
illustrates the importance of being persistent on ones efforts to secure
additional parasitic species. It also shows how technology may hasten the
control process, in this case spreading parasitoids by airplane. Finally, it
is important to judge the success of a project not by the degree of
parasitism but rather by the amount of control actually achieved. Walnut
aphid--Chromaphis juglandicola (Kaltenbach).-- The aphid was controlled in southern California with a strain
of Trioxys pallidus (Haliday) introduced
from France in 1959; and one decade later in northern California with a T. pallidus strain from Iran. The second introduction is
thought to have been a sibling species as some reproductive isolation from
the first species was detected. Complete control was achieved, as previously
discussed. This is another demonstration of the importance of multiple
introductions of different apparent strains of natural enemies from different
climatic areas. Orb-weaving spiders.--Interspecific competition between two orb-weaving spiders, Metepeira grinnelli (Coolidge) and Cyclosa turbinata
(Walckenaer), was investigated by Spiller (1986), who selectively removed the
predators. The estimated predation rate of small prey was higher when Cyclosa was alone than when
both species were present, because when Metepeira
was removed the density of Cyclosa
became higher than the combined density of both spiders. This was because the
consumption rate of small prey by Metepeira
was very low compared with that of Cyclosa.
The study suggested that a subset of predator species might be more effective
in reducing prey populations than the entire natural guild (Spiller 1984a,b,
1986). The example argues against multiple introductions. The
Importance of Single Species in Determining
the Average Density of
Plants and Animals The presence of one or two species in the
ecosystem is known to influence drastically the population density of plants
and animals (Legner 1987). The realization of this is probably of one of the most
difficult concepts to grasp for modern ecologists who through their broad
experiences in measuring density dependent and density independent forces in
nature appreciate the complexities of the ecosystem. It seems inconceivable
that in the midst of all the interacting abiotic and biotic factors, only one
or two organisms could ever be responsible for the average abundance of
another organism. Nevertheless, proof for this simplistic
assumption is available from many sources. Breaking down the world's biota
into terrestrial plants, aquatic plants, vertebrates, phytophagous insects
and insects of medical and veterinary importance, Table 1 gives selected
examples to demonstrate the importance of one or two species in accounting
for tolerably low densities of other organisms. Many of the causative agents
act as density dependent regulative forces which bear a reciprocal density
relationship to their hosts, or as limiting forces which set an upper limit
to the density that an organism can attain but which do not bear the close
relationship of reciprocity. If there are any doubts of the basic
assumption that the presence of one or two organisms account for the observed
low population densities of the various species listed in Table 1, the
question may be asked, "What would happen to the average density of the
controlled organism if the causative agent were removed?" Invariably,
the answer would be simply that a rise in density would follow the removal. It is apparent that the greatest number of
examples are found among phytophagous insects, which is a reflection of the
greater biological control effort against this group. Insects of medical and
veterinary importance are just becoming favored targets for biological
control as the desire to reduce pesticide use against them increases. Thus,
we undoubtedly will see more successful cases in years to come. Table 11.1
gives examples of the abundance of plants and animals
dependent on the presence of one or a few species of organisms in the
ecosystem. The importation of new natural enemies from abroad is the
single best approach to biological control. This approach needs much more
emphasis in current biological control attempts. The search for natural
enemies should extend throughout the entire range of distribution of the
pest. Accurate biosystematics are necessary as well as basic
ecological studies of the pest-natural enemy complex at home and abroad.
However, neither should retard the simultaneous importation of new natural
enemies. The most successful natural enemies have shown high host or
prey specificity. They are often multivoltine with respect to their prey, and
well adapted to the physical conditions of the pest habitat. They are also
good searchers. However, there is no single best natural enemy for a given
pest. In most cases of complete biological control success, only one
or two natural enemy species are involved. Different species or strains of
natural enemies are usually required when the pest is to be controlled in a
wide area with different climates. The interspecific competition between
natural enemies in the areas of overlap has not been shown to be detrimental
to regulation at a satisfactory control level, although theoretically there
is a risk (Force 1974; Ehler 1979, 1982, 1985; Turnbull & Chant 1961). Multiple importations of competing parasitoids and predators
are a practical way to practice biological control, which has not been shown
to be detrimental to overall host reduction. The so-called direct pests (e.g., olive scale) are suitable subjects
for biological control although the probability of success with direct pests
may be lower than with indirect pests. Exercise 11.1--Would you defend the multiple species introduction approach
for biological control? If so how? If not, why? Exercise 11.2--How many biological control precepts can be identified? Exercise 11.3--How is biosystematics necessary in biological control work?
Give examples. Exercise 11.4--Discuss in detail the hosts, natural enemies, and population
dynamics associated with the biological control of the following:
cottony-cushion scale, Florida red scale, California red scale, walnut aphid,
olive scale, Rhodesgrass scale, navel orangeworm. Exercise 11.5--Can you suggest a practical alternative to the designations
"complete", "substantial" and "partial" success
for biological control? REFERENCES: [Additional references may
be found at MELVYL
Library ] Bellows, T. S., Jr.
