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BIOLOGICAL PE|ST CONTROL PRECEPTS
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The Importance of Single Species & the Average Abundance of Plants |
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Table
11.1 (Examples showing host densities dependent on one or a few species) |
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[Please refer also to Selected Reviews |
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Homopterous Insects as Good 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 Demonstrating
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. |