& T. W. Fisher, (eds) 1999. Handbook of Biological Control: Principles
and Applications. Academic Press, San Diego, CA. 1046 p. DeBach, P. 1969.
Biological control of diaspine scale insects on citrus in California. Proc. 1st
Intern. Citrus Symp., Riverside, Calif. (1968) 2: 801-15. DeBach, P. 1971. The
use of imported natural enemies in insect pest management ecology. Proc. Tall
Timbers Conf. on Ecological Animal Control by Habitat Management 3: Feb.
25-27, Tallahassee, Fla. p. 211-33. DeBach,
P. (ed.) 1974. Biological
Control by Natural Enemies. Cambridge Univ. Press, London & New York. 323
p. DeBach,
P. & D. Rosen. 1971.
Biological control of coccids by introduced natural enemies. In: C. B.
Huffaker (ed.) "Biological Control." Plenum Press, N.Y. p. 165-94. Case, T. J., M. E.
Gilpin & J. M. Diamond. 1979. Overexploitation, interference competition
and excess density compensation. Amer.
Nat. 113: 843-54. Diamond,
P. 1973. The
effect of multiple parasitoid introductions upon equilibrium value of host
density. Oecologia (Berlin) 13: 279-90. Ehler,
L. E. 1979. Assessing
competitive interactions in parasite guilds prior to introduction. Environ.
Ent. 8: 558-60. Ehler, L. E. 1982.
Foreign exploration in California. Environ. Ent. 11: 525-30. Ehler, L. E. 1985.
Species-dependent mortality in a parasite guild and its relevance to
biological control. Environ. Ent. 14: 1-6. Flanders, S. E. 1969.
Herbert D. Smith's observations on citrus blackfly parasites in India and
Mexico. Canad. Ent. 101: 467-80. Force, D. C. 1974.
Ecology of insect host-parasitoid communities. Science 184: 624-32. Gonzalez, D., M.
Miyazaki, W. White, H. Takada, R. D. Dickson & J. C. Hall. 1979.
Geographical distribution of Acrythosiphon kondoi Shinji
(Homoptera: Aphididae) and some of its parasites and hyperparasites in Japan.
Kontyu, Tokyo 47(1): 1-7. Harpaz, I. & D.
Rosen. 1971. Development of integrated control programs for crop pests in
Israel. In: C. B. Huffaker (ed.), "Biological Control."
Plenum Press, N.Y. p. 458-68. Hogarth, W. L. & P.
Diamond. 1984. Interspecific competition in larvae between entomophagous
parasitoids. Amer. Nat. 124: 552-60. Huffaker,
C. B. & C. E. Kennett. 1966.
Biological control of Parlatoria oleae (Colvee) through the
compensatory action of two introduced parasites. Hilgardia 37(9): 283-335. 235. Legner, E. F. 1987. The importance of
single species in determining the average density of plants and animals. Proc. Calif. Mosq. & Vector Contr. Assoc.,
Inc. 55: 121-123. Maltby,
H. L., E. Jimenez-Jimenez & P. DeBach. 1968. Biological
control of armored scale insects in Mexico. J.
Econ. Ent. 61: 1086-88. May, R. M. & M. P.
Hassell. 1981. The dynamics of multiparasitoid-host interactions. Amer.
Nat. 117: 234-61. Quezada,
J. R. & P. DeBach. 1973.
Bioecological and population studies of the cottony cushion scale, Icerya
purchasi Mask., and its natural enemies, Rodolia cardinalis
Muls., and Cryptochaetum iceryae Will., in southern California.
Hilgardia 41(2): 631-88. Schuster, M. F., J. C.
Boling & J. J. Marony, Jr. 1971. Biological control of Rhodesgrass scale
by airplane releases of an introduced parasite of limited dispersing ability.
In: C. B. Huffaker (ed.), "Biological Control." Plenum Press,
N.Y. p. 227-50. Spiller, D. A. 1984a.
Competition between two spider species: experimental field study. Ecology 65:
909-19. Spiller, D. A. 1984b.
Seasonal reversal of competitive advantage between two spider species.
Oecologia (Berlin) 64: 322-31. Spiller, D. A. 1986.
Interspecific competition between spiders and its relevance to biological
control by general predators. Environ. Ent. 15: 177-81. Turnbull, A. L. &
D. A. Chant. 1961. The practice and theory of biological control of insects
in Canada. Canad. J. Zool. 39: 697-753. van den Bosch, R., B. D. Frazer, C. S. Davis, P. S.
Messenger & R. Hom. 1970. Trioxys pallidus--an
effective new walnut aphid parasite from Iran. Calif. Agric. 24(11): 8-10. |